Week 6

Passing an Array to a Function

To pass an array as an argument to a function simply use its name without square brackets.

myFunc(myArr);

When we pass an array to a function in C, we are really passing a pointer to the first element of the array.

This means the function can modify the original array — we are not creating a copy. That is, we are passing by reference.

When we pass an array in to a function which doesn't need to modify it, for example searching an array for max, we can pass it as const.
This is like putting up guard rails to make sure that we don't actually change the array.

Multi-Dimensional Arrays

Multidimensional arrays are arrays within arrays. The simplest kind is a 2-dimensional array.

In a 2-dimensional array, the first index is the row and the second is the column. Think of it like a spreadsheet:

However, memory is only one-dimensional. Therefore, a 2D array is stored like this:

The number of columns determines where each new row starts in memory.
This is because C stores arrays in row-major order — row by row.

We can compute the address of any element in a 2D array with this formula:

start_loc + (rowNum * numberOfColumns) + colNum

where start_loc is the address of the first element of the array.

Multi-Dimensional Arrays with Initializer Lists

When we initialize a 2d array, we can use additional sets of {} to signify rows

int arr[3][3] = {
    {0, 1, 2},
    {3, 4, 5},
    {6, 7, 8}
};

But this is just for readability — the underlying memory layout is the same as:

int arr[3][3] = {0,1,2,3,4,5,6,7,8};

Both arrays will be identical, but the first is more readable.

Passing a Multi-Dimensional Array to a Function

When passing a multi-dimensional array to a function, we must explicitly specify the size of all dimensions except the first.

This is because the compiler needs to know the number of columns to correctly calculate where each row starts in memory.

void printArray(int arr[][3], size_t nRows);

We do not need to specify the first dimension (number of rows).

Looping Over Multi Dim Arrays

Bob specifically said do not use variables like i or j when looping over arrays.

Instead use things like r, c or row, col or something else that is similarly explicit.

for (size_t row = 0; row < 3; ++row)
{
    for (size_t col = 0; col < 4; ++col)
    {
    printf("%d ", arr[row][col]);
    }
    printf("\n");
}

Variable-Length Arrays (VLAs)

A variable-length array is an array whose size is not known at compile time. It can be created using a runtime value.

int size;
scanf("%d", &size);
int arr[size];

However, not all compilers support VLAs.

Note: Bob very specifically said don't use VLAs in this course.

If needed, dynamic arrays should be created using malloc, calloc, or realloc.