8.5.1 The Uses of the Comma Operator

With commas, you can put several expressions into a place that requires just one expression—for example, in the header of a for statement. This statement

for (i = 0, j = 10, k = 20; i < n; i++)

contains three assignment expressions, to initialize i, j and k. The syntax of for requires just one expression for initialization; to include three assignments, we use commas to bundle them into a single larger expression, i = 0, j = 10, k = 20. This technique is also useful in the loop-advance expression, the last of the three inside the for parentheses.

In the for statement and the while statement (see Loop Statements), a comma provides a way to perform some side effect before the loop-exit test. For example,

while (printf ("At the test, x = %d\n", x), x != 0)