Getting output from recursion manually in C - c

Question

So, I have two questions.
Question 1) I find recursion difficult in C. And I have this one question, that I dont know how should I go about attempting it. I want to know its output, Please help me.
#include <stdio.h>
void fun (int);
int main (void)
{
int a;
a = 3;
fun(a);
printf("\n");
return 0;
}
void fun ( int n )
{
if ( n > 0 )
{
fun(--n);
printf("%d",n);
fun(--n);
}
}
How can I solve this recursion manually?
I know during recursion, the information is stored on stack. Therefore, I tried doing it by that way. Firstly, a will be decremented all the way upto 0. But then, it will exit out of the loop. So, when will it print the values?
Question 2) Also, I Wanted to know since the topic I am studying right now is functions. If I make a function and lets suppose it returns some value, then IS IT MANDATORY that I collect its value upon calling or I can call it without collecting its return value?
For eg: Let's say I made the function as,
int foo ( int a )
{
........
return b;
}
Now, if I call this function from inside main, then is it mandatory that I store the returned value in some variable?

Answer 1

You had 2 questions: the first one is what happens in your code:
To your question #1: Function fun(n) could be rewritten so that it is functionally equivalent but easier to understand, as:
void fun(n) {
if (n > 0) {
fun(n - 1);
printf("%d", n - 1);
fun(n - 2);
}
}
That is:
for fun(n)
if n > 0,
first call fun(n - 1)
then print the number n - 1
lastly call fun(n - 2)
Thus the following happens when unwinding the recursion:
fun(3) ->
fun(2) ->
fun(1) ->
fun(0) ->
n <= 0 -> exits
prints 0
fun(-1) ->
n <= 0 - exits
prints 1
fun(0) ->
n <= 0 - exits
prints 2
fun(1) ->
fun(0) ->
exits as n <= 0
prints 0
fun(-1) ->
exits as n <= 0
Execution goes from up to down sequentially - thus the output 0120 from the prints lines.
Question #2:
No, return value does not need to be stored in a variable. In fact, the printf you used returns an int, that tells the number of characters written, but you did not store that return value anywhere.

Answer 2

For no 1 - Get a note pad and a pencil.
Start off an write fun(3) - It is in Main.
You can now cross that out an instead write
if ( 3 > 0 )
{
fun(2);
printf("%d",2);
fun(1);
}
(applying the logic of --n)
Repeat with both of those fun. You can do the leg work on this one
Number 2 - You do not have to collect the return value from a function

Answer 3

I would like to answer your second question About storing the value returned by the called function.
The answer returned by the called function can be displayed in two ways..
No.1-You need not store it in any variable in the calling function and print it directly as follows:
#include<stdio.h>
#include<conio.h>
void main()
{
int a=10, b=9, add(int,int);
printf("Sum of %d and %d is : %d",a,b,add(a,b));
getch();
}
int add(int m,int n)
{
return(m+n);
}
Here,the function call has been written in the printf() function and hence the need of an extra variable has been eliminated
No.2-The other way and the code for the same-
#include<stdio.h>
#include<conio.h>
void main()
{
clrscr();
int a=10,b=9,c,add(int,int);
c=add(a,b);
printf("Sum of %d and %d is : %d",a,b,c);
getch();
}
int add(int m,int n)
{
return(m+n);
}
So,you do need a variable for the second method.Because there has to be something to catch the value thrown by the called function