Increment and Decrement Operators
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 (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 sqeeze (char [], 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 the getline function that we wrote in Chapter 1, where we can replace
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 the 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 to t */
;
}
As each character 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.
[The C Programming Language p.47-48]
