for loop missing initialization

I've seen

for(;;)

and

for ( ; *s != '\0'; s++)

Why is it blank like that. Thanks.

Solution 1:

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...

Solution 2:

It is "blank like that" because the author of the code left it blank. The author did not want/need to do anything in the corresponding section of for statement, so it was left blank.

for (;;) is a statement that iterates indefinitely (unless it is interrupted from inside cycle body).

for ( ; *s != '\0'; s++) is a statement that does not need an initialization section, since everything necessary (like the initial value of s) was already initialized before that for statement.

Solution 3:

for(;;) is an infinite loop. It is effectively the exact same as while (true).

The reason this works is because when the middle condition in a for loop is empty, it is interpreted as always being true.

for ( ; *s != '\0'; s++) is used for reading strings character-by-character. This approach works because every C string ends with a null character (\0).

Solution 4:

The parts that are blank essentially do nothing. So for (;;) creates an infinite loop that does nothing at all, and never exits because there is no condition in the loop. Your second example:

for ( ; *s != '\0'; s++)

is just a normal loop without any initialization expression. This relies on the fact that s already has an initial value and just loops until it reaches the end of the string.

Solution 5:

while(1) and while(true) are the same as for(;;)