Good Names and Constants

Naming rules

C is strict about names. A variable name:

• can use letters, digits and the underscore _,
• cannot start with a digit (2cats is not allowed, cats2 is fine),
• cannot contain spaces or symbols like - or !,
• cannot be a C keyword such as int, return or if,
• is case-sensitive: score and Score are two different variables.

Good habits

Choose names that say what the value means:

#include <stdio.h>

int main() {
    int x = 60;              // what is x? nobody knows
    int y = 3;

    int minutesPerHour = 60; // much clearer!
    int hours = 3;
    printf("%d %d\n", x * y, minutesPerHour * hours);
    return 0;
}

Output

180 180

Two popular styles for names made of several words: camelCase (totalPrice) and snake_case (total_price). Pick one and stick with it.

Constants with #define

Some values never change, like the number of days in a week. The classic C way is #define. It goes at the top, under the #include lines, and has no = and no ;:

#include <stdio.h>

#define DAYS_IN_WEEK 7
#define PI 3.14159

int main() {
    printf("Days in 4 weeks: %d\n", 4 * DAYS_IN_WEEK);
    printf("PI is about %.2f\n", PI);
    return 0;
}

Output

Days in 4 weeks: 28
PI is about 3.14

Before compiling, C replaces every DAYS_IN_WEEK in your code with 7. If the value ever changes, you only edit one line.

Constants with const

Another way is to put const in front of a normal variable. Then C refuses to let anyone change it:

#include <stdio.h>

int main() {
    const int MAX_PLAYERS = 4;
    printf("Up to %d players\n", MAX_PLAYERS);
    return 0;
}

Output (on your own computer with gcc – the simple C runner in the browser editor can't run this one)

Up to 4 players

If you try to change a const variable, the program won't compile:

#include <stdio.h>

int main() {
    const int MAX_PLAYERS = 4;
    MAX_PLAYERS = 5;
    return 0;
}

Output (the program does not compile) (on your own computer with gcc – the simple C runner in the browser editor can't run this one)

main.c:5:17: error: assignment of read-only variable ‘MAX_PLAYERS’

Tip: The simple C runner in our browser editor doesn't understand const, so in the challenges use #define for constants.

Remember: Constants are usually written in CAPITAL_LETTERS so they stand out.