Comparison Operators

The symbols < and > that you've used in conditions are comparison operators: they compare two values, and the result is either true or false. C has six of them:

• == equal to: 5 == 5
• != not equal to: 5 != 3
• > greater than: 5 > 3
• < less than: 3 < 5
• >= greater than or equal to: 5 >= 5
• <= less than or equal to: 3 <= 5

All the examples in the list are true. In !=, >= and <=, the equals sign always comes second, with no space in between. Watch the difference between > and >= at the boundary: 18 > 18 is false, but 18 >= 18 is true. “18 or older” is therefore age >= 18.

“Equal” is written with two equals signs, ==, because a single one already has a job: points = 10 assigns, which means it stores 10 in the variable points. points == 10 compares: it asks whether points holds 10, and changes nothing.

Writing = instead of == in a condition is one of the most common mistakes. On this site, the compiler catches it:

#include <stdio.h>

int main(void) {
    int points = 7;

    if (points = 10) {
        printf("Full marks!\n");
    }
    printf("Points: %d\n", points);
    return 0;
}

The compiler reports:

main.c:6:16: error: using the result of an assignment as a condition without parentheses [-Werror,-Wparentheses]
    6 |     if (points = 10) {
      |         ~~~~~~~^~~~
main.c:6:16: note: place parentheses around the assignment to silence this warning
    6 |     if (points = 10) {
      |                ^
      |         (          )
main.c:6:16: note: use '==' to turn this assignment into an equality comparison
    6 |     if (points = 10) {
      |                ^
      |                ==
1 error generated.

points = 10 in the condition doesn't compare anything. It would store 10 in points and then use that 10 as the condition, which counts as true. The block would run every time, whatever points held before. Elsewhere, C compilers usually let this through with a warning at most, and the program then quietly does the wrong thing. Here it's an error, so you find out right away. (The end of the first line, in square brackets, just names the check that found the problem.)

Below the error, the compiler adds two notes, lines with note:, that suggest fixes. The second one is the right one here: use ==. The first one, about parentheses, is for the rare case when someone really wants to store a value inside a condition, so don't follow it.

You can compare characters as well as numbers. A character goes in single quotes:

char size = 'M';

if (size == 'M') {
    printf("We have your size\n");
}

This prints:

We have your size

Upper and lower case letters are different characters, so 'm' == 'M' is false. Comparing whole texts works differently in C; you'll learn how in the lesson on strings.

Be careful with == and decimal numbers. The computer stores most of them only approximately, so even 0.1 + 0.2 == 0.3 is false. With a double, compare with <, >, <= or >= instead.