How can I store these numbers in C? - c

Question

Lets say you had an array like this
{ 1 2 5 7 2 3 7 4 2 1 }
And you wanted to store that the difference between the first half of the array and the second half is at positions 2 and 4.
The trick is, I need to use those stored numbers later in other code, so what I can't figure out is how I would store these numbers.
I have this method
int * getPositions(int *array, int size){
int * c[(size/2)];
int counter = 0;
for(int i = 0; i < size /2; i++) {
if (*(array + i) != *(array + (size - 1) - i)) {
c[counter]= (int *) i;
counter++;
}
}return (int *) c;
}
but it seems to be storing -1774298560 into every location. I know that cause when I try to print it
int c = (int) getPositions(array, size_of_array);
for(int i = 0; i < ((size_of_array/2)); i++){
printf("%d\t", c);
}
all it prints out is
-1774298560 -1774298560 -1774298560 -1774298560 -1774298560
PS: I have array and size_of_array initialized somewhere else.
PS: I have taken the comments into consideration and changed the code to the following
int * getPositions(int *array, int size){
int * c = (int *) malloc((size_t) (size/2));
int counter = 0;
for(int i = 0; i < size /2; i++) {
if (*(array + i) != *(array + (size - 1) - i)) {
c[counter]= i;
counter++;
}
}

Answer 1

If the function should return a simple int array, you need to declare a pointer-to-int, and then call malloc to reserve space for the array. Then fill in the array, and return the pointer. The calling function will need to free the memory at some point.
int *getPositions(int *array, int size)
{
int *c = malloc( (size/2) * sizeof(int) );
if ( c == NULL )
return NULL;
// put stuff in the array using array syntax, e.g.
c[0] = array[0];
return c;
}
Call the function like this
int *c = getPositions( array, size );
if ( c != NULL )
for( int i = 0; i < (size/2)); i++ )
printf( "%d\t", c[i] );
free( c );
Notes:
Yes, error checking in C is a pain, but you must do it, or your
program will randomly crash.
You are allowed to use array syntax with a pointer, just be sure you don't read or write past the end of the memory that the pointer references.
It's legal to pass a NULL pointer to free.

Answer 2

Another option.
int * getPositions(int *array, int size);
int main() {
int array[] = { 1, 2, 5, 7, 2, 3, 7, 4, 2, 1 };
int size_of_array = sizeof(array) / sizeof(int);
int *ptr = getPositions(array, size_of_array);
for(int i = 0; ptr[i] != '\0' ; i++){
printf("%d\t", *(ptr + i));
}
return 0;
}
int * getPositions(int *array, int size) {
int temp[size/2];
int counter = 0;
for (int i = 0; i < size / 2 ; i++) {
if (array[i] != array[(size - 1) - i]) {
temp[counter++] = i;
}
}
int *c = malloc(counter * sizeof(int));
for (int i = 0; i < counter; i++) {
c[i] = temp[i];
}
return c;
}

Answer 3

My compiler going mad actually...which compiler you're using? Any modern static analyzer should have warned you
aftnix#dev:~⟫ gcc -std=c11 -Wall st.c
st.c: In function ‘getPositions’:
st.c:8:25: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
c[counter]= (int *) i;
^
st.c:11:6: warning: function returns address of local variable [-Wreturn-local-addr]
}return (int *) c;
^
st.c: In function ‘main’:
st.c:16:13: warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]
int c = (int) getPositions(array, 10);
So compiler is showing all the problems of the code. And your edited code doesn't even compile :
aftnix#dev:~⟫ gcc -std=c11 -Wall st.c
st.c:4:4: error: expected identifier or ‘(’ before ‘[’ token
int[] getPositions(int *array, int size){
^
st.c: In function ‘main’:
st.c:17:5: warning: implicit declaration of function ‘getPositions’ [-Wimplicit-function-declaration]
int c = (int) getPositions(array, 10);
Couple of things you have to take into consideration.
If you need to return a storage address, never try to return a local variable. As local variables are allocated in the Stack. Stack frames have the same lifetime of the function. Use malloc and co to allocate memory at the Heap. Understanding how the Runtime data structures are allocated and used is a very essential for a C programmer as C doesn't have fancy magical stuff under the hood. C is pretty much bare metal.
Stack Vs Heap
When passing arrays/pointers around, you should first think about your design first. Do you want to change your original array "in place"? or you want a different copy of "transformed" array. Usually you should try to avoid changing a memory "in place" as there might be other users to that. But if don't have such worries, you can then design your function as a "input->output" basis. That means you will not be returning void, and will return the transformed array itself.