Seemingly simple array program yielding bizarre output

[alter.ego]

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


int main(void) {
int grades[10];
int i, sum;

for (i=0; i<=10; ++i)
{
sum = sum+grades [i];

printf("\n %d",sum);

}
return 0;
}

Output:

2009291924
2009147472
-1
2009288233
2009288258
4200880
2359128
4200982
4200880
2354536
10

Tried my best to debug & finally gave up. IMO output is irrelevant. Kindly help.

[Subsonics]

Both sum and grades are uninitialized, meaning they will contain what ever garbage happens to reside at that memory location currently.

To initialize them:

int grades[10] = {0};
int i, sum = 0;

That way they will have initial values of 0. Another thing I noticed is that your loop gives you 11 outputs, (meaning you print one value outside the array) due to the condition: i <= 10
you probably want: i < 10

This comes down to how arrays are indexed, starting from 0, so in your case valid indexes are 0-9.

[alter.ego]

Thanks! that helped a bit & I have tweaked the program a little. Why isn't value of sum changing when i is getting iterated? I am sorry if my doubts are slightly noobish; I am just working on some basic concepts of C programming.

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


int main(void) {
int grades[5] = {0};
int i, sum = 0;

for (i=0; i<5; ++i)
{
	sum = sum+grades [i];

printf("\n%d \t%d",i, sum);
}

return 0;
}

Output:

0     0
1     0
2     0
3     0
4     0

[JohnGraham]

Why isn't value of sum changing when i is getting iterated?

...
int grades[5] = {0};
...
for (i=0; i<5; ++i)
{
	sum = sum+grades [i];
}
...

The statement "grades[5] = {0}" will set all the members of grades to 0, meaning that each time through your loop you're adding 0 to sum, so it stays the same.

[Subsonics]

Yes, there are no grades in your array. If you want to sum grades, you must first input these grades into the array, how else can you sum them?

Edit: you can initialize the array with the grades if you want to.

int grades[5] = {1, 2, 3, 4, 5};

[alter.ego]

That's a great enlightment for me (if I understood correctly).

So for loop just references the memory location where the values are initialized. It doesn't actually provide the values in int grades[5]. Am I in right direction?

[Subsonics]

Yes, if you are talking about your specific for loop here, you are stepping through the array with the index and adding them one at a time to sum.

[alter.ego]

Thanks guys! Got it. Much appreciated.