CODEWITHSUNDEEP
crecursion
KevinDW
KevinDW

Reputation: 315

Recursive function in C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void recursie(int);

int main(int argc, char **argv) {
  recursie(3);
}

void recursie(int a){
  if(a==0){return;}
    recursie(a-1);
    printf("%d",a);
    recursie(a-1);
}

The output is : 1213121. Can someone explain me how i get to this output ?

Upvotes: 0

Views: 548

Answers (4)

jeremy
jeremy

Reputation: 10057

When new concepts like these seem confusing, grab a piece of notebook paper and graph everything out. You'll see that the reason that the number 3 is in the center is that you're running two recursive functions on either side of its printing, leaving an equal padding for 3 to be in the center. The next is that 2 is left in the center of its padding (1's). This is for the same reason that 3 is in the center. 1 is on either sides and there are no 0s because you said if 0, stop.

Was there a specific result you were looking for that wasn't achieved?

Upvotes: 0

David
David

Reputation: 1419

The output of calling it on n is the string of n surrounded by two copies of the output of calling it on n-1, except that the output of calling it on 0 is nothing. This means the output of calling it on 1 is "1", so the output of calling it on 2 is "121" ("2" surrounded by copies of "1"), and the output of calling it on 3 is "1213121" ("3" surrounded by two copies of "121").

Upvotes: 1

user268396
user268396

Reputation: 12006

This happens because of the order in which recursion occurs vis a vis that of the prinf(). If you work out the order of calls, ignore recursie(0) because it is a no-op (does nothing) and flatten them to a simple list of statements what you get is:

recursie(3-1);
printf("%d", 3);
recursie(3-1);

Which leads to:

recursie(2-1);
printf("%d", 2);
recursie(2-1);
printf(3);
recursie(2-1);
printf("%d", 2);
recursie(2-1);

Which in turn works out as:

printf("%d", 1);
printf("%d", 2);
printf("%d", 1);
printf("%d", 3);
printf("%d", 1);
printf("%d", 2);
printf("%d", 1);

Upvotes: 2

influjensbahr
influjensbahr

Reputation: 4078

recursie(3)
  -calls recursie(2)
   - calls recursie(1)
     -calls recursie(0) -> void
     -prints 1
     -calls recursie(0) -> void
   -prints 2
   -calls recursie 1
      -calls recursie 0 -> void
      -prints 1
      -calls recursie 0 -> void
  -prints 3
  -calls recursie(2)
   - calls recursie(1)
     -calls recursie(0) -> void
     -prints 1
     -calls recursie(0) -> void
   -prints 2
   -callse recurse 1
      -calls recusie 0 -> void
      -prints 1
      -calls recursie 0 -> void
end

Upvotes: 8

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