Day 2: Introduction to Control Flow in Python

Day 2 of The 30 Days of Python.
Introduction to Control Flow In Python
  • What is Control flow?
  • Types of Control structures.

    • sequential
    • selection: If statement, if...else statement,if..elif...else, nested if,
    • Repetition: for loop, while loop
  • What is Control flow?
    Control flow is the order in which a programs code executes based on values and logic.
    Python programming language is regulated by three types of control structures:
  • Sequential statements.
  • Selection statement.
  • Repetition statement.
  • Sequential statements.
    Are set of statements where the execution process will happen in sequence manner.
    It is not commonly used because if the logic has broken in any line, then the complete source code execution will break.
    To avoid this problem, selection and repetition statements are used
    Selection Statements.
    Are also called decision control statements or branching statements.
    Selection statements allows a program to execute instructions based on which condition is true.
    They include:
  • if Statement.
  • If statement executes a block of code based on a specified condition.
    Here is the syntax;
    if condition:
        if-block
    The if statement checks the condition first, if the condition evaluates to True the statement is executed in the if-block. Otherwise, it ignores the statements.
    Example
    age= input('Enter your age:')
    if int(age) >= 40:
        print("You are an adult")
    Output
    Enter your age:46
    You are an adult
    2.if…else statement
    It evaluates the condition and will execute the body of if if the test condition is True, and else body if the condition is false
    Syntax
    if condition:
        if-block;
    else:
        else-block;
    Example
    age = input('Enter your age:')
    if int(age) >= 40:
        print("You are an adult.")
    else:
        print("You are a child.")
    Output
    Enter your age:30
    You are a child.
    3.if…elif…else statement.
    Checks multiple conditions for true and execute a block of code as soon as one of the conditions evaluates to true.
    If no condition evaluates to true, the if...elif...else statement executes the statement in the else branch.
    The elif stands for else if.
    Syntax:
    if if-condition:
        if-block
    elif elif-condition1:
        elif-block1
    elif elif-condition2:
        elif-block2
    ...
    else:
        else-block
    Example
    age = input('Enter your age:')
    
    your_age = int(age)
    
    if your_age >= 70:
        print("Your are old")
    elif your_age >= 40:
        print("Your young")
    else:
        print("null")
    Output
    Enter your age:80
    Your are old
    Repetition statement.
    Also called loops and are used to repeat the same code multiple times.
    Python has two repetitive statements namely:
  • for Loop Used to iterate over a sequence that is either a list, set, tuple or dictionary. Syntax
  • for index in range(n):
        statement
    From the above syntax; n is the number of times the loop will execute and index is the loop counter
    Example
    # list
    numbers = [10,12,13,14,17]
    # variable to store the sum
    sum = 0
    # iterate over the list
    for index in numbers:
        sum = sum+index
    print("The sum is", sum)
    Output
    The sum is 66
    for index in range(4):
        print(index)
    Output
    0
    1
    2
    3
    2.While loop.
    Python while statement allows you to execute a block of code repeatedly until a given condition is satisfied.
    Syntax:
    while condition:  
       body
    An example to print 6 numbers from 0 to 5:
    max = 6
    counter = 0
    
    while counter < max:
        print(counter)
        counter += 1
    Output
    0
    1
    2
    3
    4
    5
    Resources.

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    Day 2: Introduction to Control Flow in Python