CODEWITHSUNDEEP
crecursionundefined-behavior
xecutioner
xecutioner

Reputation: 1

Erratic recursion in C

I ran this code on C and in Java and I got 65 and 55 respectively. I cannot fathom how C can get 65. Please help.

int recur(int count) 
{
    if(count==10)
        return count;
    else
        return (count+recur(++count));
}

void main()
{
    printf("%d\n",recur(0));
}

Upvotes: 0

Views: 59

Answers (2)

Preet Sangha
Preet Sangha

Reputation: 65506

Rewriting the code and testing on codepad with some debug statements shows what's happening. I encourage you to take this approach with running code to see what's happening.

int recur(int count) 
{
    int ret;
    if(count==10)
       ret = count;
    else {
       printf("Count Before = %d\n", count);
       ret =  (count +recur(++count));
    }
    printf("Count after = %d\n", ret);
    return ret; 
}

 void main()
 {
    printf("%d\n",recur(0));
 }

Running it gives this

Count Before = 0
Count Before = 1
Count Before = 2
Count Before = 3
Count Before = 4
Count Before = 5
Count Before = 6
Count Before = 7
Count Before = 8
Count Before = 9
Count after = 10
Count after = 20
Count after = 29
Count after = 37
Count after = 44
Count after = 50
Count after = 55
Count after = 59
Count after = 62
Count after = 64
Count after = 65
65

So you can see that recurse up to 10 first , then add 10, then 9 then 8 etc...

Changing it to i = (count + recur(count + 1))

gives

Count Before = 0
Count Before = 1
Count Before = 2
Count Before = 3
Count Before = 4
Count Before = 5
Count Before = 6
Count Before = 7
Count Before = 8
Count Before = 9
Count after = 10
Count after = 19
Count after = 27
Count after = 34
Count after = 40
Count after = 45
Count after = 49
Count after = 52
Count after = 54
Count after = 55
Count after = 55
55

But now the 10 level of nesting is only reached but added amount is still at 9.

Ie. the pre increment means you're adding an extra 10.

Upvotes: 1

alexrider
alexrider

Reputation: 4463

return (count+recur(++count));

Is Undefined Behaviour. On different compilers you may have different results, even same compiler with different compilation options may result to different output.

Upvotes: 1

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