Problem in understanding Recursion....

[isaisa]

Hi,

I am a beginner in 'C' programming. I faced one problem in understanding the following code.

Here is a code:-

int main()
{
int x,y;

x = recsum(1);
y = recsum(1);
printf("\n x = %d y =%d",x,y);
}

recsum(i)
int i;
{
static int sum =0;
if(i == 3)
return(sum);
else
{
sum += 10;
i++;
recsum(i);
}
}

output:-
x = 20 y =40

Here is the confussion for me. What i think in this case is that i am creating three copies of recsum function[ for i =1, i = 2 & i =3]. When the last recsum gets executed, it returns the sum as 20 but it is not getting collected in a function from which the recursive call is made i.e. the case at i=2. So the value should get lost. Similarly the function copy for i =1 is not collecting any value. Here all the return values should be lost. And the first call to recsum should return any int value and get assigned to x in the main.
This is what i interpret. I am not getting the correct results as per my interpretations.
So can anyone explain me how function return works and stack cleaning. I am using std. 'C' calling convention here.

Thanks in advance.
Sanjay

 

[JOLESEN]

since sum is static, all nested calls to recsum() will use the same instance of sum - which is probably not what you want ?

Try to remove the 'static'
Also the function should return a value when i != 3 ?
/JOlesen

 

[isaisa]

Hi,
Thanks for your reply. I have tried with declaring the sum as auto as int sum = 10;

And what i get is ...
x = 10 y = 10;

I am still not getting.When a function returns without any explicit "return" keyword, what it returns ?????

Thanks again.
Sanjay

 

[JOLESEN]

It will return whatever is present in the AX / EAX (Microsoft / Intel) register - that is : more or less 'random'.
More precisely : The calling function will read the register, expecting the called function to put something meaningful into it.
At least that's what I have always seen.
Your compiler should (ought to) give you a warning about this.
You MUST explicitly return a value from your function.

/JOlesen

 

[jamisar]

do it correct !!
#include <stdio.h>
int main()
{
int recs(int);
int a = recs(1);
int b = recs(1);
printf(&quot;a=%d b=%d\n&quot;,a,b);
exit(0);
}
int recs(int x)
{
static int sum; /* do not init a static */
if(x == 3) return(sum);
sum += 10;
return(recs(++x));
} vox clamantis in deserto.