Hi there! Need help understanding simple C program output.

Question

Hi! I was wondering why following function has the given output:

int main()
{
tree(3);

return 0;
}

void tree(int n)
{
if(n > 1) // edited typo
{
tree(n-1);
printf("%d\n", n);
tree(n-1);
printf("%d\n", n);
}
}

Output:
2
2
3
2
2
3

I don't understand how 3 comes up there and why it loops in an if statement! Any explanation would be appreciated!

Answer 1

It's a recursive function, tricky to understand.
See first you call
tree(3):
3>1 is true so in if block, it call a new function call tree(3-1) => tree(2). This is new call so still previous call tree(3) is pushed to stack with n=3. Note this. will be returned to this after inner recursions completed.
tree(2)

tree(2-1)
=> calls tree(1) , here 1>1 so function does nothing. Goes back to previous call tree(2-1), the next statement is print("%d\n", n); so prints 2.
Next statement is tree(2-1) so does same as tree(1) call. Come backs and prints prints("%d\n", n) ; #2
this ends tree(3-1) so next call is print 3

you have same repeating code,so

tree(3-1)
just the same repeatation for second inner recursive call

tree(3)

tree(3-1):
tree(2)
tree(2-1) => tree(1)
prints("%d",n); #2
tree(2-1) => tree(1)
prints("%d",n); #2
goes back tree(3-1) call so prints 3

tree(3-1):

tree(2)
tree(2-1) => tree(1) x
prints("%d",n); #2
tree(2-1) => tree(1) x
prints("%d",n); #2
goes back tree(3-1) call so prints 3

hope it helps...

Answer 2

Sarah Nour El Din
A function is recursive, when it calls itself recursively. There must be stop condition at some point. Otherwise infinite recursive.
See ex:

void fun(n) {
printf("%d ", n);
if( n > 0)
fun(n-1);
}

Calling this function fun(5), it prints
5 4 3 2 1 0
The function fun() calling itself within the function body so it is a recursive function call.
if you don't place if( n > 0) then it calls infinitely with negative values going on...

Answer 3

Are you sure about output?
It won't output anything..
n > n always results to false..

Answer 4

Jayakrishna 🇮🇳 omg you're so right that's a mistake on my part its supposed to be (n>1)

Answer 5

Jayakrishna 🇮🇳 Thanks so much, I'm getting closer to understanding it. Is an if statement always a recursive function? Or what marks a recursive function? I'll try to wrap my head around it to fully grasp it :)