Coding Questions VIII - 2023

Find the longest sequence of zeros in binary representation of an integer (Max count of 0s in between 1s).

9 (1001) => 2, 529 (1000010001) => 44, 20 (10100) => 1, 15 (1111) =>0, and 32 (100000) => 0.

def binaryGap(n):

  # convert an int to an array of integers (0 and 1)
  bits = []
  while n > 0:
    bits.append(n % 2)
    n = n // 2
    
  # reverse the list : for n = 20 bits = [0,0,1,0,1] => [1,0,1,0,0]
  bits = bits[::-1]

  # get max gap
  count = 0
  mxgap = 0
  for b in bits:
    if b == 1:
      mxgap = max(count, mxgap)
      count = 0
    else:
      count += 1

  return bits,mxgap

numbs = [9, 529, 20, 15, 32]
for n in numbs:
  bits, mxgap = binaryGap(n)
  print("%d => %s mxgap = %d" %(n,bits,mxgap))

Output:

9 => [1, 0, 0, 1] mxgap = 2
529 => [1, 0, 0, 0, 0, 1, 0, 0, 0, 1] mxgap = 4
20 => [1, 0, 1, 0, 0] mxgap = 1
15 => [1, 1, 1, 1] mxgap = 0
32 => [1, 0, 0, 0, 0, 0] mxgap = 0    

[1,2,3] => [() (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)]

import itertools

a = [1,2,3]
ans = []
for r in range(0,len(a)+1):
  nexts = itertools.combinations(a,r)
  for n in nexts:
    ans.append(n)
print(ans)    # [() (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)]

['ABC', 'D', 'EF'] => [A, D, E, B, F, C]:

a = ['ABC','D','EF']

# construct a dictionary - { string: current length }
# {'ABC': 3, 'D': 1, 'EF': 2}
da = { x:len(x) for x in a }

# get max length of the strings
mx = max(da.values())

ans = []

# loop with max length
for i in range(mx):
  # loop the dictionary  
  for k,v in da.items():
    if v > 0:
      # append a char starting from the first one 
      ans.append(k[-v])
      v -= 1
      da[k] = v

print(ans)    

Output:

['ABC', 'D', 'EF'] => ['A', 'D', 'E', 'B', 'F', 'C']    

'ABCDEFG', 3, 'x' --> ['ABC','DEF', 'Gxx']

def block(a,n,mark):
  ans = []
  count = len(a) // n   # full string count
  nleft = len(a) % n    # length of partial string
  nmark = n - nleft     # number of 'x's to fill

  # full string
  for i in range(count):
     ans.append(a[i*n:(i+1)*n])

  # partial string
  ans.append(a[count*n:count*n+nleft]+nmark*'x')
  return ans

a = 'ABCDEFG'
n = 3
mark = 'x'

answer = block(a,n,mark)
print(answer)    # ['ABC','DEF', 'Gxx']

[1,2,3,4,5] => [1,3,6,10,15]

def accumulate(a):
  ans = [a[0]]
  for i in range(1,len(a)):
     ans.append(a[i-1]+a[i])
  return ans

a = [1,2,3,4,5]
ans = accumulate(a)
print(ans)        # [1,3,6,10,15] 

Given list of strings, ['ABC', 'DEF'], write a function that produces A B C D E F, fn (iterable).

The following code is an example of the generator function:

def chain_from_iterable(a):
  for s in a:
    for c in s:
      yield c

a = ['ABC','DEF']

# chain_from_iterable(a) is a generator function.
for x in chain_from_iterable(a):
  print(x, end=' ')    # A B C D E F

A return sends a specified value back to its caller while yield produces a sequence of values. In other words, The difference is that while a return statement terminates a function entirely, yield statement pauses the function saving all its states and later continues from there on successive calls.
We may want to use yield when we want to iterate over a sequence instead of storing the entire sequence in memory.
The yield is using a generator and a generator function is defined like a normal function but with the yield keyword rather than return. So, whenever the body of a def contains yield, the function automatically becomes a generator function.

Make an iterator that filters elements from data returning only those that have a corresponding element in selectors that evaluates to True. Stops when either the data or selectors iterables has been exhausted.

compress('ABCDEF', [1,0,1,0,1,1]) => ACEF

def compress(data, selector):
  return (d for d,s in zip(data, selector) if s)

a = 'ABCDEF'
bit = [1,0,1,0,1,1]

for x in compress(a,bit):
  print(x, end='')    # ACEF   

Note that the "compress()" returns a generator, (d for d,s in zip(data, selector) if s).

To return just a list and process it, we do the following:

def compress(data, selector):
  return [d for d,s in zip(data, selector) if s]

a = 'ABCDEF'
bit = [1,0,1,0,1,1]

d = ''.join(compress(a,bit))
print(d)    # ACEF     

Make an iterator that drops elements from the iterable as long as the predicate is true; afterwards, returns every element. Note, the iterator does not produce any output until the predicate first becomes false, so it may have a lengthy start-up time.

def dropwhile(predicate, a):
# dropwhile(lambda x: x<5, [1,4,6,4,1]) --> 6 4 1
  iterator = iter(a)
  for x in iterator:
    if not predicate(x):   # if x > 5
      yield x
      break;

  for x in iterator:
    yield x

pred = lambda x: x<5
a = [1,4,6,4,1]
for x in dropwhile(pred,a):
  print(x, end = ' ')    # 6 4 1    

Note that within the generator function, we need to convert the iterable to an iterator via iter(iterator) so that the generator keeps the track of the iterator (to resume the iteration where it left off at the break).

Write a function that does 'AAAABBBCCD' --> AAAA BBB CC D

def groupby(a):
  prev = a[0]
  for c in a:
    # yield a blank if a new char
    if c != prev:
      yield ' '
    # otherwise, just yield the char
    yield c
    prev = c

x = ['ABC', 'ABCDD', 'AAAABBBCCD']
new_x = []
for e in x:
  out = ''
  for c in groupby(e):
    out += c
  new_x.append(out)

print("%s \n => %s" %(x,new_x))    

Output:

['ABC', 'ABCDD', 'AAAABBBCCD'] 
 => ['A B C', 'A B C DD', 'AAAA BBB CC D']    

Write a generator function, product('ABCD', '12') that yields A1, A2, B1...D2 one by one:

def product(*args):
  ar = list(args)
  pools = [list(p) for p in ar]

  print('args = %s' %(args,))
  print('pools = %s' %pools)

  for p0 in pools[0]:
    for p1 in pools[1]:
      yield p0+p1     # A1, A2, B1...D2  one by one


a = 'ABCD'
b = '12'
for p in product(a,b):
  print(p, end=' ')    

Output:

args = ('ABCD', '12')
pools = [['A', 'B', 'C', 'D'], ['1', '2']]
A1 A2 B1 B2 C1 C2 D1 D2    

Actually, we can use a list comprehension if not for the yield requirement:

a = 'ABCD'
b = '12'
c = [ x+y for x in a for y in b ]    

'ABCD' => AB AC AD BA BC BD CA CB CD DA DB DC

a = 'ABCD'
x = list(a)
n = len(x)
for i in range(n):
  for j in range(n):
    if i != j:
      print(x[i]+x[j], end=' ')    

Output:

AB AC AD BA BC BD CA CB CD DA DB DC    

With a list comprehension, we can do it as the following:

a = 'ABCD'
b = ' '.join([ x+y for x in a for y in a if x != y ])
print(b)

Output:

AB AC AD BA BC BD CA CB CD DA DB DC

Make an iterator that computes the function using arguments obtained from the iterable. Used instead of map() when argument parameters are already grouped in tuples from a single iterable (the data has been "pre-zipped").

def starmap(fn,iterable):
  for arg in iterable:
    # unpacking a tuple into two arguments 
    yield fn(*arg)

x = [(2,5),(3,2),(10,3)]
for e in starmap(pow,x):
  print(e)    

Output:

32
9
1000    

Note the difference between map() and starmap() parallels the distinction between function(a,b) and function(*c) (itertools.starmap(function, iterable)).

Make an iterator that aggregates elements from each of the iterables.

zip_longest('ABCD', 'xy', fillvalue='-') => Ax By C- D-    

If the iterables are of uneven length, missing values are filled-in with fillvalue in the StopIterator section. Iteration continues until the longest iterable is exhausted.

def zip_longest(*args, fillvalue=None):
  iterators = [iter(it) for it in args]
  num = len(iterators)
  while True:
    values = []
    for i, it in enumerate(iterators):
       try:
         value = next(it)
         print("value=%s" %value)
       except StopIteration:
         num -= 1
         if not num:
           return
         iterators[i] = iter(fillvalue)
         value = fillvalue

       values.append(value)

    yield tuple(values)

for s in zip_longest('ABCD','xy',fillvalue='-'):
    print(s[0]+s[1])    

Output:

Ax
By
C-
D-

Ref: itertools.zip_longest(*iterables, fillvalue=None).

1. def sum(a, b):
       """
       sum(4, 3)
       7
   
       sum(-4, 5)
       1
       """
       return a + b    
   




2. def sum(a, b):
       """
       >>> sum(4, 3)
       7
   
       >>> sum(-4, 5)
       1
       """
       return a + b    
   




3. def sum(a, b):
       """
       # >>> sum(4, 3)
       # 7
   
       # >>> sum(-4, 5)
       # 1
       """
       return a + b    
   




4. def sum(a, b):
       ###
       >>> sum(4, 3)
       7
   
       >>> sum(-4, 5)
       1
       ###
       return a + b    
   

Ans. #2

Ref: doctest — Test interactive Python examples

What would this expression return?

college = ['Freshman', 'Sophomore', 'Junior', 'Senior']
c = list(enumerate(college, 2021))
print(c)

Ans:

[(2021, 'Freshman'), (2022, 'Sophomore'), (2023, 'Junior'), (2024, 'Senior')]    

The enumerate() code should look like this:

def enumerate(sequence, start=0):
    n = start
    for elem in sequence:
        yield n, elem
        n += 1    

[('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)] => [('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]:

from collections import defaultdict
s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
d = defaultdict(list)
for k,v in s:
  d[k].append(v)

sd = sorted(d.items())
print(sd)

Output:

[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

(Note) If we try to access a key in a dictionary that doesn't exist, defaultdict will create a new key for us instead of throwing a KeyError.

Ref: defaultdict objects

1. self means that no other arguments are required to be passed into the method.
2. There is no real purpose for the self method; it's just historic computer science jargon that Python keeps to stay consistent with other programming languages.
3. self refers to the instance whose method was called.
4. self refers to the class that was inherited from to create the object using self.

Ans: #3

Suppose we have the following class with instance method foo():

# a.py
class A:
   message = "class message"

   @classmethod
   def cfoo(cls):
      print(cls.message)

   def foo(self, msg):
      self.message = msg
      print(self.message)

   def __str__(self):
      return self.message

Because the method foo() is designed to process instances, we normally call the method through instance:

>>> from a import A
>>> a = A()
>>> a.foo('instance call')
instance call

When we call the method, the Python automatically replace the self with the instance object, a, and then the msg gets the string passed at the call which is 'instance call'.

(Note) Instance methods can modify the state of an instance or the state of its parent class.

from collections import namedtuple
Point = namedtuple('Point', ['x', 'y'])

p = Point(10,20)
print("p[0]+p[1]=%s" %(p[0]+p[1]))

x,y = p
print("x = %s y = %s" %(x,y))

print("p = ", p)    

Output:

p[0]+p[1]=30
x = 10 y = 20
p =  Point(x=10, y=20)    

1. We can assign a name to each of the namedtuple members and refer to them that way, similarly to how we would access keys in dictionary.
2. Each member of a namedtuple object can be indexed to directly, just like in a regular tuple.
3. namedtuple are just as memory efficient as regular tuples.

What does calling namedtuple on a collection type return?

Ans: a tuple subclass with iterable named fields

Given the following three list, how would we create a new list that matches the desired output printed below?

fruits = ['Apples', 'Oranges', 'Bananas']
quantities = [5, 3, 4]
prices = [1.50, 2.25, 0.89]

# Desired output
[('Apples', 5, 1.50),
('Oranges', 3, 2.25),
('Bananas', 4, 0.89)]

The code should look like this:

fruits = ['Apples', 'Oranges', 'Bananas']
quantities = [5, 3, 4]
prices = [1.50, 2.25, 0.89]

z = list(zip(fruits, quantities, prices))
print(z)    

1. A set is an unordered collection of unique items.
2. A list is an ordered collection of non-unique items.

The __init__ method is a constructor method that is called automatically whenever a new object is created from a class. It sets the initial state of a new object.

What statement about a class methods is true?

1. A class method is a regular function that belongs to a class, but it must return None.
2. A class method can modify the state of the class, but they can't directly modify the state of an instance that inherits from that class.
3. A class method is similar to a regular function, but a class method doesn't take any arguments.
4. A class method hold all of the data for a particular class.

Ans: #2

(Note) The idea of class method is very similar to instance method, only difference being that instead of passing the instance hiddenly as a first parameter, we're now passing the class itself as a first parameter.

class MyMethods:
  def i_method(self,x):
    print(self,x)

  @staticmethod
  def s_method(x):
    print(x)

  @classmethod
  def c_method(cls,x):
    print(cls,x)

obj = MyMethods()

obj.i_method(1)
obj.s_method(3)
obj.c_method(5)  

Output:

<__main__.MyMethods object at 0x1038dc0a0> 1
3
<class '__main__.MyMethods'> 5    

Since we're passing only a class to the method, no instance is involved. This means that it can't directly modify the state of an instance.

1. Permutations
   ['AB', 'AC', 'BA', 'BC', 'CA', 'CB']


2. Multiset Permutations
   ['AA', 'AB', 'AC', 'BA', 'BB', 'BC', 'CA', 'CB', 'CC']


3. Combinations
   ['AB', 'AC', 'BC']


4. Combinations with repeatition
   ['AA', 'AB', 'AC', 'BB', 'BC', 'CC']

The code using itertools should look like this:

import itertools

a = ['A','B','C']

def merge(input):
  ans = []
  for x in input:
    ans.append(x[0]+x[1])
  return ans

print("Permutations")
pm = list(itertools.permutations(a,2))
print(merge(pm))

print("Multiset Permutations")
pd = list(itertools.product(a,repeat=2))
print(merge(pd))

print("Combinations")
cb = list(itertools.combinations(a,2))
print(merge(cb))

print("Combinations with Repetition")
cbr = list(itertools.combinations_with_replacement(a,2))
print(merge(cbr))    

Sort the following list of dictionaries by 'age':

animals = [
     {'type': 'Crow', 'name': 'clair', 'age': 16},
     {'type': 'Elephant', 'name': 'ivory', 'age': 4},
     {'type': 'Lion', 'name': 'Leo', 'age': 7},
 ]    

import operator

animals = [
     {'type': 'Crow', 'name': 'clair', 'age': 16},
     {'type': 'Elephant', 'name': 'ivory', 'age': 4},
     {'type': 'Lion', 'name': 'Leo', 'age': 7},
 ]

# using lambda for key
sd = sorted(animals, key = lambda x: x['age'])
print(sd)

# using operator.itemgetter for key
sd2 = sorted(animals, key = operator.itemgetter('age'))
print(sd2)

Output:

[{'type': 'Elephant', 'name': 'ivory', 'age': 4}, {'type': 'Lion', 'name': 'Leo', 'age': 7}, {'type': 'Crow', 'name': 'clair', 'age': 16}]
[{'type': 'Elephant', 'name': 'ivory', 'age': 4}, {'type': 'Lion', 'name': 'Leo', 'age': 7}, {'type': 'Crow', 'name': 'clair', 'age': 16}]    

Write a function returning a list of unique words.

def uniq_words(ws):
  wset = set()
  for w in ws.split():
    wset.add(w)
  return list(wset)

sentence = "There is no strife no prejudice no national conflict in outer space as yet"
print(uniq_words(sentence))    

Output:

['national', 'There', 'is', 'in', 'yet', 'as', 'strife', 'outer', 'prejudice', 'conflict', 'space', 'no']    

Note that we're not using a list to processing if a word is already in the list. Rather we're using set() from the onset.

"Sets are significantly faster when it comes to determining if an object is present in the set (as in x in s), but are slower than lists when it comes to iterating over their contents."- Check the efficiencies (list vs set) from Python Sets vs Lists

Hash functions take an arbitrary amount of data and return a fixed-length bit string. The output of the function is called secure hash or message digest.

Hash functions are widely used in cryptography.

We want to write a function that hashes a file (lorem.txt) and return the secure hash:

Lorem ipsum dolor sit amet, consectetur adipisicing elit, 
sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. 
Ut enim ad minim veniam, 
quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. 
Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. 
Excepteur sint occaecat cupidatat non proident,
sunt in culpa qui officia deserunt mollit anim id est laborum.    

The code should look like this:

import hashlib

def hashing(fname):
  m = hashlib.sha256()
  with open("lorem.txt", "rb") as f:
    line = f.readline()
    m.update(line)
  # return(m.digest())
  return(m.hexdigest())

secure_hash = hashing("lorem.txt")
print(secure_hash)    

Output:

ea7ead7f16fd6b4d2314312bab772ea74aae2700f0c243171d89bf9b1e75c2e1    

Note that the hash functions are meant to be one-way. So, we cannot convert the output back to the original text.

If we use digest() instead of hexdigest() method for the hash objet m, we'll get the following output:

b"\xea~\xad\x7f\x16\xfdkM#\x141+\xabw.\xa7J\xae'\x00\xf0\xc2C\x17\x1d\x89\xbf\x9b\x1eu\xc2\xe1"    

We can read certain amount of byte one at a time. For example, 512 bytes per read, the code should look like this:

import hashlib

def hashing(fname):
  m = hashlib.sha256()
  with open("lorem.txt", "rb") as f:
    chunk = 0
    while chunk != b'':
      chunk = f.read(512)
      m.update(chunk)
  return(m.hexdigest())

secure_hash = hashing("lorem.txt")
print(secure_hash)    

Output:

6ef2bb967fcd5de4d5d0c1be2226496d89a1d15c6343e11cdf8c216a337517df    

Methods of io.TextIOWrapper:

1. f.read(): reads whole file and returns a string
   

   >>> with open('lorem.txt','r') as f:
   ...   f.read()
   ... 
   'Lorem ipsum dolor sit amet, consectetur adipisicing elit, \nsed do eiusmod tempor incididunt ut labore et dolore magna aliqua. \nUt enim ad minim veniam, \nquis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. \nDuis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. \nExcepteur sint occaecat cupidatat non proident,\nsunt in culpa qui officia deserunt mollit anim id est laborum.\n'    
   



2. f.readline(): reads a line one by one attaching '\n' at the end of the line
   

   >>> f = open('lorem.txt','r')
   >>> f.readline()
   'Lorem ipsum dolor sit amet, consectetur adipisicing elit, \n'
   >>> f.readline()
   'sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. \n'
   >>> f.readline()
   'Ut enim ad minim veniam, \n'    
   



3. f.readlines(): reads all lines as a list. Same as list(f)
   

   >>> f = open('lorem.txt','r')
   >>> f.readlines()
   ['Lorem ipsum dolor sit amet, consectetur adipisicing elit, \n', 'sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. \n', 'Ut enim ad minim veniam, \n', 'quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. \n', 'Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. \n', 'Excepteur sint occaecat cupidatat non proident,\n', 'sunt in culpa qui officia deserunt mollit anim id est laborum.\n']
   

Here is the code using the methods of io.TextIOWrapper:

fname = 'csv.txt'

with open(fname,'r') as f:
  print("-- for line in f --")
  for line in f:
    print(line)
print()

with open(fname,'r') as f:
  print("-- read() --")
  s = f.read()
  print(s)
print()

with open(fname,'r') as f:
  print("-- f.readlines() --")
  s = f.readlines()
  print(s)
print()

with open(fname,'r') as f:
  print("-- f.readline() --")
  while True:
    s = f.readline()
    if not s:
      break;
    print(s, end='')
print()    

Input (csv.txt):

Los Angeles,34°03′N,118°15′W
New York City,40°42′46″N,74°00′21″W
Paris,48°51′24″N,2°21′03″E      

Output:

-- for line in f --
Los Angeles,34°03′N,118°15′W

New York City,40°42′46″N,74°00′21″W

Paris,48°51′24″N,2°21′03″E


-- read() --
Los Angeles,34°03′N,118°15′W
New York City,40°42′46″N,74°00′21″W
Paris,48°51′24″N,2°21′03″E


-- f.readlines() --
['Los Angeles,34°03′N,118°15′W\n', 'New York City,40°42′46″N,74°00′21″W\n', 'Paris,48°51′24″N,2°21′03″E\n']

-- f.readline() --
Los Angeles,34°03′N,118°15′W
New York City,40°42′46″N,74°00′21″W
Paris,48°51′24″N,2°21′03″E    

If we want to get the number of bytes of a string, we can use encode() method of string:

>>> s = "12345 7"
>>> s.encode('utf-8')
b'12345 7'
>>> len(s.encode('utf-8'))
7

where the utf stands for Unicode Transformation Format and the 8 means 8 bits.

Input: csv.txt:

Los Angeles,34°03′N,118°15′W
New York City,40°42′46″N,74°00′21″W
Paris,48°51′24″N,2°21′03″E    

Code:

import csv

with open('csv.txt', 'r') as f:
  reader = csv.reader(f, delimiter=',')
  for row in reader:
    print(row)    

Output:

['Los Angeles', '34°03′N', '118°15′W']
['New York City', '40°42′46″N', '74°00′21″W']
['Paris', '48°51′24″N', '2°21′03″E']    

Input (capital.txt):

Abc d
Ef
Ghi
Jkl mn
Opq rstuv
Wxyz    

Code:

with open('capital.txt','r') as f:
  count = sum(1 for line in f for c in line if c.isupper())

print(count)  # output => 6    

What's the difference between is and ==?

1. is checks if the two are referencing the same object
2. == checks if the two are the same value

So, a = [1,2,3] and b = [1,2,3]. Then, a is b : False while a == b : True:

>>> a = [1,2,3]
>>> b = [1,2,3]
>>> a is b
False
>>> a == b
True    

Note: integers are a little different:

>>> small_a = 100
>>> small_b = 100
>>> small_a is small_b
True
>>> small_b == small_b
True

>>> big_a = 257
>>> big_b = 257
>>> big_a is big_b
False
>>> big_a == big_b
True

Actually, in Python, numbers from -5 up to and including 256 have singleton instances. That means Python creates singletons for the most commonly used integers!

The code (https://www.interviewcake.com/python-interview-questions ):

import copy

question_template = {
    "title": "default title",
    "question": "default question",
    "answer": "default answer",
    "hints": []
}

def make_new_question(title, question, answer, hints=None):
    # new_q = copy.copy(question_template)
    new_q = copy.deepcopy(question_template)

    # always require title, question, answer
    new_q["title"] = title
    new_q["question"] = question
    new_q["answer"] = answer

    # sometimes there aren't hints, that's fine. Otherwise, add them:
    if hints is not None:
        new_q["hints"].extend(hints)

    return new_q

question_1 = make_new_question("title1", "question1", "answer1", ["q1 hint1", "q1 hint2"])
question_2 = make_new_question("title2", "question2", "answer2")
question_3 = make_new_question("title3", "question3", "answer3", ["q3 hint1"])

print(question_1)
print(question_2)
print(question_3)    

Output:

{'title': 'title1', 'question': 'question1', 'answer': 'answer1', 'hints': ['q1 hint1', 'q1 hint2']}
{'title': 'title2', 'question': 'question2', 'answer': 'answer2', 'hints': []}
{'title': 'title3', 'question': 'question3', 'answer': 'answer3', 'hints': ['q3 hint1']}    

If we use copy.copy() which is doing a shallow copy, we get a wrong output:

{'title': 'title1', 'question': 'question1', 'answer': 'answer1', 'hints': ['q1 hint1', 'q1 hint2', 'q3 hint1']}
{'title': 'title2', 'question': 'question2', 'answer': 'answer2', 'hints': ['q1 hint1', 'q1 hint2', 'q3 hint1']}
{'title': 'title3', 'question': 'question3', 'answer': 'answer3', 'hints': ['q1 hint1', 'q1 hint2', 'q3 hint1']}    

iterator = [i for i in range(1, 4)]
matrix = [[y+3*(x-1) for y in iterator] for x in iterator]

Given a list of binary numbers, write a function that converts it to an integer and returns True if the integer is the power of 2:

import collections

def toDec(b):
  # converts b to dec
  dec = 0
  for i,c in enumerate(b):
    dec += int(c)*2**(len(b)-1-i)
  return dec

def powerOf2(b):
  # counts 0s and 1s
  d = collections.defaultdict(int)
  for bit in b:
    d[bit] += 1

  # returns dec, True if only one '1', otherwise False
  return d['1'] ==  1

bin = ['00000001', '00000011', '00000100', '00000101','00000111',
      '00001000', '00001001', '00010000', '00100000', '01000000',
      '10000000','11111111']

for b in bin:
  print(b,toDec(b), powerOf2(b))

Output:

00000001 1 True
00000011 3 False
00000100 4 True
00000101 5 False
00000111 7 False
00001000 8 True
00001001 9 False
00010000 16 True
00100000 32 True
01000000 64 True
10000000 128 True
11111111 255 False

Write a code to read public data returned from URL, and parse the JSON to dictionary object.

urllib:

from urllib.request import urlopen

url = "http://date.jsontest.com"
webUrl = urlopen(url)
if (webUrl.getcode() == 200):
  data = webUrl.read().decode('utf-8')
  print(f'data = {data}')
else:
  raise ConnectionError("Error {} from server, cannot retrieve results ".format(str(webUrl.getcode())))

Output:

data = {
   "date": "04-30-2021",
   "milliseconds_since_epoch": 1619742724035,
   "time": "12:32:04 AM"
}

requests:

import json
import requests
from requests.exceptions import HTTPError

try:
    url = 'http://date.jsontest.com'
    response = requests.get(url)

    # If the response was successful, no Exception will be raised
    response.raise_for_status()

    data = response.content
    print(f'data = {data}')
    jd = json.loads(data) 
    print(f'type(jd) = {type(jd)}')
    print(f'jd = {jd}')
except HTTPError as http_err:
    print(f'HTTP error occurred: {http_err}')  # Python 3.6
except Exception as err:
    print(f'Other error occurred: {err}')  # Python 3.6
else:
    print('Success!')

Output:

data = b'{\n   "date": "04-30-2021",\n   "milliseconds_since_epoch": 1619742814896,\n   "time": "12:33:34 AM"\n}\n'
type(jd) = <class 'dict'>
jd = {'date': '04-30-2021', 'milliseconds_since_epoch': 1619742814896, 'time': '12:33:34 AM'}
Success!

Ref: json.load(s) & json.dump(s)

Given an input file (games.txt) that indicates the results of a series of matches. The results are of the form A M B N which means that team A beat team B by the score of M to N.

o 14 i 6
l 17 h 12
j 13 d 8
i 5 n 4
...

Write a program that reads in this file and outputs the following statistics:

1. The average margin of victory across all games.
2. The team that has the largest average margin of victory.
3. The team that has the largest average losing margin.
4. Find the teams with the top 10 largest average margins of victory.
5. The top 10 largest average losing margin.

import collections
import operator

# games log list
games = []

with open("game.txt","r") as f:
    for line in f:
      games.append(line.split())

# margins
s = 0
for g in games:
  s += abs(int(g[1])-int(g[3]))
average_margin = s / len(games)
print("average_margin = %s" %average_margin)

# dictionaries of teams - winners & losers 
w = collections.defaultdict(list)
for g in games:
  k,v = g[0],int(g[1])-int(g[3])
  w[k].append(round(v,2)) 
winners = dict(w)

l = collections.defaultdict(list)
for g in games:
  k,v = g[2],int(g[3])-int(g[1])
  l[k].append(round(v,2)) 
losers = dict(l)

# find a team with largest margin of victory
# loop through winners dict
mx = 0
team = None 
w_margin = {}
for k,v in winners.items():
  av = sum(v)/len(v)
  w_margin[k] = round(av,2)
  if av > mx:
    team = k
    mx = av
print("The team with the largest average margin of victory is '%s' with a av-margin of %f " %(team, mx))

# find a team with largest margin of lost
# loop through winners dict
mn = 0
l_team = None 
l_margin = {}
for k,v in losers.items():
  av = sum(v)/len(v)
  l_margin[k] = round(av,2)
  if av < mn:
    team = k
    mn = av
print("The team with the largest average margin of lost is '%s' with a av-margin of %f " %(team, mn))

w10 = sorted(w_margin.items(), key = operator.itemgetter(1), reverse = True)
print("top 10 = %s" %w10[:10])
l10 = sorted(l_margin.items(), key = operator.itemgetter(1), reverse = False)
print("bottom 10 = %s" %l10[:10])    

Output:

average_margin = 6.245
The team with the largest average margin of victory is 'e' with a av-margin of 10.000000 
The team with the largest average margin of lost is 'o' with a av-margin of -8.555556 
top 10 = [('e', 10.0), ('f', 8.5), ('c', 8.5), ('k', 8.25), ('v', 7.85), ('m', 7.8), ('w', 7.7), ('g', 6.8), ('r', 6.75), ('j', 6.5)]
bottom 10 = [('o', -8.56), ('t', -8.5), ('e', -8.27), ('s', -8.0), ('q', -7.6), ('g', -7.36), ('v', -7.17), ('l', -7.16), ('c', -6.75), ('m', -6.71)]    

Pawn race is a game for two people, played on an ordinary 8 × 8 chessboard. The first player has a white pawn, the second one - a black pawn. Initially the pawns are placed somewhere on the board so that the 1st and the 8th rows are not occupied. Players take turns to make a move.

White pawn moves upwards, black one moves downwards. The following moves are allowed:

1. one-cell move on the same vertical in the allowed direction
2. two-cell move on the same vertical in the allowed direction, if the pawn is standing on the 2nd (for the white pawn) or the 7th (for the black pawn) row. Note that even with the two-cell move a pawn can't jump over the opponent's pawn
3. capture move one cell forward in the allowed direction and one cell to the left or to the right.

Actually, the code is available from Python: Pawn race

Virtually the same, here is my code:

def file_rank(x):   
    """given position(x), returns column & row"""
    return ord(x[0])-ord('a'), int(x[1])

def capture(w_rank, w_file, b_rank, b_file, ):
    """return True if can be captured""" 
    return abs(b_file - w_file) == 1 and \
            abs(w_rank - b_rank) == 1 and \
            w_rank < b_rank

def pawn_chess(white, black, to_move):
    """return a winner for an input : w_position, b_position, turn"""   
    w_file, w_rank = file_rank(white)
    b_file, b_rank = file_rank(black)

    # if on the same file while black is on a higher rank,
    # they'll bump and will not be able to pass each other
    if(w_file == b_file and b_rank > w_rank):
        return 'draw'

    # loop through all other cases
    while True:
        # if can capture the piece: return a winner
        if capture(w_rank, w_file, b_rank, b_file):
            return "white" if to_move == 'w' else "black" 
     
        # w to move
        if to_move == 'w':
            # if rank !=2 or can be captured with advance 2, then just advance 1
            if w_rank != 2 or capture(w_rank + 2, w_file, b_rank, b_file):
                w_rank += 1
            # advance 2
            else:
                w_rank += 2

        # b to move
        if to_move == 'b':
            # if rank !=7 or can be captured with advance 2, then just advance 1
            if b_rank != 7 or capture(w_rank, w_file, b_rank - 2, b_file ):
                b_rank -= 1
            # otherwise, advance 2
            else:
                b_rank -= 2

        # if a pawn is on a promotion square: return a winner
        if w_rank == 8 or b_rank == 1:
            return "white" if to_move == 'w' else "black" 

        # take turns
        to_move = 'w' if to_move == 'b' else 'b'

# play the game
white, black, toMove = "e2", "e7",'w'
print(white, black, toMove, ' : ', pawn_chess(white, black, toMove))

white, black, toMove = "e3", "d7",'b'
print(white, black, toMove, ' : ', pawn_chess(white, black, toMove))

white, black, toMove = "a7", "h2",'w'
print(white, black, toMove, ' : ', pawn_chess(white, black, toMove))   

Output:

e2 e7 w  :  draw
e3 d7 b  :  black
a7 h2 w  :  white    

Write a code to decode a string, for example, 'a1b2z3' => 'abbzzz', 'k2b1y4' => 'kkbzzzz':

def decode(s):
    decoded_str = ""
    i = 0
    while i < len(s):
        ch = s[i]
        i += 1
        count = 0
        while i < len(s) and s[i].isdigit():
            count = 10 * count + int(s[i])
            i += 1
        decoded_str += ch*int(count)
    return decoded_str

if __name__ == '__main__':
    s = "a1b2z3"
    print(f"{s} => {decode(s)}")

    s = "k2b1y4"
    print(f"{s} => {decode(s)}")
          
    s = "w10b1z4"
    print(f"{s} => {decode(s)}")    

Output:

a1b2z3 => abbzzz
k2b1y4 => kkbyyyy
w10b1z4 => wwwwwwwwwwbzzzz    

We can make a class and make the code as @staticmethod:

class Decoder:
    @staticmethod
    def decode(s):
        i = 0
        decoded_str = ""
        while i < len(s):
            ch = s[i]
            i += 1
            count = 0
            while i < len(s) and s[i].isdigit():
                count = 10*count + int(s[i])
                i += 1
            decoded_str += ch*count
                
        return decoded_str, len(decoded_str)
            
if __name__ == '__main__':
    s = "z1y2x3"
    decoded, decoded_length = Decoder.decode(s)
    print(f"s = {s}, decoded = {decoded}, decoded_length = {decoded_length}")
    
    s = "x4y2z10"
    decoded, decoded_length = Decoder.decode(s)
    print(f"s = {s}, decoded = {decoded}, decoded_length = {decoded_length}")    

Note that the @staticmethod decorator indicates that the decode method is a static method, providing class-level functionality without requiring an instance of the class.

Being a static method, decode can be called on the class itself (Decoder.decode('a1b2z3')) without needing to create an instance of the Decoder class. Since it's a static method, it doesn't have access to instance-specific attributes or methods. In this case, it doesn't matter whether an instance of Decoder is created or not; the method behaves the same.

Output:

s = z1y2x3, decoded = zyyxxx, decoded_length = 6
s = x4y2z10, decoded = xxxxyyzzzzzzzzzz, decoded_length = 16    

1. Python Coding Questions I
2. Python Coding Questions II
3. Python Coding Questions III
4. Python Coding Questions IV
5. Python Coding Questions V
6. Python Coding Questions VI
7. Python Coding Questions VII
8. Python Coding Questions VIII
9. Python Coding Questions IX
10. Python Coding Questions X

List of codes Q & A

1. Merging two sorted list
2. Get word frequency - initializing dictionary
3. Initializing dictionary with list
4. map, filter, and reduce
5. Write a function f() - yield
6. What is __init__.py?
7. Build a string with the numbers from 0 to 100, "0123456789101112..."
8. Basic file processing: Printing contents of a file - "with open"
9. How can we get home directory using '~' in Python?
10. The usage of os.path.dirname() & os.path.basename() - os.path
11. Default Libraries
12. range vs xrange
13. Iterators
14. Generators
15. Manipulating functions as first-class objects
16. docstrings vs comments
17. using lambdda
18. classmethod vs staticmethod
19. Making a list with unique element from a list with duplicate elements
20. What is map?
21. What is filter and reduce?
22. *args and **kwargs
23. mutable vs immutable
24. Difference between remove, del and pop on lists
25. Join with new line
26. Hamming distance
27. Floor operation on integers
28. Fetching every other item in the list
29. Python type() - function
30. Dictionary Comprehension
31. Sum
32. Truncating division
33. Python 2 vs Python 3
34. len(set)
35. Print a list of file in a directory
36. Count occurrence of a character in a Python string
37. Make a prime number list from (1,100)
38. Reversing a string - Recursive
39. Reversing a string - Iterative
40. Reverse a number
41. Output?
42. Merging overlapped range
43. Conditional expressions (ternary operator)
44. Packing Unpacking
45. Function args
46. Unpacking args
47. Finding the 1st revision with a bug
48. Which one has higher precedence in Python? - NOT, AND , OR
49. Decorator(@) - with dollar sign($)
50. Multi-line coding
51. Recursive binary search
52. Iterative binary search
53. Pass by reference
54. Simple calculator
55. iterator class that returns network interfaces
56. Converting domain to ip
57. How to count the number of instances
58. Python profilers - cProfile
59. Calling a base class method from a child class that overrides it
60. How do we find the current module name?
61. Why did changing list 'newL' also change list 'L'?
62. Constructing dictionary - {key:[]}
63. Colon separated sequence
64. Converting binary to integer
65. 9+99+999+9999+...
66. Calculating balance
67. Regular expression - findall
68. Chickens and pigs
69. Highest possible product
70. Implement a queue with a limited size
71. Copy an object
72. Filter
73. Products
74. Pickle
75. Overlapped Rectangles
76. __dict__
77. Fibonacci I - iterative, recursive, and via generator
78. Fibonacci II - which method?
79. Fibonacci III - find last two digits of Nth Fibonacci number
80. Write a Stack class returning Max item at const time A
81. Write a Stack class returning Max item at const time B
82. Finding duplicate integers from a list - 1
83. Finding duplicate integers from a list - 2
84. Finding duplicate integers from a list - 3
85. Reversing words 1
86. Parenthesis, a lot of them
87. Palindrome / Permutations
88. Constructing new string after removing white spaces
89. Removing duplicate list items
90. Dictionary exercise
91. printing numbers in Z-shape
92. Factorial
93. lambda
94. lambda with map/filter/reduce
95. Number of integer pairs whose difference is K
96. iterator vs generator
97. Recursive printing files in a given directory
98. Bubble sort
99. What is GIL (Global Interpreter Lock)?
100. Word count using collections
101. Pig Latin
102. List of anagrams from a list of words
103. lamda with map, filer and reduce functions
104. Write a code sending an email using gmail
105. histogram 1 : the frequency of characters
106. histogram 2 : the frequency of ip-address
107. Creating a dictionary using tuples
108. Getting the index from a list
109. Looping through two lists side by side
110. Dictionary sort with two keys : primary / secondary keys
111. Writing a file downloaded from the web
112. Sorting csv data
113. Reading json file
114. Sorting class objects
115. Parsing Brackets
116. Printing full path
117. str() vs repr()
118. Missing integer from a sequence
119. Polymorphism
120. Product of every integer except the integer at that index
121. What are accessors, mutators, and @property?
122. N-th to last element in a linked list
123. Implementing linked list
124. Removing duplicate element from a list
125. List comprehension
126. .py vs .pyc
127. Binary Tree
128. Print 'c' N-times without a loop
129. Quicksort
130. Dictionary of list
131. Creating r x c matrix
132. Transpose of a matrix
133. str.isalpha() & str.isdigit()
134. Regular expression
135. What is Hashable? Immutable?
136. Convert a list to a string
137. Convert a list to a dictionary
138. List - append vs extend vs concatenate
139. Use sorted(list) to keep the original list
140. list.count()
141. zip(list,list) - join elements of two lists
142. zip(list,list) - weighted average with two lists
143. Intersection of two lists
144. Dictionary sort by value
145. Counting the number of characters of a file as One-Liner
146. Find Armstrong numbers from 100-999
147. Find GCF (Greatest common divisor)
148. Find LCM (Least common multiple)
149. Draws 5 cards from a shuffled deck
150. Dictionary order by value or by key
151. Regular expression - re.split()
152. Regular expression : re.match() vs. re.search()
153. Regular expression : re.match() - password check
154. Regular expression : re.search() - group capturing
155. Regular expression : re.findall() - group capturin
156. Prime factors : n = products of prime numbers
157. Valid IPv4 address
158. Sum of strings
159. List rotation - left/right
160. shallow/deep copy
161. Converting integer to binary number
162. Creating a directory and a file
163. Creating a file if not exists
164. Invoking a python file from another
165. Sorting IP addresses
166. Word Frequency
167. Printing spiral pattern from a 2D array - I. Clock-wise
168. Printing spiral pattern from a 2D array - II. Counter-Clock-wise
169. Find a minimum integer not in the input list
170. I. Find longest sequence of zeros in binary representation of an integer
171. II. Find longest sequence of zeros in binary representation of an integer - should be surrounded with 1
172. Find a missing element from a list of integers
173. Find an unpaired element from a list of integers
174. Prefix sum : Passing cars
175. Prefix sum : count the number of integers divisible by k in range [A,B]
176. Can make a triangle?
177. Dominant element of a list
178. Minimum perimeter
179. MinAbsSumOfTwo
180. Ceiling - Jump Frog
181. Brackets - Nested parentheses
182. Brackets - Nested parentheses of multiple types
183. Left rotation - list shift
184. MaxProfit
185. Stack - Fish
186. Stack - Stonewall
187. Factors or Divisors
188. String replace in files 1
189. String replace in files 2
190. Using list as the default_factory for defaultdict
191. Leap year
192. Capitalize
193. Log Parsing
194. Getting status_code for a site
195. 2D-Array - Max hourglass sum
196. New Year Chaos - list
197. List (array) manipulation - list
198. Hash Tables: Ransom Note
199. Count Triplets with geometric progression
200. Strings: Check if two strings are anagrams
201. Strings: Making Anagrams
202. Strings: Alternating Characters
203. Special (substring) Palindrome
204. String with the same frequency of characters
205. Common Child
206. Fraudulent Activity Notifications
207. Maximum number of toys
208. Min Max Riddle
209. Poisonous Plants with Pesticides
210. Common elements of 2 lists - Complexity
211. Get execution time using decorator(@)
212. Conver a string to lower case and split using decorator(@)
213. Python assignment and memory location
214. shallow copy vs deep copy for compound objects (such as a list)
215. Generator with Fibonacci
216. Iterator with list
217. Second smallest element of a list
218. *args, **kargs, and positional args
219. Write a function, fn('x','y',3) that returns ['x1', 'y1', 'x2', 'y2', 'x3', 'y3']
220. sublist or not
221. any(), all()
222. Flattening a list
223. Select an element from a list
224. Circularly identical lists
225. Difference between two lists
226. Reverse a list
227. Split a list with a step
228. Break a list and make chunks of size n
229. Remove duplicate consecutive elements from a list
230. Combination of elements from two lists
231. Adding a sublist
232. Replace the first occurence of a value
233. Sort the values of the first list using the second list
234. Transpose of a matrix (nested list)
235. Binary Gap
236. Powerset
237. Round Robin
238. Fixed-length chunks or blocks
239. Accumulate
240. Dropwhile
241. Groupby
242. Simple product
243. Simple permutation
244. starmap(fn, iterable)
245. zip_longest(*iterables, fillvalue=None)
246. What is the correct way to write a doctest?
247. enumerate(iterable, start=0)
248. collections.defaultdict - grouping a sequence of key-value pairs into a dictionary of lists
249. What is the purpose of the 'self' keyword when defining or calling instance methods?
250. collections.namedtuple(typename, field_names, *, rename=False, defaults=None, module=None)
251. zipped
252. What is key difference between a set and a list?
253. What does a class's init() method do?
254. Class methods
255. Permutations and combinations of ['A','B','C']
256. Sort list of dictionaries by values
257. Return a list of unique words
258. hashlib
259. encode('utf-8')
260. Reading in CSV file
261. Count capital letters in a file
262. is vs ==
263. Create a matrix : [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
264. Binary to integer and check if it's the power of 2
265. urllib.request.urlopen() and requests
266. Game statistics
267. Chess - pawn race
268. Decoding a string
269. Determinant of a matrix - using numpy.linalg.det()
270. Revenue from shoe sales - using collections.Counter()
271. Rangoli
272. Unique characters
273. Valid UID
274. Permutations of a string in lexicographic sorted order
275. Nested list
276. Consecutive digit count
277. Find a number that occurs only once
278. Sorting a two-dimensional array
279. Reverse a string
280. Generate random odd numbers in a range
281. Shallow vs Deep copy
282. Transpose matrix
283. Are Arguments in Python Passed by Value or by Reference?
284. re: Is a string alphanumeric?
285. reversed()
286. Caesar's cipher, or shift cipher, Caesar's code, or Caesar shift
287. Every other words
288. re: How can we check if an email address is valid or not?
289. re: How to capture temperatures of a text
290. re.split(): How to split a text.
291. How can we merge two dictionaries?
292. How can we combine two dictionaries?
293. What is the difference between a generator and a list?
294. Pairs of a given array A whose sum value is equal to a target value N
295. Adding two integers without plus
296. isinstance() vs type()
297. What is a decorator?
298. In Python slicing, what does my_list[-3:2:-2] slice do?
299. Revisit sorting dict - counting chars in a text file
300. re: Transforming a date format using re.sub
301. How to replace the newlines in csv file with tabs?
302. pandas.merge
303. How to remove duplicate charaters from a string?
304. Implement a class called ComplexNumber
305. Find a word frequency
306. Get the top 3 most frequent characters of a string
307. Just seen and ever seen
308. Capitalizing the full name
309. Counting Consequitive Characters
310. Calculate Product of a List of Integers Provided using input()
311. How many times a substring appears in a string
312. Hello, first_name last_name
313. String validators
314. Finding indices that a char occurs in a list
315. itertools combinations