Problem in result of two version of C code

Question

What's difference between result of these codes :

#include <stdio.h>

int main()
{
int n = 10;
int a, S, i;

i = 0;
S = 0;
while(i<n)
{
printf("Enter a number: ");
scanf("%d",&a);
S+=a;
i++;
}

printf("\nSum of numbers = %d\n\n",S);

return 0;
}

------------------------------------------------------------------------------------------------------

#include <stdio.h>

int main()
{
int n = 10;
int a, S, i;

i = 0;
while(i<n)
{
printf("Enter a number: ");
scanf("%d",&a);
S+=a;
i++;
}

printf("\nSum of numbers = %d\n\n",S);

return 0;
}

------------------------------------------------------------------------------------------------------

for example if you give 1, 2, 3, 4, 5, 6, 7, 8, 9, 10
to "a" :

code #1 give you 55.
code #2 give you 57 or another number.

in other word
what changes when there is "S = 0;" in code?

Answer 1

Since S in first code in not initialized that means it could be filled with a garbage value. So there's a change that S is not start from 0. Thats why we need to initialize the variable

Answer 2

thank you
but what's garbage value ?
How is S filled when we did not initialize it?

Answer 3

Its just leftover from another procress in memory