Recursive call to myfunc(int x) - printing backwards?

[jimhakans]

hello every1.
i have got this code from somewhere but i can not understand its output

#include<stdio.h>
void myfunc(int x)
{
   if(x>0)
      myfunc(--x);
   printf("%d,",x);
}
 
int main()
{
   my func(5);
   return 0;
}
 

The output is 0,0,1,2,3,4,. According to my logic the output should be 4.
And one more thing i am a little confuse about the last line of the code that is return 0; plz explain what this return means.
thanx in advance.

[sicarie]

It's a recursive function, each time passing x, and then decrementing it when it comes back. (So it's called with 5, but then goes back down to 4, 3, 2, 1, etc... and to 0 where it finally fails the condition, and then is printed out on the way back up, as many times as it was called.)

The return statement is the standard in both C and C++ for the return of int main(). It tells the compiler that everything ended correctly. There are different return codes for different scenarios (most of them are errors).

PS - Per the Posting Guidelines, SMS/'leet speak (plz, thx), are not proper English and should not be used. Thanks

[jimhakans]

It's a recursive function, each time passing x, and then decrementing it when it comes back. (So it's called with 5, but then goes back down to 4, 3, 2, 1, etc... and to 0 where it finally meets the condition, and then is printed out on the way back up.)

The return statement is the standard in both C and C++ for the return of int main(). It tells the compiler that everything ended correctly. There are different return codes for different scenarios (most of them are errors).

PS - Per the Posting Guidelines, SMS/'leet speak (plz, thx), are not proper English and should not be used. Thanks

Okay. I will take care about that from now.
But in that program the output should be 4,3,2,1,0,. (If i am not wrong)
Please correct me if i am wron.g

[sicarie]

Well, the concept is there, but the finer points are fuzzy. Look at the code:

void myfunc(int x)
{
   if(x>0)
      myfunc(--x);
   printf("%d,",x);
}
 

So let's trace this out

myfunc(5)     (the call is made and passed a value)
if 5 > 0      (the if statement is executed)
true          (expression results in true condition)
myfunc(--5)   (so function is called with new value. This is the pre-decrement operator, so it's done before the call is made, giving)
myfunc(4)     (at this point, the print statement hasn't been called due to the recursive function call)
if 4 > 0
true
myfunc(--4)
myfunc(3)
blah blah blah until
myfunc(0)
if 0>0
false            (false condition finally allows if statement to stop executing, and the next call after the if statement is)
print 0          (value is printed)
 

Then, you have to realize that the call returns itself to it's calling function. This is myfunc(--x), and as the x with the 0 value was local to that specific instance of that call to recursive myfunc(), the x value of the myfunc() that called it was, well, 0. but the next one was 1. And so each returns from the recursive myfunc(), and moves on to the print statement.


Does that make sense? Because the print statement is after the recursive call, each separate recursive call has to return for the print statement to occur. It counts down 4,3,2,1,0, but then counts back up as the calls return.

[jimhakans]

Well, the concept is there, but the finer points are fuzzy. Look at the code:

void myfunc(int x)
{
   if(x>0)
      myfunc(--x);
   printf("%d,",x);
}
 

So let's trace this out

myfunc(5)     (the call is made and passed a value)
if 5 > 0      (the if statement is executed)
true          (expression results in true condition)
myfunc(--5)   (so function is called with new value. This is the pre-decrement operator, so it's done before the call is made, giving)
myfunc(4)     (at this point, the print statement hasn't been called due to the recursive function call)
if 4 > 0
true
myfunc(--4)
myfunc(3)
blah blah blah until
myfunc(0)
if 0>0
false            (false condition finally allows if statement to stop executing, and the next call after the if statement is)
print 0          (value is printed)
 

Then, you have to realize that the call returns itself to it's calling function. This is myfunc(--x), and as the x with the 0 value was local to that specific instance of that call to recursive myfunc(), the x value of the myfunc() that called it was, well, 0. but the next one was 1. And so each returns from the recursive myfunc(), and moves on to the print statement.


Does that make sense? Because the print statement is after the recursive call, each separate recursive call has to return for the print statement to occur. It counts down 4,3,2,1,0, but then counts back up as the calls return.

Thanks, i got it mow. It was really helpful. Thanks again.
many regards

[sicarie]

Thanks, i got it mow. It was really helpful. Thanks again.
many regards

Glad I could help. I see you've already posted again - we're more than happy to help. Also, if you could help us and take a look at the Posting Guidelines, a lot of the stuff we ask you to do is in there - including how to form your question so it is answered quickly. Thanks!