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Conditional Statements

if Statement

Let's explore the idea of conditional statements by solving a simple problem:

Problem

Accept an integer as input from the user. If the number is greater than or equal to zero, print: non-negative.

Solution

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x = int(input())
if x >= 0:
    print('non-negative')

The if is a keyword in Python. The expression next to if is a boolean expression, also called the if-condition or just the condition. The body of the if statement is indented, indicating which lines are to be executed if the condition evaluates to True.

The following diagram captures the control flow of the if statement:

if Statement Flowchart

Code Snippet

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x = int(input())
if x >= 0:
    print('non-negative')

Note that line-3 in the solution code is indented. In this case, the indentation corresponds to four spaces, and it's crucial to keep this consistent throughout the program.

Example

To understand how indentation works, consider the following code blocks:

Left

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x = 1
if x >= 0:
    print('non-negative')
    print('inside if')
print('outside if')

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x = -1
if x >= 0:
    print('non-negative')
    print('inside if')
print('outside if')

Output:

  • Left:
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2
3
non-negative
inside if
outside if
  • Right: 
    1
    outside if

Lines 3-5 in the left code make up the if-block. Whenever the if-condition evaluates to True, the interpreter executes the body of the if.

if-else Statement

Let's add another level of complexity to the problem.

Problem

Accept an integer as input from the user. If the number is greater than or equal to zero, print: non-negative. If the number is less than zero, print negative.

Solution

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x = int(input())
if x >= 0:
    print('non-negative')
else:
    print('negative')

else is another keyword in Python. When the if condition is True, the statements inside the if block are executed. When the condition is False, the statements inside the else block are executed.

Points to Remember

  • if and else must be at the same level of indentation.
  • else cannot exist independently of an if statement.
  • else cannot have a condition associated with it.

if-elif-else Statement

Time for another bump in complexity.

Problem

Accept an integer as input from the user. If the number is greater than zero, print: positive. If the number is less than zero, print negative. If the number is equal to zero, print zero.

Solution

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x = int(input())
if x > 0:
    print('positive')
elif x == 0:
    print('zero')
else:
    print('negative')

elif is a shorthand for else-if.

Example Inputs and Outputs

Input Output
x = 1 positive
x = 0 zero
x = -1 negative

The general syntax for if-elif-else is:

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if <condition-1>:
    <statement-1>
elif <condition-2>:
    <statement-2>
else:
    <statement-3>

Features to Note:

  • Exactly one of the three statements gets executed.
  • If an if or elif condition evaluates to True, the body of that block is executed, and the flow exits the entire if-elif-else block.
  • There can be multiple elif conditions after the if.

Nested Conditional Statements

Consider the following problem:

Problem

Accept three distinct integers as input from the user. If the numbers have been entered in ascending order, print in ascending order. If not, print not in ascending order.

Incomplete Solution

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x = int(input())
y = int(input())
z = int(input())

if x < y:
    print('in ascending order')
else:
    print('not in ascending order')

The issue with this solution is that it doesn't check if y < z. The complete solution is given below:

Complete Solution

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x = int(input())
y = int(input())
z = int(input())

if x < y:
    if y < z:
        print('in ascending order')
    else:
        print('not in ascending order')
else:
    print('not in ascending order')

Indentation

Whenever a new if block is introduced, its body should have exactly one level of indentation with respect to its if-condition.

Defining Variables Inside if

Consider the following snippet of code:

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x = int(input())
if x % 5 == 0:
    output = 'the number is divisible by 5'
print(output)

Observation

Run the code multiple times, varying the input each time. When the input is a multiple of 5, the code runs without any error. However, when the input is not divisible by 5, the code throws a NameError because the variable output is referenced even when it has not been defined.