for loop missing initialization

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The for statement works like:

for (initialization; test-condition; update)

And any or all of those three can be omitted (left blank). So:

  • for (;;) is an infinite loop1 equivalent to while (true) because there is no test condition. In fact, for (int i=0; ;i++) would also be an infinite loop1.

  • for ( ; *s != '\0'; s++) is a loop with no initialization. s will point to the beginning of (probably) a string and is incremented until it reaches the null character '\0' denoting end-of-string. This essentially means loop through all characters of the string s

1 The loop will still be interrupted if there’s a break statement in the loop body, or a call to exit(), etc…

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