Chapter 4 — Operators

What you'll learn

• 1Use arithmetic operators correctly, especially integer division and modulo.
• 2Distinguish pre- and post-increment.
• 3Combine boolean conditions with `&&`, `||`, `!`.
• 4Apply explicit type casts.

Core Concepts

1) Arithmetic

7 / 2    // → 3   (integer division!)
7 % 2    // → 1
7.0 / 2  // → 3.5 (promoted to double)

2) Comparison and logical

`a == b` is the comparison; `a = b` is assignment — one of C's classic typo bugs.

3) Increment / decrement

int x = 5;
int y = x++;  // y=5, x=6   (post: use then increment)
int z = ++x;  // x=7, z=7   (pre: increment then use)

4) Compound assignment

x += 3;   // same as x = x + 3
x *= 2;   // same as x = x * 2

5) Type cast

int a = 7, b = 2;
double r = (double)a / b;   // 3.5

Without the cast both operands are `int` and integer division kicks in.

6) Precedence cheat sheet (highest → lowest, simplified)

unary (! ++ --)  >  * / %  >  + -  >  < <= > >=  >  == !=  >  &&  >  ||  >  = += -=

When unsure, **add parentheses**. Clearer code beats clever code.

Examples

Example 1 — `ex01_arith.c`: integer vs floating-point division

printf("%d\n", 7 / 2);
printf("%d\n", 7 % 2);
printf("%f\n", 7.0 / 2);

**Output**

3
1
3.500000

Key: int / int truncates; promote at least one side to float/double.

Example 2 — `ex02_increment.c`: pre vs post

int x = 5;
printf("%d\n", x++);  // prints 5 then becomes 6
printf("%d\n", x);    // 6
printf("%d\n", ++x);  // becomes 7, prints 7

**Output**

5
6
7

Key: never combine multiple `++` on the same variable in one expression — undefined behavior.

Example 3 — `ex03_logical.c`: short-circuit evaluation

int a = 0, b = 5;
if (a != 0 && b / a > 1) {  // safe: a != 0 false, second never runs
    printf("never\n");
} else {
    printf("avoided divide-by-zero\n");
}

**Output**

avoided divide-by-zero

Key: `&&` stops at the first false; use that to guard dangerous ops.

Example 4 — `ex04_cast.c`: explicit cast

int a = 7, b = 2;
printf("int div   = %d\n", a / b);
printf("float div = %.2f\n", (double)a / b);

**Output**

int div   = 3
float div = 3.50

Key: cast **before** the division, not after.

Common mistakes

1. **`=` vs `==`** — `if (x = 0)` always assigns and is false; turn on `-Wall`.
2. **Integer division surprise** — `1/2 == 0`.
3. **Operator precedence** — `a & b == c` parses as `a & (b == c)`. Parenthesize.
4. **Double increment in one expression** — `i = i++ + ++i;` is undefined.

Recap

• `/` and `%` on ints truncate; cast to get float behavior.
• `&&` and `||` short-circuit — leverage that.
• When in doubt, add parentheses.

Try it

cd src
gcc -std=c11 -Wall -o ex01 ex01_arith.c && ./ex01
gcc -std=c11 -Wall -o ex02 ex02_increment.c && ./ex02
gcc -std=c11 -Wall -o ex03 ex03_logical.c && ./ex03
gcc -std=c11 -Wall -o ex04 ex04_cast.c && ./ex04

💻 Examples

Compilable, runnable examples — see the output yourself.

#include <stdio.h>

int main(void) {
    int a = 17, b = 5;

    printf("a = %d, b = %d\n", a, b);
    printf("a + b = %d\n", a + b);
    printf("a - b = %d\n", a - b);
    printf("a * b = %d\n", a * b);
    printf("a / b = %d  ( )\n", a / b);
    printf("a %% b = %d  ()\n", a % b);
    printf("a / (double)b = %.3f\n", a / (double)b);

    return 0;
}

3
1
3.500000

#include <stdio.h>

int main(void) {
    int a = 5;
    int b;

    b = a++;       // : a   
    printf(": a=%d, b=%d\n", a, b);   // a=6, b=5

    a = 5;
    b = ++a;       // : a  
    printf(": a=%d, b=%d\n", a, b);   // a=6, b=6

    a += 10;       // a = a + 10
    printf("a += 10 -> %d\n", a);

    a *= 2;
    printf("a *= 2  -> %d\n", a);

    return 0;
}

5
6
7

#include <stdio.h>

int main(void) {
    int a = 10, b = 20;

    printf("a == b : %d\n", a == b);
    printf("a != b : %d\n", a != b);
    printf("a <  b : %d\n", a <  b);
    printf("a >= b : %d\n", a >= b);

    int age = 25;
    int has_license = 1;

    printf("Adult  : %d\n", (age >= 18) && has_license);
    printf("   : %d\n", (age < 18) || !has_license);

    return 0;
}

avoided divide-by-zero

#include <stdio.h>

int main(void) {
    int total = 95, count = 4;

    double avg_wrong = total / count;
    double avg_right = (double)total / count;

    printf(" ( ): %.3f\n", avg_wrong);  // 23.000
    printf(" ():       %.3f\n", avg_right); // 23.750

    double pi = 3.99;
    int trunc = (int)pi;
    printf("(int)3.99 = %d  ( )\n", trunc);

    return 0;
}

int div   = 3
float div = 3.50

📝 Exercises

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

#include <stdio.h>

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

    printf("sum:   %d\n", a + b);
    printf("diff:  %d\n", a - b);
    printf("prod:  %d\n", a * b);
    if (b != 0) {
        printf("quot:  %d\n", a / b);
        printf("rem:   %d\n", a % b);
        printf("float div: %.2f\n", (double)a / b);
    } else {
        printf("Cannot divide by zero.\n");
    }
    return 0;
}

#include <stdio.h>

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

    int even     = (n % 2 == 0);
    int positive = (n > 0);
    int negative = (n < 0);

    printf("even=%d, pos=%d, neg=%d\n", even, positive, negative);
    return 0;
}

#include <stdio.h>

int main(void) {
    double f;
    printf("Fahrenheit: ");
    scanf("%lf", &f);

    double c = (f - 32) * 5.0 / 9.0;
    printf("Celsius: %.2f\n", c);
    return 0;
}