C Constants

A constant in C is a value that should not change while the program runs. Constants help you give important fixed values clear names, such as a maximum score, tax rate, or number of days in a week.

Using const

The most common beginner-friendly way to create a named constant is to add const before a variable declaration. This tells the compiler that the value should stay the same after it is initialized.

#include <stdio.h>

int main(void)
{
    const int days_in_week = 7;

    printf("There are %d days in a week.\n", days_in_week);
    return 0;
}

Output:

There are 7 days in a week.

The constant days_in_week is initialized with 7. After that line, assigning a new value to days_in_week would be an error because it was declared with const.

Constants Make Code Clearer

A named constant is often easier to understand than a number written directly inside a calculation. A number like 60 may be correct, but the name minutes_per_hour explains what the value means.

#include <stdio.h>

int main(void)
{
    const int minutes_per_hour = 60;
    int hours = 3;
    int total_minutes = hours * minutes_per_hour;

    printf("Hours: %d\n", hours);
    printf("Minutes: %d\n", total_minutes);

    return 0;
}

Output:

Hours: 3
Minutes: 180

Here, minutes_per_hour makes the multiplication easier to read. If the same fixed value is used in several places, a named constant also helps avoid repeated unexplained numbers.

Literal Constants

C also has literal constants, which are fixed values written directly in source code. Examples include 42, 3.14, 'A', and string text such as "Hello".

Literal	Meaning
100	An integer constant
2.5	A decimal constant, usually treated as double
2.5f	A float constant because of the f suffix
'Y'	A character constant
"Yes"	A string literal

Literal constants are useful, but names are better when the value has a special meaning in your program.

Using #define

C also supports constants created with the preprocessor directive #define. A #define name is replaced before the compiler checks the program.

#include <stdio.h>

#define MAX_SCORE 100

int main(void)
{
    int score = 86;

    printf("Score: %d out of %d\n", score, MAX_SCORE);
    return 0;
}

Output:

Score: 86 out of 100

Many C programs write #define constants in uppercase, such as MAX_SCORE. For simple typed values inside functions, prefer const because it gives the compiler type information.

Common Beginner Mistakes

• Initialize a const variable when you declare it.
• Do not try to assign a new value to a const variable later.
• Use descriptive names for values with meaning, such as seconds_per_minute.
• Remember that #define lines do not end with a semicolon.

The key idea is that constants give fixed values a clear, protected name. Next, you will learn how operators let you calculate, compare, and combine values.