Constants in C: Uses, Types & Rules

In programming, a constant is just a label given to a variable whose values don't change. Constants are similar to variables in the context of the C programming language, but they differ in that they maintain a single value during program execution. 

• This suggests that once a constant's value is established, it is fixed in its given state throughout the program. 
• The 'const' keyword is used to specify a constant in the C language. 
• The const keyword, referred to as a const type qualifier, is used at the beginning of the variable declaration to designate that variable as a constant.

Key Terms: Variables, Integer constant, Floating point constant, Real constant, String and character constant

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Use of the Constants in C

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In the context of programming, a constant is a specified storage location in a program that keeps a single value constant over its entire runtime. Among other data kinds, these constants can also be character, floating-point, string, double, and integer. 

• Constants are divided into two groups in C programming: primary constants and secondary constants. 
• Character, real, and integer constants are considered primary constants, while structures, arrays, pointers, unions, and related kinds are considered secondary constants.

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What are Literals in C?

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Literals are values that we assign to variables, and they retain their constancy throughout a program's execution. In most cases, we can use the terms "literal" and "constant" interchangeably. For example, in the expression "const int = 9;", an entire expression is a form of constant expression, and we specifically call the value 9 a constant integer literal.

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Types of Constants in C

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The types of constraints in C are mentioned below in the tabular format:

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More About Types of Constants in C

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Types of constants used in C are mentioned below. 

Integer Constants

It could be an octal integer, a decimal integer, or a hexadecimal integer. It represents a decimal integer directly as a plain integer value. For octal integer values, we indicate them by prefixing with 'o,' and for hexadecimal integer values, we use '0x' as a prefix.

• In programming, integer constants can also be of an unsigned or long type. 
• Unsigned constant values are marked with 'u' as a suffix and long integer constant values with 'l' as a suffix. 
• Furthermore, an unsigned long integer constant is indicated by 'ul' as a suffix.

Floating Point Constants / Real Constants

This kind of constant should have integral and fractional components. 

• An exponential component is sometimes included in a floating-point constant. 
• When a floating-point constant is expressed in exponential form, the suffix 'E' or 'e' must be used to indicate the value of the constant.

Character Constants

Character constants are symbols enclosed within single quotation marks. These character quotations have a maximum length of just one character.

Example,

‘B’

‘5’

‘+’

String Constants

String constants are composed of a variety of special symbols, digits, characters, and escape sequences, all enclosed within double quotation marks.

The definition of a string constant occurs in a single line:

"You go there"

It can be defined with the use of constant multiple lines.

"You \

go \

there"

The definition can also occur using white spaces.

"You" "go" "there"

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Rules of Constructing Constants in C

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Rules to construct these constants in a given program:

Integer Constants

It is required to contain at least one digit. The presence of a decimal point is strictly prohibited. Blank spaces and commas are not permissible.

• An integer constant can have either a positive or negative sign.
• In the absence of a sign in front of the constant, it is considered positive by default.
• This type of constant is valid within the range of -32768 to 32767.

Real Constants

This constant type should contain at least one digit. It should not contain a decimal point.

• It can be either positive or negative.
• It cannot include any spaces or commas.
• If there is no sign in front of the constant, we consider it positive.

String and Character Constants

This category of constant can be a solitary digit, an individual alphabet, or even a lone special symbol, all of which remain enclosed within single quotation marks.

• Double quotation marks encompass string constants.
• The maximum length for this kind of constant is just one character.

Backslash Character Constants

These represent certain character types with unique significance within the C language.

• These constant types require a preceding backslash symbol to enable the program to utilize these distinct functions.
• Below is a comprehensive compilation of all special characters employed in the C language, along with their respective functions:

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Creation and Use of Constants in C

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In C programming constants can be created by two concepts.

• By using the ‘#define’ preprocessor
• By using the ‘const’ keyword.

Use #define to Define Constants in C

When creating constants using define directives, it's essential to define them at the outset of the program. This is necessary because all preprocessor directives must be written before any global declarations.

Syntax of Constant in C using #define

#define const_name value

Example

// C Program to define a constant using #define#include <stdio.h>#define pi 3.14 int main(){ printf("The value of pi: %.2f", pi); return 0;}

Use of the ‘const’ Keyword

The 'const' keyword is employed to generate a constant of any data type within a program. To create such a constant, the variable declaration must be preceded by the 'const' keyword. Below is the standard syntax we adhere to when employing the 'const' keyword:

Syntax of Constant in C using #const

const <data_type> <var_name> = <value>;

Example 

#include <stdio.h>

int main(){ const int STUDENT_ID = 27; const int COURSE_CODE = 502; printf("Student ID: %d is taking the class %d\n", STUDENT_ID, COURSE_CODE);

return 0;}

# OutputStudent ID: 27 is taking the class 502

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Things To Remember

• Constants are fixed, unchanging values in C.
• They can be categorized as literal constants (hardcoded values) or symbolic constants (user-defined with names).
• Symbolic constants are typically defined using #define or const.
• Naming conventions for symbolic constants often involve uppercase letters and underscores.
• Constants can be used in expressions, just like variables.
• Constants can also be used as function parameters to indicate immutability.
• The 'const' keyword is employed to generate a constant of any data type within a program.