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Program Problem

Last Modified: 2010-04-15
I am new to C and I am trying to figure out how it works. The program I am working on now creates two dynamically allocated arrays, one being the exact opposite of the other. Then the program is supposed to add the the two two dimensional arrays. This is what I thought should work, unfortunately it doesn't. Could you please helpe me. Thanks in advance.

int addmatrix(int *array1, int *array2, int cells){
      int i, *array;
      for(i=0; i<cells; i++){
            *(array + i) = *(array2 + i) + *(array1 + i);
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Hi burttonboarder,

"int *array" is a pointer to an int data type, you are not creating an actual array here.
"return (array); You are not returning an array (that's impossible), but the pointer to the "array"

Even if you created the array as a 'real' array it would be a local variable to the function. This means it would be created on the stack and therefore it would go away when the function finishes (after the return).

Since you are learning just a few hints:

create the array as a global variable in your main() function (How did you define your two arrays array1 and array2?) and pass it into addmatrix().
If you want to use pointers instead of declaring the array like " int array[100]" or if you want to create it dynamic use the malloc() function.

Have fun with C


I created array1 and array2 and sum, the pointer that gets the return value, using malloc().

As I told you, you cannot create an array as a local variable in your function and return a pointer to the calling program. The array for the sum has to exist BEFORE you call addmatrix() because local variables are created on the stack which will become invalid after the return().

Data Warehouse / Database Architect
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Thanks, I actually came up with the exact same answer as your third suggestion just minutes ago, I really appreciate the input though. Thanks.

I tried to avoid to explain to a beginner why it is possible to create 'persitant' variables in a function. Oh well, YOU tell him about the heap now :-)
Kent OlsenData Warehouse / Database Architect


Hi griessh,

The heap -- that's that big pile over in the corner, isn't it?  :)

Seriously, I suspect that those questions won't be long in coming....

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