Decisions: if, else and switch

Introduction

An if statement runs its block only when its condition is true. In C, a condition is just a number: 0 means false and anything else means true.

Programs become useful when they can choose. Conditions in C are unusually simple, because they are only numbers, and that simplicity is the source of one of the language's best-known traps.

How it works

The = trap

if (x = 5) does not compare. It stores 5 in x, and since 5 is not 0 the block always runs. Real C accepts this silently; StepVioz stops and explains, because it is almost always a typo for ==.

switch

When you compare one value against many fixed choices, switch is clearer:

#include <stdio.h>

int main(void) {
    int day = 6;
    switch (day) {
    case 1:
        printf("Sunday: classes\n");
        break;
    case 6:
    case 7:
        printf("Weekend\n");
        break;
    default:
        printf("A weekday\n");
    }
    return 0;
}

Each case needs a break, or C carries on into the next case. That "fall through" is occasionally useful, as in case 6: case 7: sharing one result.

Syntax

if (x > 0) {
    ...
} else if (x == 0) {
    ...
} else {
    ...
}

switch (n) {
case 1:
    ...
    break;
default:
    ...
}

Example

#include <stdio.h>

int main(void) {
    int marks = 75;
    char grade;
    if (marks >= 80) {
        grade = 'A';
    } else if (marks >= 70) {
        grade = 'B';
    } else if (marks >= 60) {
        grade = 'C';
    } else {
        grade = 'F';
    }
    printf("Marks %d: grade %c\n", marks, grade);
    if (marks >= 40 && marks <= 100) printf("Passed\n");
    return 0;
}

C checks the conditions from the top. The first one that is true wins, and the rest are skipped. Step through and watch the calculation box: it shows each comparison with the real values, such as marks >= 80 → 75 >= 80 → false.

Line by line

1. marks is 75, and grade has no value yet.
2. marks >= 80 is false, so C goes on to the next test.
3. marks >= 70 is true, so grade becomes 'B' and the rest of the chain is skipped.
4. printf shows the marks and the grade.
5. The last if uses &&, and both of its sides are true.

Expected result

Marks 75: grade B
Passed

In the real world

A thermostat, a login check and a game's rules are all built from conditions. A misplaced = inside an if has caused real security bugs, which is why compilers offer a warning for it.

Common mistakes

• = instead of == in a condition.
• A ; right after the condition: if (x > 0); ends the if there, so the block below always runs.
• Forgetting break in a switch.
• Writing if (0 < x < 10): C works out 0 < x first (0 or 1), then compares that with 10. Write x > 0 && x < 10.

Best practices

• Always use braces, even for one line.
• Compile with warnings turned on, such as -Wall.
• Prefer switch when comparing one value with many fixed choices.

Summary

if chains run the first true branch. Comparisons give 0 or 1, and &&, || and ! combine them. == compares and = assigns. switch picks among fixed cases, each ended by break.

Mark lesson complete