#include <stdio.h>
void modifyArray(int arr[]) {
arr = (int[]){6, 7, 8, 9, 10}; // Trying to modify the array
}
int main() {
int arr[] = {1, 2, 3, 4, 5};
modifyArray(arr);
for (int i = 0; i < 5; i++) {
printf("%d ", arr[i]); // Outputs 1 2 3 4 5, not 6 7 8 9 10
}
}
Expected Output: 6 7 8 9 10
Actual Output: 1 2 3 4 5
Why does the original array remain unchanged, and how can I modify it correctly?
An array, whenever used in a function parameter declaration, gets adjusted to a pointer to the first element. In your case the code is equivalent to void modifyArray(int* arr).
And that's why the code works in the first place - C does not allow assignments of arrays. Instead arr = (int[]){6, 7, 8, 9, 10}; re-assigns the local variable arr from pointing at the passed array, to pointing at a compound literal. When the function ends, the local variable is gone, so the caller side remains unaffected.
To modify the array you'd have to use memcpy or similar - to actually access the contents of the pointed-at array.
When you passed the array variable (typically the address of the first element), the function would just let's say copy its value into a local variable of its own, and all it does is changing the value of that variable which doesn't affect the original arr you want to change. In order to do that you need to access the data pointed to by that address and change each number value at a time, you could iterate over it or use something like memcpy() , that'll do the job.
#include <stdio.h>
#include <string.h>
void modifyArray(int arr[]) {
int tempArr[] = {6, 7, 8, 9, 10}; /* Creating a temporary array to copy from */
size_t sizeOfArray = 5; /* This is just the number of elements in the array */
memcpy(arr, tempArr, sizeof(int) * sizeOfArray); /* copying from the tempArr to the original arr */
}
int main() {
int arr[] = {1, 2, 3, 4, 5};
modifyArray(arr);
for (int i = 0; i < 5; i++) {
printf("%d ", arr[i]); // Outputs 1 2 3 4 5, not 6 7 8 9 10
}
}
there are many other ways to do it ofc. hope I helped somehow.
memcpy(arr, tempArr, sizeof(int) * sizeOfArray);, consider memcpy(arr, tempArr, sizeof arr[0] * sizeOfArray);. It is easier to write correctly, review and maintain.
memcpy(arr, tempArr, sizeof tempArr);