Printing loop variable changes var value

Question

Here are my two structures :

int *data = malloc(sizeof(int) * 512 * 512);
int **sortedData = malloc(sizeof(int) * 512);

The data var contains integers from a file. sortedData is supposed to be a 2D version of data. Here is how I initialize it :

for (int i = 0; i < dimensions[0]; i++)
    {
        sortedData[i] = (data + i * 512);
    }

For testing the values I use :

    printf("%d\n", data[0]);
    printf("%d\n", sortedData[0][0]);`

The problem is that the outcome of the second printf is an address, so the values are :

81
44766304

I tried to print the value of (data + i * 512) like this :

for (int i = 0; i < dimensions[0]; i++)
    {
        printf("%d\n", data + i * 512);
        sortedData[i] = (data + i * 512);
    }

And here is the thing : The printf returns an address for each i, but now, the outcome of the program is what I am looking for :

81
81

I can't really understand how the printf would change the returned value and why my initial solution is wrong.

EDIT 1 : It still doesn't work after changing sortedData from

... = malloc(sizeof(int) * 512);

To

... = malloc(sizeof(int *) * 512);

`int **sortedData = malloc(sizeof(int) * 512);` => `int **sortedData = malloc(sizeof(int *) * 512);` your elements are pointers not integers — Jean-François Fabre, May 23 '18 at 14:45
if someone finds a better duplicate, be my guest to edit. But it's a common error, so I'm closing, my answer explains the exact same issue — Jean-François Fabre, May 23 '18 at 14:48
I changed the malloc as you said, but it still doesn't work. The printf is still the only way to get what I am looking for. — Neodia, May 23 '18 at 15:03
could you create a [mcve] that runs and demonstrates the issue? it's not obvious by the current bits of code. — Jean-François Fabre, May 23 '18 at 16:09
So, while doing what you asked from me, I cleaned some of the code to get it smaller to paste here. It now works after cleaning the code. Seems like the problem was coming from an unused var that I used for testing. Any ideas why it would cause trouble ? — Neodia, May 23 '18 at 16:35
not right away. You can completely change your question by posting your example that works & the one that doesn't (both as [mcve]) and I'll have a look — Jean-François Fabre, May 23 '18 at 17:25
After some further research, it seems that is was one of my string declarations. Before it was : `char str[10];` After : `char str[10] = "";` — Neodia, May 24 '18 at 08:09
both aren't equivalent. In the first case `str` value isn't initialized/nul-terminated — Jean-François Fabre, May 24 '18 at 08:10

0 Answers0