If-Else

So far you’ve seen sequential programs: instructions execute one after another. But sometimes that’s not enough.

Often a program needs to choose between two or more paths based on a condition.

That’s where the if-else statement appears.

In this lesson you will learn:

• what it means to make a decision in a program
• what if does
• what if-else does
• how to use code blocks with braces
• what nested statements are
• common mistakes when working with decisions

Key idea: if-else breaks the simple linear sequence to choose which block to execute based on a condition.

What is a decision in programming?

A decision occurs when the program must evaluate a condition and, from that, execute one block or another.

For example:

• if the age is 18 or more, display “adult”
• if the grade is less than 6, display “failed”
• if the number is 0, display “zero”

Reminder: what is a condition?

A condition is an expression that can result in true or false.

Examples:

age >= 18
number == 0
grade < 6

These expressions use relational operators, which you saw in the previous lesson.

The if statement

The if statement executes a block of code only if the condition is true.

Syntax

if (condition) {
    instructions;
}

Simple example

#include <stdio.h>

int main() {
    int age = 20;

    if (age >= 18) {
        printf("Is adult\n");
    }

    return 0;
}

If the condition is true, the block executes. If it’s false, that block is skipped.

The if-else statement

The if-else structure allows executing one block if the condition is true and a different block if it’s false.

Syntax

if (condition) {
    instructions_if_true;
} else {
    instructions_if_false;
}

Example

#include <stdio.h>

int main() {
    int number = 5;

    if (number % 2 == 0) {
        printf("The number is even\n");
    } else {
        printf("The number is odd\n");
    }

    return 0;
}

What role do braces play?

The braces {} delimit the block of instructions belonging to if or else.

if (age >= 18) {
    printf("Adult\n");
    printf("Can continue\n");
}

Here both statements belong to the same block.

Although in C there’s the possibility of writing an if without braces when there’s only one instruction, for teaching and writing clear code it’s best to always use braces.

Nested statements

A nested statement is a control statement placed inside another.

For example, an if inside another if.

Example

#include <stdio.h>

int main() {
    int number = 0;

    if (number >= 0) {
        if (number == 0) {
            printf("The number is zero\n");
        } else {
            printf("The number is positive\n");
        }
    } else {
        printf("The number is negative\n");
    }

    return 0;
}

When is nesting worth it?

Nesting is worth it when a second decision depends on the first already being met.

But don’t overdo it either.

If you start nesting too many levels, the code becomes hard to read.

Program flow with if-else

Look at this example:

int age = 16;

if (age >= 18) {
    printf("Adult\n");
} else {
    printf("Minor\n");
}

Flow

1. age >= 18 is evaluated
2. since it’s false, the if block doesn’t execute
3. the else block executes

The key is understanding that the program doesn’t execute both paths: it chooses one.

Common mistakes when starting

1. Confusing = with ==

if (age = 18) {

That’s conceptually wrong for a comparison. To compare, use ==.

2. Forgetting braces in blocks that grow

Today you have one line, tomorrow you add two more and chaos appears.

3. Not thinking about the flow

It’s not enough to write the condition. You have to understand which block executes when the condition is true and which when it’s false.

4. Nesting without need

Nesting is useful, but abusing it makes code harder.

Summary

• if executes a block only if the condition is true
• if-else allows choosing between two paths
• braces delimit the block of instructions
• nested statements allow decisions inside other decisions
• the program chooses one path based on the result of the condition

Final idea

if-else is the first great break from simple sequence.

From here on, the program no longer always does the same thing: it starts adapting based on the data.

And that’s exactly what makes a huge number of real programs useful.