C recursion code

Question

I need help with understanding this code using recursion.

    int power(int n1,int n2);
    int main()
    {
        int base, exp;
        printf("Enter base number: ");
        scanf("%d",&base);
        printf("Enter power number(positive integer): ");
        scanf("%d",&exp);
        printf("%d^%d = %d", base, exp, power(base, exp));
        return 0;
   }
   int power(int base,int exp)
   {
       if ( exp!=1 )
       return (base*power(base,exp-1));
   }

The code works fine but I don't understand why it still manages to return a value when exp is 1 even when the condition in the function body evaluates to false, so I think it shouldn't return anything then.

For example, if the function is called in the main() function and I assign the values of 5 and 1 to base and exp respectively, I expect that I shouldn't get a return value because exp is 1 and my function returns something only if exp !=1 and also, there is no else part in the function body to cover the case when exp is 1. I know about C returning garbage values when the return value is not specified but in this case, it returns the right answer and I tested it several times. For example, when I call it in the main function with base=7 and exp=1, it returns 7 as my answer and even with other "base" values, it always returns the base which is the right answer. And that exactly is my source of confusion - How is C managing to return the right answer to me when the IF clause of the function body evaluates to false... I guess I'm missing something here.

Your code invokes Undefined Behavior as you don't return an `int` when `exp` is 1. The function will return a "random" , "garbage" value from memory. So anything can be the result. — Spikatrix, Jun 24 '15 at 07:07
It does not return when exp = 1, but if you catch the return value when call the function, it will be junk value from memory — marcadian, Jun 24 '15 at 07:07
I know about C returning garbage values when the return value is not specified but in this case, it returns the right answer and I tested it several times. For example, when I call it in the main function with base = 7 and exp =1, it returns 7 as my answer and even with other "base" values, it always returns the base which is the right answer. And that exactly is my source of confusion - How is C managing to return the right answer to me when the IF clause of the function body evaluates to false... — Ayo, Jun 24 '15 at 07:21

score 3 · Answer 1 · answered Jun 24 '15 at 07:08

3

Actually your function is Undefined Behaviour, because of is not returning nothing in case of exp == 1

The value you have in return to your main function is something random.

answered Jun 24 '15 at 07:08

LPs

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mazhar islam · Answer 2 · 2015-06-24T07:15:03.043

1

You may warned by the compiler in the declaration int power(int base,int exp)

No return, in function returning non-void

Hence, you will get undefined behavior, when you call power() with exp = 1.

Note that: Your power() will fall into recursion trap if exp is negative.

edited Jun 24 '15 at 07:15

answered Jun 24 '15 at 07:08

mazhar islam

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Sourav Ghosh · Answer 3 · 2015-06-24T07:15:38.463

1

In your case, for exp having value 1, there is no return statement. If the return value of that call is used in the caller function, program shows UB.^{See note below}

That said, the issue will also hold true in case you're calling power() with exp value something other than 1, as power() will end up calling itself with exp as 1 at some point of time.

Finally, exp being an int, it can accept a negative value and go into infinite loop, only to create stack overflow by infinite recursion.

Note: As per the C11 standard, if the function is terminated by reaching the closing } and the returned value is used, it invokes invokes undefined behaviour.

Ref: Chapter §6.9.1, paragraph 12,

If the } that terminates a function is reached, and the value of the function call is used by the caller, the behavior is undefined.

edited Jun 24 '15 at 07:15

answered Jun 24 '15 at 07:10

Sourav Ghosh

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I know about C returning garbage values when the return value is not specified but in this case, it returns the right answer and I tested it several times. For example, when I call it in the main function with base = 7 and exp =1, it returns 7 as my answer and even with other "base" values, it always returns the base which is the right answer. And that exactly is my source of confusion - How is C managing to return the right answer to me when the IF clause of the function body evaluates to false... – Ayo Jun 24 '15 at 07:55
@AyorindeKomolafe well, undefined behaviour is undefined. It includes working "as expected" case also. – Sourav Ghosh Jun 24 '15 at 07:57
really? I didn't know that. I thought if it was undefined, it might throw different answers to me each time I run it, but it was giving me consistently right answers. – Ayo Jun 24 '15 at 08:55

C recursion code

3 Answers3