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if · else · switch · ternary

Chapter 1 — Conditionals

`if / else if / else`, the ternary, and `switch` — the four ways C branches.

ifswitchternary
Duration
⏱ 1-2 hours
Level
📊 Beginner+
Prerequisite
🎯 Basic 4
Outcome
Express branches cleanly

What you'll learn

  • 1Write nested `if/else` cleanly.
  • 2Use the ternary `? :` for short, value-producing branches.
  • 3Choose `switch` when matching a single value to many cases.
  • 4Watch out for fallthrough.

Core Concepts

1) `if / else if / else`

c
if (score >= 90)       grade = 'A';
else if (score >= 80)  grade = 'B';
else                   grade = 'F';

Style tips: brace single-line bodies anyway; future edits won't bite you.

2) Ternary operator

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

Read as "condition ? value-if-true : value-if-false". Good for short value selection; avoid for side-effects.

3) `switch`

c
switch (op) {
    case '+': r = a + b; break;
    case '-': r = a - b; break;
    default:  r = 0;
}

Each `case` must end with `break;` — otherwise execution **falls through** to the next case. Sometimes that's what you want; usually it's a bug.

4) Truthiness

In C, any **non-zero** integer is true; `0` is false.

c
if (x) { ... }       // same as if (x != 0)

Examples

Example 1 — `ex01_if.c`: simple grade classifier

c
int score = 85;
if (score >= 90) printf("A\n");
else if (score >= 80) printf("B\n");
else if (score >= 70) printf("C\n");
else printf("F\n");

**Output**

text
B

Key: only one branch runs — the **first** match.

Example 2 — `ex02_ternary.c`: max of two

c
int a = 7, b = 4;
int max = (a > b) ? a : b;
printf("max = %d\n", max);

**Output**

text
max = 7

Key: ternary returns a value; `if` is a statement. Use ternary for assignments.

Example 3 — `ex03_switch.c`: tiny calculator

c
char op = '+';
int a = 3, b = 4, r;
switch (op) {
    case '+': r = a + b; break;
    case '-': r = a - b; break;
    case '*': r = a * b; break;
    default:  r = 0;
}
printf("%d\n", r);

**Output**

text
7

Key: never forget `break` unless fallthrough is intentional.

Example 4 — `ex04_nested_if.c`: leap-year test

c
int y = 2024;
if (y % 400 == 0)        printf("leap\n");
else if (y % 100 == 0)   printf("not leap\n");
else if (y % 4 == 0)     printf("leap\n");
else                     printf("not leap\n");

**Output**

text
leap

Key: order matters — most specific condition first.

Common mistakes

  1. **`=` instead of `==`** — assigns instead of compares.
  2. **Forgot `break`** — case fallthrough.
  3. **Misnested braces** — `else` binds to the nearest `if`.
  4. **Float equality** — `if (x == 0.1)` rarely works as expected.

Recap

  • `if/else` for general branching; `switch` for one-value-many-cases.
  • Brace every body even when one statement.
  • Always `break` unless you actually want fallthrough.

Try it

bash
cd src
gcc -std=c11 -Wall -o ex01 ex01_if.c && ./ex01
gcc -std=c11 -Wall -o ex02 ex02_ternary.c && ./ex02
gcc -std=c11 -Wall -o ex03 ex03_switch.c && ./ex03
gcc -std=c11 -Wall -o ex04 ex04_nested_if.c && ./ex04

💻 Examples

Compilable, runnable examples — see the output yourself.

ex01_if.c— simple grade classifier
CODE
#include <stdio.h>

int main(void) {
    int score;
    printf("Score (0-100): ");
    scanf("%d", &score);

    if (score < 0 || score > 100) {
        printf("Out of range.\n");
        return 1;
    }

    if (score >= 90)      printf("Grade: A\n");
    else if (score >= 80) printf("Grade: B\n");
    else if (score >= 70) printf("Grade: C\n");
    else if (score >= 60) printf("Grade: D\n");
    else                  printf("Grade: F\n");

    return 0;
}
▶ Output
B
ex02_ternary.c— max of two
CODE
#include <stdio.h>

int main(void) {
    int a, b;
    printf("Two ints: ");
    scanf("%d %d", &a, &b);

    int max = (a > b) ? a : b;
    int min = (a < b) ? a : b;
    const char *parity = (a % 2 == 0) ? "even" : "odd";

    printf("max: %d\n", max);
    printf("min: %d\n", min);
    printf("a is %s\n", parity);

    return 0;
}
▶ Output
max = 7
ex03_switch.c— tiny calculator
CODE
#include <stdio.h>

int main(void) {
    double a, b, result = 0;
    char op;

    printf("  (: 3 + 4): ");
    scanf("%lf %c %lf", &a, &op, &b);

    int ok = 1;
    switch (op) {
        case '+': result = a + b; break;
        case '-': result = a - b; break;
        case '*': result = a * b; break;
        case '/':
            if (b == 0) { printf("Cannot divide by zero.\n"); ok = 0; }
            else result = a / b;
            break;
        default:
            printf("  : %c\n", op);
            ok = 0;
    }

    if (ok) printf(": %.3f\n", result);
    return 0;
}
▶ Output
7
ex04_nested_if.c— leap-year test
CODE
#include <stdio.h>

/*  : 4 ,  100  , 400   */
int main(void) {
    int year;
    printf("Year: ");
    scanf("%d", &year);

    int leap;
    if (year % 4 == 0) {
        if (year % 100 == 0) {
            leap = (year % 400 == 0);
        } else {
            leap = 1;
        }
    } else {
        leap = 0;
    }

    printf("%d %s\n", year, leap ? "is a leap year." : "is not a leap year.");
    return 0;
}
▶ Output
leap

📝 Exercises

Try them yourself first, then open the solution to compare.

Exercise 1

Problem 1 (hw01.c)

Goal: Read a month (1-12) and print the season.

Requirements
  • Filename: hw01.c
▶Toggle solution
SOLUTION
#include <stdio.h>

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

    int max = a;
    if (b > max) max = b;
    if (c > max) max = c;

    printf("max: %d\n", max);
    return 0;
}
Exercise 2

Problem 2 (hw02.c)

Goal: Implement rock-paper-scissors logic with `switch`.

Requirements
  • Filename: hw02.c
▶Toggle solution
SOLUTION
#include <stdio.h>

int main(void) {
    int age;
    printf("Age: ");
    scanf("%d", &age);

    if (age < 0)        printf("Invalid input\n");
    else if (age <= 7)  printf("Infant\n");
    else if (age <= 13) printf("Child\n");
    else if (age <= 19) printf("Teen\n");
    else if (age <= 64) printf("Adult\n");
    else                printf("Senior\n");

    return 0;
}
Exercise 3

Problem 3 (hw03.c)

Goal: Read weight (kg) and height (m), compute BMI = w / h², classify (underweight/normal/overweight/obese).

Requirements
  • Filename: hw03.c
▶Toggle solution
SOLUTION
#include <stdio.h>

int main(void) {
    int month;
    printf("Month (1-12): ");
    scanf("%d", &month);

    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:
            printf("Invalid input\n");
            return 1;
    }

    printf("%d has %d days.\n", month, days);
    return 0;
}
Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗