21.1.1. What is the IF Statement?

The if statement is used for decision-making. It allows a program to perform a specific action only when a certain condition is true.

The IF statement is a fundamental building block in programming that facilitates decision-making. It checks whether a condition is true, and if it is, it executes the code or actions specified. If the condition evaluates to false, the program skips the actions within the IF block. This is similar to how humans make choices based on present circumstances; if something is true, we proceed with that option.

Imagine a student deciding whether to go outside based on the weather. If it's raining, they decide to stay in (condition is false). If it's sunny, they choose to go for a walk (condition is true). Similarly, an IF statement in programming checks if a condition is met before deciding on the next action.

Syntax:

if condition:
    statement(s)

The syntax for the IF statement includes the keyword 'if' followed by a condition and a colon. After the colon, an indented block of code represents the actions that will execute if the condition is met. This indentation is crucial in Python, as it defines which statements belong to the IF block.

Think of it like a question on a quiz. If the question states: 'If you answer correctly, you will pass,' then only the students who answered correctly will see that they passed (the indented actions).

Example:

age = 18
if age >= 18:
    print("You are eligible to vote.")

In this example, we assign the variable 'age' a value of 18. The IF statement checks if 'age' is greater than or equal to 18. Since this condition is true, the action prints "You are eligible to vote." If age were less than 18, the print statement would not execute.

This is similar to a club policy: if a person is 18 years or older, they can enter. If someone is 17, they are not permitted (no action is taken for them).

IF-ELSE Statement: If the condition is false, an alternative set of instructions can be executed using else.

if condition:
    # Executes if condition is true
else:
    # Executes if condition is false

The IF-ELSE statement helps to provide alternative actions when the initial condition is not true. The ELSE block contains the code that runs when the IF condition evaluates to false, allowing for a complete decision-making structure.

Imagine a game where you can only enter if you have a valid ticket. If you do have a ticket (condition is true), you enter; if not (condition is false), you are turned away—this reflects how the IF-ELSE structure operates, deciding what happens next based on whether the ticket condition holds.

Example:

age = 16
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")

In this example, the variable 'age' is set to 16. The IF statement checks if 'age' is 18 or older, which is false. Therefore, the ELSE block executes, printing "You are not eligible to vote." This illustrates how the IF-ELSE statement provides a clear outcome based on the evaluated condition.

Consider a scenario where a person checks if they have enough money to buy lunch. If they do (condition true), they buy it; if not (condition false), they choose to save their money. This reflects the decision-making process supported by the IF-ELSE structure in programming.

IF-ELIF-ELSE Ladder: To check multiple conditions in a sequence, use elif.

marks = 85
if marks >= 90:
    print("Grade A")
elif marks >= 75:
    print("Grade B")
elif marks >= 50:
    print("Grade C")
else:
    print("Grade D")

The IF-ELIF-ELSE ladder allows checking several conditions in order. Each condition is evaluated one after the other. If one condition is found to be true, the corresponding action is executed, and the remaining conditions are ignored. This structure is efficient for grading systems or multi-choice scenarios where several outcomes are possible but only one can be selected.

Imagine a teacher grading students: if the score is above 90, they receive an A; if it's 75 or above but less than 90, they get a B; and so on. This hierarchical checking resembles the IF-ELIF-ELSE structure, enabling the determination of grades based on specific criteria.

--