Constants

Some values should never change: the number of days in a week, a tax rate, the size of a buffer. C gives you three ways to name them — const variables, #define macros and enum constants — and each has its place.

const variables

const makes a variable read-only. The compiler rejects any attempt to change it:

#include <stdio.h>

int main(void) {
    const double VAT_RATE = 0.20;
    VAT_RATE = 0.25;
    printf("%.2f\n", VAT_RATE);
    return 0;
}

Compiler error

main.c:5:14: error: cannot assign to variable 'VAT_RATE' with const-qualified type 'const double'

A const variable has a type, follows normal scope rules and shows up in the debugger — the safest choice for most constants.

#include <stdio.h>

int main(void) {
    const double VAT_RATE = 0.20;
    const int DAYS_PER_WEEK = 7;
    double net = 50.0;
    printf("Gross: %.2f\n", net * (1 + VAT_RATE));
    printf("Weeks in 365 days: %d\n", 365 / DAYS_PER_WEEK);
    return 0;
}

Output

Gross: 60.00
Weeks in 365 days: 52

#define macros

The preprocessor replaces a macro name with its text before compiling (lesson 4). Macros have no type and no scope, but they work anywhere a literal value is needed, such as array sizes in older C:

#include <stdio.h>

#define MAX_NAME 32
#define PI 3.14159265358979

int main(void) {
    char name[MAX_NAME] = "Ada";
    double r = 2.0;
    printf("%s: room for %d characters\n", name, MAX_NAME);
    printf("Circle area: %.3f\n", PI * r * r);
    return 0;
}

Output

Ada: room for 32 characters
Circle area: 12.566

No = and no semicolon in a #define: #define MAX_NAME = 32; would paste = 32; everywhere. By convention macro names are all capitals.

enum constants

An enum defines a set of named integer constants. They're ideal for related values and are real constant expressions:

#include <stdio.h>

enum { BOARD_SIZE = 8, MAX_PLAYERS = 4 };
enum weekday { MONDAY = 1, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY };

int main(void) {
    int board[BOARD_SIZE];
    printf("%zu squares in a row\n", sizeof board / sizeof board[0]);
    printf("Wednesday is day %d\n", WEDNESDAY);
    printf("Up to %d players\n", MAX_PLAYERS);
    return 0;
}

Output

8 squares in a row
Wednesday is day 3
Up to 4 players

Values count up from the previous one, so TUESDAY is 2. Lesson 43 covers enums fully.

Which to use

	`const`	`#define`	`enum`
Has a type	Yes	No	`int`
Respects scope	Yes	No — applies to the rest of the file	Yes
Usable as an array size at file scope / in `case` labels	No (before C23's `constexpr`)	Yes	Yes
Non-integer values	Yes	Yes	No
Best for	Typed values, especially `double` and strings	Conditional compilation, values shared through headers	Integer constants and sets of choices

C23 adds constexpr, which combines the advantages of const and true constant expressions (lesson 55).

Literals

A value written directly in code is a literal: 42, 3.5, 'x', "hello". Numbers with no explanation are called magic numbers — if (temp > 37.5) is clearer as if (temp > FEVER_THRESHOLD). Name the constant once, and changing it later means editing one line.

Try it yourself

Replace the magic numbers with named constants.

Output

2 days, 7 hours, 33 minutes, 20 seconds

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