Operators and Expressions

Computing with operators

An operator is a symbol that performs an operation on values. C has a rich set, and they fall into a few clear families. The arithmetic operators are familiar:

int a = 17, b = 5;
printf("%d\n", a + b);   // 22
printf("%d\n", a - b);   // 12
printf("%d\n", a * b);   // 85
printf("%d\n", a / b);   // 3  -- integer division!
printf("%d\n", a % b);   // 2  -- remainder (modulo)

The integer division trap

Look carefully at a / b above: 17 divided by 5 gives 3, not 3.4. This is the single biggest surprise for C beginners. When you divide two integers, C throws away the decimal part entirely — it does integer division. The fractional part vanishes.

To get a real decimal answer, at least one operand must be a floating-point number:

int a = 17, b = 5;
printf("%d\n", a / b);              // 3   -- both ints
printf("%f\n", (float)a / b);       // 3.4 -- a converted to float first
printf("%f\n", 17.0 / 5);           // 3.4 -- 17.0 is already a float

That (float)a is a cast — it temporarily treats a as a float for that calculation. Understanding when C uses integer versus floating-point division will save you hours of debugging mysterious wrong answers.

Comparison operators

These ask true/false questions. In C, the result is an integer: 1 for true, 0 for false (C has no separate boolean type in its classic form):

int x = 10, y = 20;
printf("%d\n", x > y);    // 0 (false)
printf("%d\n", x < y);    // 1 (true)
printf("%d\n", x == 10);  // 1 -- double equals compares
printf("%d\n", x != y);   // 1 -- not equal

As in Python, beware the = versus == trap — and in C it's especially dangerous, because if (x = 5) is valid code that assigns 5 to x and is always true. The compiler often won't stop you. Always double-check you've used == for comparison.

Logical operators

Combine conditions with && (and), || (or), and ! (not):

int age = 25;
int has_license = 1;

if (age >= 18 && has_license) {
    printf("Can drive\n");
}

if (age < 13 || age > 65) {
    printf("Discount ticket\n");
}

if (!has_license) {
    printf("Apply for a license\n");
}

Note the doubled symbols: && not &, || not |. The single versions are different operators (bitwise) that do something else entirely — a classic source of subtle bugs.

Assignment shortcuts and increment

C is famous for its concise shortcuts. Updating a variable based on its current value is so common that C gives you compound operators:

int score = 100;
score += 10;   // same as score = score + 10  -> 110
score -= 5;    // 105
score *= 2;    // 210
score /= 3;    // 70

And the increment/decrement operators add or subtract exactly one — you'll see these constantly, especially in loops:

int i = 5;
i++;    // increment: i is now 6
i--;    // decrement: i is now 5 again

There's a subtle distinction between i++ (use the value, then increment) and ++i (increment, then use) that occasionally matters, but for now just know that i++ is the everyday way to add one. These operators are the building blocks of the loops we'll write in the next chapter, where we finally make programs that repeat and decide.