Control Flow: Decisions and Loops · Lesson 10 of 35

if, else and switch in C

4 min read Updated Nov 7, 2026 Saurabh Samir

On this page

if and else
The else-if ladder
Nested if: largest of three
switch
Practice
Summary

Programs make decisions: pass or fail, discount or no discount, which menu option the user chose. C's decision-making statements are if, else if, else and switch. They're the first thing every lab exam tests – grading programs, largest of three numbers, calculators.

In this lesson

if, else and the else-if ladder
Nested conditions and the dangling else
switch with case, break and default
Choosing between if and switch

if and else

#include <stdio.h>

int main(void) {
    int n;
    printf("Enter a number: ");
    scanf("%d", &n);

    if (n % 2 == 0) {
        printf("%d is even\n", n);
    } else {
        printf("%d is odd\n", n);
    }
    return 0;
}

Output

Enter a number: 17
17 is odd

The else-if ladder

#include <stdio.h>

char grade_for(int marks) {
    if (marks >= 90)      return 'O';
    else if (marks >= 75) return 'A';
    else if (marks >= 60) return 'B';
    else if (marks >= 40) return 'C';
    else                  return 'F';
}

int main(void) {
    int tests[] = {95, 76, 60, 39};
    for (int i = 0; i < 4; i++) {
        printf("%d marks -> grade %c\n", tests[i], grade_for(tests[i]));
    }
    return 0;
}

Output

95 marks -> grade O
76 marks -> grade A
60 marks -> grade B
39 marks -> grade F

How the ladder is checked

marks >= 90?true → 'O', the rest is skipped
otherwise marks >= 75?true → 'A'
…and so on downthe first true condition wins
none truethe final else → 'F'

Nested if: largest of three

#include <stdio.h>

int main(void) {
    int a = 42, b = 87, c = 63;
    if (a >= b) {
        if (a >= c) printf("Largest: %d\n", a);
        else         printf("Largest: %d\n", c);
    } else {
        if (b >= c) printf("Largest: %d\n", b);
        else         printf("Largest: %d\n", c);
    }
    return 0;
}

Output

Largest: 87

Common mistakes

A stray semicolon: if (x > 0); { … } – the ; is an empty statement, so the block always runs.
Missing braces: without braces only the first statement belongs to the if. Always use braces.
Dangling else: an else belongs to the nearest unmatched if, whatever the indentation suggests.
Chained comparisons: if (1 < x < 10) means (1 < x) < 10, which is always true. Write x > 1 && x < 10.

switch

switch compares one integer (or character) expression against constant values:

#include <stdio.h>

int main(void) {
    double a = 12, b = 4;
    char ops[] = {'+', '-', '*', '/', '%'};
    for (int i = 0; i < 5; i++) {
        char op = ops[i];
        switch (op) {
            case '+': printf("%.0f + %.0f = %.2f\n", a, b, a + b); break;
            case '-': printf("%.0f - %.0f = %.2f\n", a, b, a - b); break;
            case '*': printf("%.0f * %.0f = %.2f\n", a, b, a * b); break;
            case '/':
                if (b != 0) printf("%.0f / %.0f = %.2f\n", a, b, a / b);
                else        printf("Cannot divide by zero\n");
                break;
            default:  printf("Unknown operator %c\n", op);
        }
    }
    return 0;
}

Output

12 + 4 = 16.00
12 - 4 = 8.00
12 * 4 = 48.00
12 / 4 = 3.00
Unknown operator %

Fall-through

Without break, execution continues into the next case. Usually that's a bug, but it's handy when several cases share code:

#include <stdio.h>

int main(void) {
    int months[] = {2, 4, 7};
    for (int i = 0; i < 3; i++) {
        int m = months[i], days;
        switch (m) {
            case 2:
                days = 28;
                break;
            case 4: case 6: case 9: case 11:     // several cases, one block
                days = 30;
                break;
            default:
                days = 31;
        }
        printf("Month %d has %d days\n", m, days);
    }
    return 0;
}

Output

Month 2 has 28 days
Month 4 has 30 days
Month 7 has 31 days

Use if / else if when…	Use switch when…
testing ranges (`marks >= 75`)	matching one value against fixed constants
conditions involve several variables	building menus and command handlers
comparing strings or doubles	the value is an int, char or enum

Practice

Try it yourself

Read a character and say whether it's a vowel, a consonant or not a letter.

Show solution

#include <stdio.h>
#include <ctype.h>

int main(void) {
    char ch;
    printf("Enter a character: ");
    scanf(" %c", &ch);
    if (!isalpha((unsigned char)ch)) {
        printf("'%c' is not a letter\n", ch);
    } else {
        switch (tolower((unsigned char)ch)) {
            case 'a': case 'e': case 'i': case 'o': case 'u':
                printf("'%c' is a vowel\n", ch); break;
            default:
                printf("'%c' is a consonant\n", ch);
        }
    }
    return 0;
}

Output

Enter a character: E
'E' is a vowel

Quick check

What happens if you leave out break in a case?

Execution falls through into the next case.

Can a switch case use a string or a range?

No – case labels must be integer constant expressions (ints, chars, enums).

Which if does an else belong to?

The nearest unmatched if before it.

Summary

if/else if/else tests conditions in order; the first true branch runs.
Always use braces; watch for stray semicolons and chained comparisons.
switch matches an integer against constant cases; remember break and default.

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