CODEWITHSUNDEEP
crecursion
user1208523
user1208523

Reputation: 73

Why does this recursive function loop infinitely rather than printing numbers in reverse?

int reverse(int);
void main()
{
     int no =5;
     reverse(no);
}

int reverse(int no)
{

 if(no == 0)
      return 0;
else 
 printf("%d",no);

reverse(no--);
}

This program goes in infinite loop.Why is that so? I am not able to get the desired ouput. Desired output should be 5 4 3 2 1.Thank you in advance

Upvotes: 4

Views: 298

Answers (6)

Austin
Austin

Reputation: 154

You want to do 

int reverse(int);
int main()
{
    int no =5;
    reverse(no);
}

int reverse(int no)
{
    if(no == 0)
        return 0;
    else 
        printf("%d",no);
    return reverse(--no);
}

So that you're returning a number every time you call reverse and you decrement no before you use it.

Upvotes: 0

Alejandro Iván
Alejandro Iván

Reputation: 4051

You are using post-decrement. First evaluates, then reduces value.

Change this:

reverse(no--);

to this:

return reverse(--no);

--no is pre-decrement. First decrease "no", then pass the value to reverse.

Note that I'm RETURNING the result value, you function always have to return an int, due to its declaration.

Upvotes: 0

Krishnabhadra
Krishnabhadra

Reputation: 34265

n-- is post increment which first use n as argument of the function and then decrement n.

So n-- essentially does is

reverse(n);
n = n - 1;

What you want is --n

Upvotes: 0

MatthewD
MatthewD

Reputation: 2534

Change:

reverse(no--);

to:

reverse(no-1);

no-- returns the original value before the decrement occurred. So this line will always call reverse(5). Hence, infinite loop.

Upvotes: 0

templatetypedef
templatetypedef

Reputation: 372764

In this recursive call:

reverse(no--);

You are passing the value of no--. Since this uses the postfix -- operator, this means "decrement no, then pass the original value of no into the recursive call to reverse." This causes infinite recursion, since you are constantly making a recursive call with reverse set to the same value.

To fix this, change this to read

reverse(no - 1);

Notice that there's no reason to decrement no here, since within the current recursive call you will never read no again. You can just pass the value of no - 1 into the function. In fact, you might want to in general avoid using -- in recursive calls, since it almost always indicates an error in the code.

Hope this helps!

Upvotes: 12

icktoofay
icktoofay

Reputation: 129001

You're using the post-decrement operator on no. That first gets the value of no and uses that as the value of the expression and then decrements no (still using the undecremented value as the value of the expression). You probably want to use --no or even just no - 1. (The former will modify no, whereas the latter will not, but it does not matter because no is not referenced after that point.)

Upvotes: 3

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