Constants in C: const, #define, enum and Literals

A constant is a value that does not change while the program runs, such as a tax rate, a maximum size or the number of days in a week. Naming constants makes code easier to read and change.

const variables#

const makes a variable read-only after it is initialised:

#include <stdio.h>

int main(void) {
    const double GST_RATE = 0.18;
    const int DAYS_IN_WEEK = 7;

    printf("%.2f\n", 1000 * GST_RATE);
    printf("%d hours\n", DAYS_IN_WEEK * 24);
    return 0;
}

180.00
168 hours

Trying to change a const is a compile error:

#include <stdio.h>

int main(void) {
    const int MAX_USERS = 100;
    MAX_USERS = 200;
    printf("%d\n", MAX_USERS);
    return 0;
}

GCC reports error: assignment of read-only variable 'MAX_USERS'.

#define macros#

The preprocessor replaces every use of a #define name with its text before compiling:

#include <stdio.h>

#define PI 3.14159
#define MAX_NAME 50

int main(void) {
    double r = 2.0;
    char name[MAX_NAME];
    printf("Area: %.2f\n", PI * r * r);
    printf("Buffer size: %zu\n", sizeof name);
    return 0;
}

Area: 12.57
Buffer size: 50

• No = and no semicolon: #define PI 3.14159.
• By convention, macro names are UPPER_CASE.
• Macros are useful for array sizes and compile-time settings, as above.

const or #define?#

Prefer const for typed values in modern C, and use #define (or enum) where you need a true compile-time constant, such as the size of a global array. Macros are covered in The Preprocessor and Macros.

enum constants#

An enum defines a group of named integer constants:

#include <stdio.h>

enum Weekday { MON, TUE, WED, THU, FRI, SAT, SUN };

int main(void) {
    enum Weekday today = WED;
    printf("%d\n", today);
    printf("%d\n", SUN - MON + 1);
    return 0;
}

2
7

Values start at 0 unless you set them. Enums are covered in Unions and Enums.

Literals#

Values written directly in code are literals:

printf("%d %d %d\n", 0x1F, 017, 31);
printf("%.0f\n", 1e6);

31 15 31
1000000

Note the octal trap: 017 is 15, not 17. Never write leading zeros on decimal numbers.

Avoid magic numbers#

if (age >= 18 && marks > 39) { ... }            /* what do 18 and 39 mean? */

#define VOTING_AGE 18
const int PASS_MARK = 40;
if (age >= VOTING_AGE && marks >= PASS_MARK) { ... }

Named constants explain themselves and need changing in only one place.

Practice#

#include <stdio.h>

#define LIGHT_KM_PER_S 299792

int main(void) {
    const int SECONDS_PER_MINUTE = 60;
    long long km = (long long)LIGHT_KM_PER_S * SECONDS_PER_MINUTE;
    printf("%lld km\n", km);
    return 0;
}

17987520 km

Key takeaway#