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Why Pointers?

To illustrate the usefulness of pointers, let's pretend we want to pass a collection of data into a function. Within that function, we want to modify the data. In Python, we could use a class to store the data, and pass an instance of that class into the function:

class Coordinate:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z


def update_coordinate_x(coord, new_x):
    coord.x = new_x


c = Coordinate(1, 2, 3)
print(c.x)  # 1
update_coordinate_x(c, 4)
print(c.x)  # 4

Assignment

Now let's do the same thing, but using a struct in C.

Complete the coordinate_update_x and coordinate_update_and_return_x functions.

coordinate_update_x should update the x field with the provided new_x value. It returns void and should not update the caller's struct.
coordinate_update_and_return_x should update the x field with the provided new_x value, and then return the updated coordinate struct.

Remember, you can access the field of a struct with a . operator, like so:

car.tires = 4;

What Happened?

After passing the assignment, open up main.c and take a look at the test cases. You'll notice that coordinate_update_x doesn't update anything, but coordinate_update_and_return_x does because it returns a new copy of the struct.

In C, structs are passed by value. That's why updating a field in the struct does not change the original struct from the main function.
To get the change to "persist", we needed to return the updated struct from the function (a new copy).
The memory address of the struct that went in to coordinate_update_and_return_x was not the same as the address of the struct that was returned. Again, because we created a copy.