Learning Objectives

• if statement

• if-else statement

• if-elif-else statement

Tutorial on Conditional Statements

If Statement

• Syntax (how to write If statement in Python?)- statement(s)

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  if test expression/condition:
     do this
  

• Here, the program evaluates the test expression and will execute statement(s) only if the test expression/condition is True.

• If the test expression is False, the statement(s) is not executed.

• In Python, the body of the if statement is indicated by the indentation. The body starts with an indentation, and the first unindented line marks the end.

• Python interprets non-zero values as True (even negative values).

• None and 0 are interpreted as False.

• Empty strings, empty lists, empty dicts, empty tuples, and empty sets are also interpreted as False.

Flowchart:

Example: Python program to detect if a number is even. (% sign tells us the remainder of an expression. Any number with a remainder of 0 after dividing by 2 must be even.)

If-else Statement

• Syntax:

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if test expression:
  Body of if
else:
  Body of else

• The if-else statement evaluates test expression and will execute the body of if only when the test condition is True.
• If the condition is False, the body of else is executed. Indentation is used to separate the blocks.

Flowchart:

Example:

The test expression is False as 5 is not an even number. So the body of if statement is not executed. The body of else is executed.

if-elif-else Statement

• Syntax:

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if test expression:
  Body of if
elif test expression:
  Body of elif
else:
  Body of else

• The elif is short for else if. It allows us to check for multiple expressions.

  • If the condition for if is False, it checks the condition of the next elif block and so on.

• If all the conditions are False, the body of else is executed.

• Only one block among the several if-elif-else blocks is executed according to the condition.

• The if block can have only one else block. But it can have multiple elif blocks.

Flowchart:

Example:

5 is neither divisible by 2 nor by 3. So the if and elif statements are False. The else block will be executed here.

Now, let’s try to understand the given example.

• We first assigned the value 5 to z.
• The control shifts to the next line where z%2 is checked. 5 is not divisible by 2, so the control doesn’t shift to the body of if.
• Then, the elif statement: z%3 is executed. Since 5 is not divisible by 3, the body of elif is not executed.
• Finally, the else statement is executed. The control shifts to the body of else, and “z is neither divisible by 2 nor by 3” is printed.
• Point to be noted: The conditions are checked in a top to bottom order. If any of the above if or elif condition is True, it’ll be executed, and no further conditions will be checked.
• Can you figure out what z=6 will print in the given example?