if - elif - ... - else

• There can be zero or more elif.
• There can be nesting to suit given conditions.

    # example
    if num > 0 :
        print("positive")
    elif num < 0 :
        print("negative")
    else:
        print("zero")

    # shorthand if-else (can be used as ternary operator)
    even = True if i%2 else False
        

Chaining comparison operator

a op1 b op2 c is equivalent to a op1 b and b op2 c

    if a < b < c : # equivalent to if a < b and b < c
          pass
        

For Loop

    ''' syntax '''
    for var in iterable:
        pass

    ''' example '''
    l1 = [1, 2.3, "hi"]
    for ele in l1:
        print(ele)

        

While loop

    ''' example with list '''
    l1 = [1, 2.3, "hi"]
    while a:
        print(a.pop())

    ''' single statement while block '''
    count = 0
    while (count < 5): count += 1; print("Hello!!")

    ''' sentinel controlled while loop '''
    a = int(input('Enter a number (-1 to quit): '))
    while a != -1: # no need of counter
        a = int(input('Enter a number (-1 to quit): '))
        

Loop control statements

Controlling loops

    ''' examples are with for, but can also be achieved using while '''

    # continue, break, pass
    moves = ["start", "straight", "left", "straight", "skip", "right", "end", "left", "straight"]
    for move in moves:
        if move == "skip":
            continue # skips execution from here and moves to next loop
        elif move == "end":
            break # breaks the loop
        else:
            print(move)
    for move in moves:
        pass # passes the entire execution

    # with else
    alphabets = ["x", "y", "z"]
    for alphabet in alphabets:
        if isVowel(alphabet):
            break
    else: # executes when no break statement is called in the for loop
        print("the loop didn't break immaturely")
        

Pass

• Pass is a statement which does nothing when executed - Because it is a null statement.
• The statement is not ignored by the interpreter, but the statement results in no operation.
• It is used when we do not want anything to be executed, but a statement is required.

Looping techniques

Using enumerate(): index number along with value present in that index

    for key, value in enumerate(l1):
        print(key, value)
        

zip() is used to combine 2 similar containers(list-list or dict-dict) printing the values sequentially.

    questions = ['name', 'colour', 'shape']
    answers = ['apple', 'red', 'a circle']
    for question, answer in zip(questions, answers):
        print('What is your {0}?  I am {1}.'.format(question, answer))
        

iteritem() is used to loop through the dictionary printing the dictionary key-value pair sequentially which is used before Python 3 version.

    d = {"name": "apple", "colour": "red", "shape": "a circle"}
    for i, j in d.iteritem():
        print(i, j)
        

items() performs the similar task on dictionary as iteritems() but have certain disadvantages when compared with iteritems().

• It is very time-consuming. Calling it on large dictionaries consumes quite a lot of time.
• It takes a lot of memory. Sometimes takes double the memory when called on a dictionary.

    d = {"name": "apple", "colour": "red", "shape": "a circle"}
    for i, j in d.items():
        print(i, j)
        

sorted() is used to print the container is sorted order. It doesn't sort the original list, just returns one in sorted order.

    lis = [1, 3, 5, 6, 2, 1, 3]
    for i in sorted(lis):
        print(i)
        

reversed() is used to print the values of the container in the reversed order

    lis = [1, 3, 5, 6, 2, 1, 3]
    for i in reversed(lis):
        print(i)