Question

I’m trying to learn more about arrays since I’m new to programming. So I was playing with different parts of code and was trying to learn about three things, sizeof(). How do I find the length of an array, and also how do I put arrays into functions (as a parameter)?

My code is:

#include <iostream>

using namespace std;

void arrayprint(int inarray[])
{
    for (int n_i = 0 ; n_i < (sizeof(inarray) / sizeof(int)) ; n_i++)
    {
        cout << inarray[n_i] << endl;
    }
}

int main()
{
    int n_array[5];
    for (int n_i = 0 ; n_i < (sizeof(n_array) / sizeof(int)) ; n_i++ )
    {
        cin >> n_array[n_i];
    }
    arrayprint(n_array);
    return 0;
}

It seems the sizeof(inarray) / sizeof(int) works in the main function, but not in the arrayprint function. In the array print function, it evaluates to 1, while in main it evaluates to 5. Why?

So I guess what I want to know is how they are different? And why?

Answer 1

This

void arrayprint(int inarray[])

Is equivalent to this:

void arrayprint(int *inarray)

So you are getting the size of an int pointer on your machine. Even if your question is about C++ and your function looks a bit different, it boils down to this C FAQ entry.

So, inside main n_array is actually a true honest array. But when passed to a function, it “decays” into a pointer. And there’s no way to know the real size. All you can do is pass additional arguments to your function.

void arrayprint(int inarray[], int len)
{
    for (int n_i = 0 ; n_i < len; n_i++)
    /* .... */

And call it like this:

arrayprint(n_array, sizeof(n_array) / sizeof(n_array[0]));

Of course, the real solution would be to use vectors since you’re using C++. But I wouldn’t know much about C++.