A Little Course in C. Part 3 – Constants and #define

A constant is an identifier to which a value is assigned but whose value (unlike the value of a variable) should never change during the execution of your program. The traditional way of defining a constant is C is to use the preprocessor directive #define followed by an identifier and a value to be substituted for that identifier. Here, for example, is how I might define a constant named PI with the value 3.141593

#define PI 3.141593

In fact, the value of a constant defined in this way is not absolutely guaranteed to be immune from being changed by having a different value associated with the identifier. In C, the following code is legal (though your compiler may show a warning message):

Modern C compilers provide an alternative way of defining constants using the keyword const, like this:

const double PI = 3.14159;

If I try to assign a new value to this type of constant the compiler won’t let me. It shows an error message, so this is not permitted:

Probably most C programmers would use the #define version (as this has become the ‘traditional’ way of defining C constants). But in fact only the value assigned to the constant defined as a const is completely protected from being altered subsequently.

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