Chapter 3 · Lesson 1

Conditionals

Conditionals in C

C provides if/else and switch for branching.

if / else if / else Evaluates conditions sequentially:

if (x > 0) { puts("positive"); }
else if (x < 0) { puts("negative"); }
else { puts("zero"); }

Any non-zero value is truthy in C; zero is false.

switch / case with fall-through Unlike many modern languages, C's switch falls through from one case to the next unless you break:

switch (day) {
    case 1: printf("Mon\n"); break;
    case 2: printf("Tue\n"); break;
    default: printf("Other\n");
}

Fall-through is sometimes intentional — e.g., grouping cases:

case 'a':
case 'e':
case 'i': puts("vowel"); break;

Ternary operator

int max = (a > b) ? a : b;

Key points:

C has no boolean type in C89; use int (0 = false, non-zero = true). C99 added _Bool and <stdbool.h>.
Always include break in switch cases unless fall-through is intentional.
switch only works on integer types (int, char, enum).

Code Examples

if/else if/elsec

#include <stdio.h>

int main(void) {
    int score = 78;
    char grade;

    if (score >= 90)      grade = 'A';
    else if (score >= 80) grade = 'B';
    else if (score >= 70) grade = 'C';
    else if (score >= 60) grade = 'D';
    else                  grade = 'F';

    printf("Score %d -> Grade %c\n", score, grade);

    // Ternary
    const char *result = (score >= 60) ? "Pass" : "Fail";
    printf("%s\n", result);
    return 0;
}

In C, character literals use single quotes. The ternary operator produces a value and can be used in expressions.

switch with fall-throughc

#include <stdio.h>

int main(void) {
    int month = 4;
    int days;

    switch (month) {
        case 1: case 3: case 5: case 7:
        case 8: case 10: case 12:
            days = 31; break;
        case 4: case 6: case 9: case 11:
            days = 30; break;
        case 2:
            days = 28; break;
        default:
            days = -1;
    }

    if (days == -1) {
        printf("Invalid month\n");
    } else {
        printf("Month %d has %d days\n", month, days);
    }
    return 0;
}

Multiple case labels with no code between them intentionally fall through, grouping months with the same number of days.

Quick Quiz

1. What is the truth value of 0 in C?

2. What happens in a C switch when you omit `break` from a case?

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