If Statements

If statements give us the ability to execute different blocks of code depending on the outcome of a logical statement (or boolean value).

The syntax for an if statement is:

if condition:
    block of code

where block of code following the : and indented is considered as the code inside the if statement. block of code will only be executed if condition is/evaluates to true.

Consider an if statement with code around it:

code block before

if condition:
    code block inside

code block after

here code block before will be executed first, then condition will be evaluated. If condition is true, code block inside will be executed. Finally code block after will be executed (regardless of whether or not condition is true). Illustrated in a control flow diagram:

Fig. 7 Control flow diagram of the pseudo code if statement example.

Worked Example

Let’s consider the problem where we want to check if one variable is greater than the other. One solution using an if statement is:

a = 3
b = 2

if a > b:
    print(a, 'is greater than', b)

3 is greater than 2

If we ran the code above but with

a = 2
b = 2

then we would see nothing printed out as a > b would evaluate to False and the code block contained in the if statement would not be executed.

Else

What if you wanted to execute a code block if a statement is true; and another if it’s false? The else part of an if statement can be used for this:

if condition:
    code_block_1
else:
    code_block_2

If condition evaluates to True then code_block_1 will be executed. If, on the other hand, condition evaluates to False, code_block_2 will be executed.

This pseudo code can be illustrated by the control flow diagram:

Fig. 8 Control flow diagram of the pseudo code if statement with an else part example.

Note

The else statement cannot stand by itself. It requires a preceding if statement or loop for context.

Worked Example

Let’s take our first example and add an else part to it:

a = 3
b = 2

if a > b:
    print(a, 'is greater than', b)
else:
    print(a, 'is less than or equal to', b)

3 is greater than 2

a = 1
b = 2

if a > b:
    print(a, 'is greater than', b)
else:
    print(a, 'is less than or equal to', b)

1 is less than or equal to 2

Elif

Now, what if we had 2 conditions which are mutually exclusive (if one is true the other is necessarily false) and the one isn’t just the converse of the other. For this we can use the elif part of the if statement (to be read “else if”):

if condition1:
    code_block_1
elif condition2:
    code_block_2

If condition 1 is false, condition 2 will be evaluated. If condition 2 is found to be true, then code block 2 will be executed, if not then control will move from the if statement. Illustrated as a control flow diagram:

Fig. 9 Control flow diagram of the pseudo code example of an if statement with an elif part.

Worked Example

Let’s continue with our worked example and change the else part to be more specific:

a = 1
b = 2

if a > b:
    print(a, 'is greater than', b)
elif a < b:
    print(a, 'is less than', b)

1 is less than 2

a = 1
b = 1

if a > b:
    print(a, 'is greater than', b)
elif a < b:
    print(a, 'is less than', b)

Else After an Elif

Now, if we want to catch the case where both the conditions in the if and elif parts of the if statement are false, we can use an else part at the very end of the if statement. In pseudo code:

if condition1:
    code_block_1
elif condition2:
    code_block_2
else:
    code_block_3

Illustrated in a control flow diagram:

Fig. 10 Control flow diagram of the pseudo code example of an if statement with an elif and else part.

Warning

The else must always be the last part of the if statement and there can only be one.

Worked Example

Now, let’s re-introduce an else part to our worked example:

a = 1
b = 1

if a > b:
    print(a, 'is greater than', b)
elif a < b:
    print(a, 'is less than', b)
else:
    print(a, 'is equal to', b)

1 is equal to 1

Multiple Elif Parts

Though you may only use one else part in an if statement, you are not limited by how many elif parts you wish to use. For example:

if condition1:
    code_block_1
    
elif condition2:
    code_block_2

elif condition3:
    code_block_3

else:
    code_block_4

This can be illustrated using a control flow diagram:

Fig. 11 Control flow diagram of the pseudo code example of an if statement with two elif parts and an else part.

Worked Example

As an example of a script with multiple elif parts, lets write a script that checks if a variable is a multiple of 2, 3, or 5:

var = 21

if var % 2 == 0:
    print('Variable is a multiple of 2')
elif var % 3 == 0:
    print('Variable is a multiple of 3')
elif var % 5 == 0:
    print('Variable is a multiple of 5')
else:
    print('Variable is not a multiple of 2, 3 or 5')

Variable is a multiple of 3

Note that if we put in a value that is both a multiple of 2 and 3, the script will only print out that it is a multiple of 2:

var = 6

if var % 2 == 0:
    print('Variable is a multiple of 2')
elif var % 3 == 0:
    print('Variable is a multiple of 3')
elif var % 5 == 0:
    print('Variable is a multiple of 5')
else:
    print('Variable is not a multiple of 2, 3 or 5')

Variable is a multiple of 2

This is because the check to so if it’s a multiple of 2 is placed before the check for 3 in the if statement.