Hi, I was wondering if someone could help me with an issue I'm having in my code.

``````#include <stdio.h>
#define SZ 7
int* a[SZ];
int x, y, z;

void populate() {
x = 1;
y = 2;
z = 3;
a[0] = &x;
a[1] = &y;
a[2] = &z;
a[3] = a[0];
a[4] = a[1];
a[5] = a[2];
a[6] = a[3];
}

void printall() {
int i;
printf("x=%1d, y=%1d, z=%1d", x, y, z);
}

int i;
for( i = 0; i < SZ; i++)	{
printf("\na[%1d] = *(%p) = %1d\n", i, a[i], *(a[i]));
a[i] += 1;
printf("a[%d]+1 = %d = 0x%x\n", i, *(a[i]), *(a[i]));
}
}

int main(int argc, char* argv[]) {
populate();
printall();
return 0;
}``````

I expect the output to look something like this:

``````x=1, y=2, z=3
a[0] = *(0x804a01c) = 1
a[0]+1 = 2 = 0x2

a[1] = *(0x804a03c) = 2
a[1]+1 = 3 = 0x3

a[2] = *(0x804a040) = 3
a[2]+1 = 4 = 0x4

a[3] = *(0x804a01c) = 1
a[3]+1 = 2 = 0x2

a[4] = *(0x804a03c) = 2
a[4]+1 = 3 = 0x3

a[5] = *(0x804a040) = 3
a[5]+1 = 4 = 0x4

a[6] = *(0x804a01c) = 1
a[6]+1 = 2 = 0x2``````

but it turns out like this:

``````x=1, y=2, z=3
a[0] = *(0x804a01c) = 1
a[0]+1 = 134520864 = 0x804a020

a[1] = *(0x804a03c) = 2
a[1]+1 = 3 = 0x3

a[2] = *(0x804a040) = 3
a[2]+1 = 0 = 0x0

a[3] = *(0x804a01c) = 1
a[3]+1 = 134520864 = 0x804a020

a[4] = *(0x804a03c) = 2
a[4]+1 = 3 = 0x3

a[5] = *(0x804a040) = 3
a[5]+1 = 0 = 0x0

a[6] = *(0x804a01c) = 1
a[6]+1 = 134520864 = 0x804a020``````

I see two issues: it seems to cap at 3 (when I'm expecting 4, it returns to 0), and a[0]+1 only gives me the value of the address of a[0] (0x804a020), instead of 2.

Can someone tell me why this is? I've been fiddling with this code for a while now and still don't understand why.
Thanks for your help!

All 3 Replies

First thing you need to know is that array a in your code is an array of addresses and not values.

1. Its just your luck that this code is even running as in the last iteration of for loop when it does

``a[6] += 1;``

it will be pointing to a[7] which you haven't allocated and hence is a segmentation fault in most descent compilers.
2. Talking about your expectation of value "4" it should never come as you aren't adding values when you do a += 1; but incrementing the address.

What you want is a[i+1] rather than a+=1 and as stated in point 1 this will crash on the last iteration of for loop which you need to handle. Happy coding :)

First thing you need to know is that array a in your code is an array of addresses and not values.

1. Its just your luck that this code is even running as in the last iteration of for loop when it does

``a[6] += 1;``

it will be pointing to a[7] which you haven't allocated and hence is a segmentation fault in most descent compilers.
2. Talking about your expectation of value "4" it should never come as you aren't adding values when you do a += 1; but incrementing the address.

What you want is a[i+1] rather than a+=1 and as stated in point 1 this will crash on the last iteration of for loop which you need to handle. Happy coding :)

Ah, thanks for the pointer.. It turns out what I needed was a pointer to the addresses: `*(a[i]) += 1;`

Close the thread once you are done so that people can look into threads which need attention.

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