Making decisions with if

The order in which a program's statements run is its flow of control. So far, statements have run in sequence: line 1, then line 2, then line 3. Python also offers selection (choosing between paths) and repetition (doing things again).

Selection uses if. The line starts with if, then a condition, and ends with a colon (:). The statements after it form a block, which runs only when the condition is true. A short block can sit on the same line, after the colon; a longer block goes on the following lines, indented. For example, if marks >= 33: print("Pass") prints Pass only when marks is 33 or more.

if..else gives two paths: the if block runs when the condition is true, and the else block runs when it is false. For example, to check whether a number is even or odd:

• num = int(input("Enter a number: "))
• if num % 2 == 0: print("Even")
• else: print("Odd")

Enter 24, and it prints Even; enter 11, and it prints Odd.

Another example always prints the positive difference between two numbers, whichever is bigger: if num1 > num2, the difference is num1 - num2; otherwise it is num2 - num1.

A block can have any number of statements. Its indentation (the spaces at the start of each line) shows which statements belong to it.