Why this output 0000 for this program

Question

3.00/5 (1 vote)

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[

C#

#include <stdio.h>
#include <stdlib.h>

int main()
{
    static int i=5;
    if(--i)
    {
    main();
    printf("%d\n",i);
    }
    return 0;
}

]

Posted 24-Dec-15 0:13am

Anupkumar_vj

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2 solutions

Solution 1

What you have is head recursion. See The difference between head & tail recursion[^]. As long as the static variable i is not zero, the recursion takes place (4 times). While unwinding the recursion (in the tail of each recursive call), the current value of the static variable is printed. If it was a local variable instead of a static variable, the result was different --> a stack overflow since the stop condition for the recursion was never met.
Cheers
Andi

Posted 24-Dec-15 0:42am

Andreas Gieriet

Comments

Anupkumar_vj 24-Dec-15 12:58pm

I have gone through your give link. Still I didn't get unwinding the recursion the current value of static variable.

Andreas Gieriet 24-Dec-15 15:44pm

Go ahead and write the recursion down on paper. That's the only way to learn how it works.
Make a column for the static variable and one column of each level of recursion of the main function. Write down the value of i from the beginning to the end of the recursion and write down in the respective value plus in the recursion row the current statement of execution.
This will teach you the machinery of recursion.
E.g.
level 0: i=5 static int i = 5;
level 0: i=4 if (--i)
level 0: i=4 main()
level 1: i=3 if (--i)
level 1: i=3 main()
level 2: i=2 if (--i)
level 2: i=2 main()
level 3: i=1 if (--i)
level 3: i=1 main()
level 4: i=0 if (--i)
level 4: i=0 return 0
level 3: i=0 printf(0)
level 3: i=0 return 0
level 2: i=0 printf(0)
level 2: i=0 return 0
level 1: i=0 printf(0)
level 1: i=0 return 0
level 0: i=0 printf(0)
level 0: i=0 return 0
Cheers
Andi

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Gevorg Hai · Accepted Answer · 2015-12-26T07:16:00

Solution 2

This happens because, when it comes to "printf", the value of a output variable "i" became zero.
You can put "printf" before "main".
More better to write recursion like this:

C++

void recuot(int n)
{
	if (n > 1)
		recuot(n-1);
	printf("%d\n",n);;
}
int main()
{
	recuot(5);
	return 0;
}

Posted 26-Dec-15 7:16am

Gevorg Hai