C: Constants

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A constant is not just something that "does not change".

A constant is a named literal

In mathematics, a constant is a number that does not change.

π or pi is 3.1415.,., the ratio of the circumference to the diameter of a circle.
e is the exponential constant 2.71828..., the number whose function derivative is itself (a fixed point).
... and so on ...

In physics, a constant as a number that is assumed not to change.

speed of light
Planck's constant
... and so on ...

In computer science, a constant is a named literal.

Since literals do not change while a program is running, neither do constants. Change the literal (outside the running of the program) and the constant changes.

One can think of a computer science constant as a variable whose value is not allowed to change (during the running of the program). That is, the variable is read-only.

A pseudo constant is a variable that is used by the programmer to act as a constant. However, since the pseudo constant is actually a variable, it can be changed which could cause problems in the program. A literal value cannot be simplified, although the value it represents may be represented in another way.

Since a constant names a literal, the value of a constant cannot be changed within a program, as the value of a literal cannot be changed. Good programming style dictates that constants be used to avoid repeated literals with similar meanings. Such literals are often called magic literals.

Even if a magic literal appears only once, it is good programming practice to use a constant in place of the literal.

A literal is an expressly written entity that represents a value.

The literal 6.0 represents the (abstract) value of six. For example, a state tax rate.

A constant is a named literal.

The constant salesTax could be used in place of the literal 6.0.

A constant is used just like a variable except that the value of a constant never changes during program execution.

Here is the C code.

#include <stdio.h>
#include <math.h>

int main() {
	int i1 = 12;
	const int i2 = 34;
	printf("i1=[%d]\n",i1);
	printf("i2=[%d]\n",i2);
	printf("\n");
	printf("pi=[%lf]\n",M_PI);
	return 0;
	}

Here is the output of the C code.

i1=[12]
i2=[34]

pi=[3.141593]

Trying to assign a constant results in an error since the value is read-only.

Here is the C code.

#include <stdio.h>

int main() {
	const int i2 = 34;
	i2 = 56;
	printf("i2=[%d]\n",i2);
	return 0;
	}

Here is the error text of the C code.

D:\W\PROGRAM1.C\code16-02.c: In function 'main':
D:\W\PROGRAM1.C\code16-02.c:6:5: error: assignment of read-only variable 'i2'
  i2 = 56;
     ^

In the math.h library, the constant M_PI is defined as follows. It is not actually a constant, but a defined literal.

#define M_PI 3.14159265358979323846264338327950288

In this case, a define directive is used to create the constant, as if you had typed the value of pi at each and every place where it is used in the program.