How to write a for loop without variable?

Question

I want to write a for loop directly without any variable. Is it possible?

Example:

for (1; 1 <= 4; ++) {
  printf ("Loop working\n");
}

Expected output:

Loop working
Loop working
Loop working
Loop working

Possible duplicate of [C++11 range-based for loops without loop variable](https://stackoverflow.com/questions/17711655/c11-range-based-for-loops-without-loop-variable) — Epion, Feb 13 '19 at 13:49
Why would someone want to do that? Unless initialized, it would be an infinite loop. When you want it to iterate over 4 times, let it know that way. — WedaPashi, Feb 13 '19 at 13:55
Well you need a variable to keep the iteration count. Without a count you can only get an infinite loop. — rustyx, Feb 13 '19 at 13:55
is loop unrolling an option ? (possibly [aided with macro's or template functions](https://stackoverflow.com/questions/28231743/self-unrolling-macro-loop-in-c-c)) — Sander De Dycker, Feb 13 '19 at 14:03
What problem are you _actually_ trying to resolve? This could be an [XY Problem](http://xyproblem.info) — Jabberwocky, Feb 13 '19 at 14:14
@WedaPashi -- this would mean "do this four times", without the distraction of having to name a loop-control variable, and perhaps without the distraction of some compiler writer complaining that you created a variable that you never used. — Pete Becker, Feb 13 '19 at 15:47
@PeteBecker: True that. But I am trying to foresee a use-case for one such implementation.. — WedaPashi, Feb 14 '19 at 06:23

pmg · Answer 1 · 2019-02-13T15:01:55.920

You need a variable to keep count, but you can hide it.

#include <stdio.h>

#define JOIN(a, b) (a ## b)
// WARNING: don't LOOP in the same line
#define LOOP(n) for (unsigned JOIN(HIDDEN, __LINE__) = 0; JOIN(HIDDEN, __LINE__) < n; JOIN(HIDDEN, __LINE__)++)

int main(void) {
    LOOP(4) {
        printf("foo");
        LOOP(2) printf(" bar");
        puts("");
    }
    return 0;
}

See code running on ideone

Output

foo bar bar
foo bar bar
foo bar bar
foo bar bar

haccks · Answer 2 · 2019-02-13T13:54:27.223

-1

Yes, it is possible.

for(; ;);

But this will be an infinite loop. Add a break statement and it will run only once

for(; ; )
    break;

Or you can use increment and conditional statements inside the loop

int i = 0;

for(; ;){
    printf ("Loop working\n");
    ++i;
    if( i >= 4)
        break;
}

edited Feb 13 '19 at 13:54

answered Feb 13 '19 at 13:49

haccks

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same as `while(1){}` – Duck Dodgers Feb 13 '19 at 13:49
Or better yet, `while(true);` ;-) – Krzysiek Karbowiak Feb 13 '19 at 13:51
I don't want an infinite loop. Just print "Loop working " (4 times). How to increment? – Biswajit Feb 13 '19 at 13:52
Yes. But he is interested in `for`! – haccks Feb 13 '19 at 13:52
OP wants no variable – Stoogy Feb 13 '19 at 13:54
3

@Stoogy; What if OP wants to compile the program without any compiler? – haccks Feb 13 '19 at 13:55
2

@Biswajit if you want to loop only 4 times, you ought to specify that in the question. – William Pursell Feb 13 '19 at 13:57
1

@Biswajit "How to increment?" Increment what? You have no variable! – William Pursell Feb 13 '19 at 14:00
@WilliamPursell I already specify that (I want 4 iterations) inside loop. Yes! there is no variable that's why I am asking here. – Biswajit Feb 13 '19 at 14:11
2

@Biswajit No, you did not specify. You asked a very generic question, and provided an example that happened to iterate 4 times. – William Pursell Feb 13 '19 at 14:57

score -1 · Answer 3 · answered Feb 13 '19 at 13:57

To have the loop terminate, you need to keep state which says at which point you are.

Usually, you use loop variable for this.

You can also use recursion, in which case the state is encoded differently, but still needed:

void loop(int i)
{
    if (i < 4) {
        printf ("Loop working\n");
        loop(++i); // recursive call
    }
}

int main()
{
    loop(0);
}

How to write a for loop without variable?

3 Answers3