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Sunday, 4 March 2012

What is an identifier?

What is an identifier?
An identifier is a set of characters used to name variables, functions, macros and enums.

Valid characters in C language identifiers:
Alpha characters[a-z][A-Z]
Decimal numbers[0-9]
Underscore (_)
Dollar symbol($)

Valid and invalid identifiers in C language:
Numeric character should not be used as the first character of identifer.  Space is not allowed in the name of the identifier.  Below are some of the invalid identifier.

9num, 1val, #val

Identifiers are case sensitive.  Consider the below example,
All the above identifiers are not same, but unique.

First letter of an identifier can be $ or _.  Below are few valid identifiers

$, _, $num, _num, num

$ cannot be used as the first character of function name.

Examples for valid identifiers in c:
Variable names: $, _, $val, _val, v1, v1aa
Function names: add, sub, $add, _add, sub9
Macros        : PI, PI9, pi9, $pi, _pi

  #include <stdio.h>
  #define $pi 3.14  // pi is an identifer
  void _calc_area(int radius) { // calc_area and radius are identifiers
        printf("Area of circle: %f\n", $pi * radius * radius);

  int main() {
        int $, _, radius; // $, _, radius are identifiers
        _ = 5;
        $ = 10;
        radius = 10;
        printf("Value of _: %d\n", _);
        printf("Value of $: %d\n", $);
        _calc_area(radius);  // calc_area & radius are identifiers
        return 0;

  jp@jp-VirtualBox:~/$ ./a.out
  Value of _: 5
  Value of $: 10
  Area of circle: 314.000000

$, _, radius, $pi, _calc_area are the identifiers used in the above sample program.

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