CODEWITHSUNDEEP
c++recursionfactorial
amruth polavarapu
amruth polavarapu

Reputation: 53

I am trying to implement a program recursively. I just don't understand why my last line is being executed twice

Why is Hi being printed 4 times. It will just be executed once before the function finally returns the complete value. 

#include <stdio.h>
int k = 0;
int factorial (unsigned int i) {
  if (i <= 1) {
    return 1;
  } else {
    k = i * factorial(i - 1);
  }
  printf("hi"); // Why is Hi being printed 4 times. 
  return k;
}

int main() {
  int i = 5;
  printf("Factorial of %d is %d\n", i, factorial(i));
  return 0;
}

Upvotes: 0

Views: 77

Answers (3)

Gaurav Ojha
Gaurav Ojha

Reputation: 1177

Your printf's are probably getting queued as the function is trying to call itself.

Let's examine it iteration by iteration -

i = 5

Iteration 1:

k = 5 * factorial(4);

Iteration 2:

k = 4 * factorial(3);

Iteration 3:

k = 3 * factorial(2);

Iteration 4:

k = 2 * factorial(1);

At this point, factorial(1) actually returns 1. Note that printf line is not reached in case.

Now, this value is fed to k in iteration 4 and its returned back to its caller while doing a printf.

The same happens in iteration 3, it receives a value in k and returns it to its caller i.e. iteration 2 while doing another printf

Now, iteration 2 receives a value for k and returns it to its caller i.e. iteration 1 while doing yet another printf (3rd time!)

Finally, iteration 1 receives a value for k and returns it, and along with that it does a final printf (Total 4)

I hope that explains your issue! For ref: http://ideone.com/VgIwKX

Upvotes: 1

Saurav Sahu
Saurav Sahu

Reputation: 13934

Because your code gets executed in this fashion:

k = 5 * factorial(5 - 1);
    k = 4 * factorial(4 - 1);
        k = 3 * factorial(3 - 1);
            k = 2 * factorial(2 - 1);
               return 1;
            printf("hi"); 
        printf("hi"); 
    printf("hi"); 
printf("hi");

Upvotes: 5

acornagl
acornagl

Reputation: 521

The function factorial() is called five times:

  • the first time by the main() method and
  • four times recursively.

All the times that the i parameter is > 1 the function prints "hi". Even if you are using the recursion, the execution if a program (or function) ends when the end point is reached, for a function the end point is the return statement.

Upvotes: 0

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