If...

Introduction:

  • Python supports the usual logical conditions from mathematics:
    • Equals: a == b
    • Not Equals: a != b
    • Less than: a < b
    • Less than or equal to: a <= b
    • Greater than: a > b
    • Greater than or equal to: a >= b
  • These conditions can be used in "if statements" and loops.

If Statement:

  • An "if statement" is written using the if keyword.

    python
    a = 33
b = 200
if b > a:
    print("b is greater than a")

Syntax of If Statement:

python
if boolean_expression:
    statement(s)

  • If the boolean expression returns True, the statement(s) inside the if block are executed.

Examples:

  1. Simple If Statement:

    • Using a relational operator (<) in the if statement condition.

      python
      a = 2
b = 5

if a < b:
    print(a, 'is less than', b)
    • Output: 2 is less than 5
       
  2. If Statement with False Condition:

    • The statement inside the if-block is not executed if the condition is false.

      python
      a = 24
b = 5

if a < b:
    print(a, 'is less than', b)
    • Output: (No output)
       
  3. If Statement with Compound Condition:

    • Using logical operators (and) to join multiple conditions.

      python
      a = 2
b = 5
c = 4

if a < b and a < c:
    print(a, 'is less than', b, 'and', c)
    • Output: 2 is less than 5 and 4
       
  4. If Statement with Condition Evaluating to a Number:

    • If the expression evaluates to a non-zero number, the statement(s) inside the if-block are executed.

      python
      a = 2

if a:
    print(a, 'is not zero')
    • Output: 2 is not zero
       
  5. If Statement with Multiple Statements:

    • There can be multiple statements inside the if-block, all with the same indentation.

      python
      a = 10

if a > 2:
    print(a, 'is not zero')
    print('And this is another statement')
    print('Yet another statement')

    • Output:

      10 is not zero
And this is another statement
Yet another statement

  6. Short Hand If:

    For one-line if statements:

    python
    if a > b: print("a is greater than b")
  7. Nested If Statement:

    • Writing an if statement inside another if statement.

      python
      a = 2

if a != 0:
    print(a, 'is not zero.')
    if a > 0:
        print(a, 'is positive.')
    if a < 0:
        print(a, 'is negative.')

    • Output:

      2 is not zero.
2 is positive.

Exercises:

  1. Basic If Statement:

    • Write a program to check if a number entered by the user is positive.

      python
      number = int(input("Enter a number: "))

if number > 0:
    print("The number is positive.")
  2. If Statement with Compound Condition:

    • Write a program to check if a number is between 1 and 10.

      python
      number = int(input("Enter a number: "))

if number > 1 and number < 10:
    print("The number is between 1 and 10.")

Summary:

  • Python if statements allow you to control the flow of your program based on conditions. Practice writing if statements to become proficient in using conditional logic in your programs