Programming Tutorials

Increment and Decrement Operators in C

By: Baski  

C provides two unusual operators for incrementing and decrementing variables. The increment operator ++ adds 1 to its operand, while the decrement operator -- subtracts 1. We have frequently used ++ to increment variables, as in

   if (c == '\n')
       ++nl;

The unusual aspect is that ++ and -- may be used either as prefix operators (before the variable, as in ++n), or postfix operators (after the variable: n++). In both cases, the effect is to increment n. But the expression ++n increments n before its value is used, while n++ increments n after its value has been used. This means that in a context where the value is being used, not just the effect, ++n and n++ are different. If n is 5, then

   x = n++;

sets x to 5, but

   x = ++n;

sets x to 6. In both cases, n becomes 6. The increment and decrement operators can only be applied to variables; an expression like (i+j)++ is illegal.

In a context where no value is wanted, just the incrementing effect, as in

   if (c == '\n')
       nl++;

prefix and postfix are the same. But there are situations where one or the other is specifically called for. For instance, consider the function squeeze(s,c), which removes all occurrences of the character c from the string s.

   /* squeeze:  delete all c from s */
   void squeeze(char s[], int c)
   {
      int i, j;

      for (i = j = 0; s[i] != '\0'; i++)
          if (s[i] != c)
              s[j++] = s[i];
      s[j] = '\0';
   }

Each time a non-c occurs, it is copied into the current j position, and only then is j incremented to be ready for the next character. This is exactly equivalent to

   if (s[i] != c) {
       s[j] = s[i];
       j++;
   }

Another example of a similar construction comes from changing the following lines of code

   if (c == '\n') {
       s[i] = c;
       ++i;
   }

by the more compact

   if (c == '\n')
      s[i++] = c;

As a third example, consider the standard function strcat(s,t), which concatenates the string t to the end of string s. strcat assumes that there is enough space in s to hold the combination. As we have written it, strcat returns no value; the standard library version returns a pointer to the resulting string.

   /* strcat:  concatenate t to end of s; s must be big enough */
   void strcat(char s[], char t[])
   {
       int i, j;

       i = j = 0;
       while (s[i] != '\0') /* find end of s */
           i++;
       while ((s[i++] = t[j++]) != '\0') /* copy t */
           ;
   }

As each member is copied from t to s, the postfix ++ is applied to both i and j to make sure that they are in position for the next pass through the loop.