How Can I Get the Size of an Array in C When Using Pointers?

Getting Size of Array from Pointer in C

When working with arrays in C , one common task is to determine the size or number of elements within an array. However, this can be tricky when accessing the array through a pointer.

Issue with Determining Array Size from Pointer

Consider the following function header:

int largest(int *list, int highest_index);

This function is designed to return the largest integer in an array of integers. The problem arises when trying to determine the number of elements in the 'list' array using the pointer.

Incorrect Methods

Some incorrect methods that have been attempted include:

int i = sizeof list/sizeof(int); //returns incorrect value
int i = list.size(); // does not compile

These methods either return an incorrect value or do not compile because they assume the array is inherently known by the pointer.

Correct Approach: Passing Array Size as Argument

The correct approach is to pass the array size as an additional argument to the function. This allows the function to determine the number of elements in the array.

For example:

static const size_t ArraySize = 5;
int array[ArraySize];
func(array, ArraySize);

In this example, the function 'func' takes the array pointer and the array size as arguments.

Rationale

C allows arrays to decay to pointers when passed as function arguments. However, pointers by themselves do not contain information about the array size. By passing the array size as an argument, the function is able to correctly determine the number of elements in the array.

Conclusion

When working with arrays through pointers, it is essential to consider that pointers do not have inherent information about the array size. The solution is to pass the array size as an additional argument to functions that need this information.

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