C: Looping without using looping statements or recursion

Question

I want to write a C function that will print 1 to N one per each line on the stdout where N is a int parameter to the function. The function should not use while, for, do-while

Answer 1

int x=1;

void PRINT_2(int);

void PRINT_1(int n)
{ if(x>n)
    return;
  printf("%d\n",x++);
  PRINT_2(n);  
}

void PRINT_2(int n)
{ if(x>n)
    return;
  printf("%d\n",x++);
  PRINT_1(n);  
}

int main() 
{   int n;
    scanf("%d",&n);
    if(n>0)
      PRINT_1(n);
    system("pause");
}

Answer 2

You can use setjmp and logjmp functions to do this as shown in this C FAQ

For those who are curious to why someone have a question like this, this is one of the frequently asked questions in India for recruiting fresh grads.

Answer 3

N is not fixed, so you can't unrole the loop. And C has no iterators as far as I know.

You should find something that mimics the loop.

Or thinking outside the box:

(for example N is limited to 1000, but it is easy to adapt)

int f(int N) {
    if (N >= 900) f100(100);
    if (N >= 800) f100(100);
    if (N >= 700) f100(100);
    ...

    f100(n % 100);
}

int f100(int N) {
    if (N >= 90) f10(10);
    if (N >= 80) f10(10);
    if (N >= 70) f10(10);
    ...

    f(n % 10);
}

int f10(int N) {
    if (N >= 9) func();
    if (N >= 8) func();
    if (N >= 7) func();
    ...
}

Answer 4

write all possible output to a string first, and null terminate it where the output should stop.
this is a rather dirty solution, but given the limitations, all I can think of,
except for using assembler, off course.

char a[]="1\n2\n3\n4\n5\n6\n7\n8\n9\n10\n11\n12\n13\n14\n15\n16\n17\n"/*...*/;
main(n,v)char**v;{n=atoi(v[1]);
#define c(x)(n>x?n-x:0)
a[n+c(1)+c(9)+c(99)+c(999)+c(9999)+c(99999)+c(999999)+c(9999999)/*+...*/]=0;
puts(a);}

Given that MAX_INT==2147483647 on popular architectures, we only need to go up to +c(999999999). Typing out that initial string might take a while, though...

Answer 5

If you know the upper limit of N you can try something like this ;)

void func(int N)
{
    char *data = " 1\n 2\n 3\n 4\n 5\n 6\n 7\n 8\n 9\n10\n11\n12\n";
    if (N > 0 && N < 12)
        printf("%.*s", N*3, data);
    else
        printf("Not enough data. Need to reticulate some more splines\n");
}

Joke aside, I don't really see how you can do it without recursion or all the instructions you mentioned there. Which makes me more curious about the solution.

Edit: Just noticed I proposed the same solution as grombeestje :)

Answer 6

Another thingy (on linux) would be to do as below where 7 is N

int main() {
    return system("seq 7");
}