7.4 Increment and Decrement Operators

The operators ‘++’ and ‘--’ are the increment and decrement operators. When used on a numeric value, they add or subtract 1. We don’t consider them assignments, but they are equivalent to assignments.

Using ‘++’ or ‘--’ as a prefix, before an lvalue, is called preincrement or predecrement. This adds or subtracts 1 and the result becomes the expression’s value. For instance,

#include <stdio.h>   /* Declares printf. */

int
main (void)
{
  int i = 5;
  printf ("%d\n", i);
  printf ("%d\n", ++i);
  printf ("%d\n", i);
  return 0;
}

prints lines containing 5, 6, and 6 again. The expression ++i increments i from 5 to 6, and has the value 6, so the output from printf on that line says ‘6’.

Using ‘--’ instead, for predecrement,

#include <stdio.h>   /* Declares printf. */

int
main (void)
{
  int i = 5;
  printf ("%d\n", i);
  printf ("%d\n", --i);
  printf ("%d\n", i);
  return 0;
}

prints three lines that contain (respectively) ‘5’, ‘4’, and again ‘4’.