send dynamic array of pointer to function in c++

Question

I trying to send array to function, but my program gets stuck

int main()
{
    int n, i;
    bool random;

    cout << "number of elements in array:"; cin >> n;
    cout << "use random values?"; cin >> random;

    int* arr = new int[n]; //create int array with n size
    init_array(random, n, arr); //fill array using function

    for (i = 0; i <= n; i++) //display array
        cout << " " << arr[i];

    return 0;
}

This function should fill array with random number or input from keyboard

void init_array(bool random, int n, int* arr)
{
    int i;
    if (random)
    {
        srand(time(0)); //initialize random;
        for (i = 0; i < n; i++)
            arr[i] = rand() % 100;
    }
    else
        for (i = 0; i<n; i++)
            cout << "arr[" << i << "]: "; cin >> arr[i];
}

Is there any way send dynamic array to function?

for (i = 0; i <= n; i++) ... i<n just as in every other loop ;) — dyp
– dyp, Commented Nov 10, 2013 at 18:57
There is no difference between passing dynamic and static arrays by pointers. What do you mean by stuck? — Darth Hunterix
– Darth Hunterix, Commented Nov 10, 2013 at 18:58
Oh, btw: There's no need to declare int i; at the beginning of the function. for(int i = 0; i < n; i++) works as well, and limits the scope of i to where it is actually required. — dyp
– dyp, Commented Nov 10, 2013 at 18:59
Thanks all, problem fixed. I forgot {} brackes in second for cycle in function. — mixaill
– mixaill, Commented Nov 10, 2013 at 19:08

Zac Howland · Accepted Answer · 2013-11-10 19:15:47Z

When you do not use brackets after your for-loop, only the first statement is used as a loop:

else
    for (i = 0; i<n; i++)
        cout << "arr[" << i << "]: "; cin >> arr[i];

This loop will attempt to print "arr[#]" n times, and then ask for an input (which will attempt to be placed in the item 1 after the last element in your array (UB).

What you want is this:

else
{
    for (i = 0; i<n; i++)
    {
        cout << "arr[" << i << "]: "; 
        cin >> arr[i];
    }
}

You also have a problem with your output:

for (i = 0; i < n; i++) // <= would attempt to print 1 more item than exists in the array

And just for completeness, most of these issues go away when you use a container that does all of this for you:

int main()
{
    int n = 0;
    bool useRandom = false;
    std::cout << "number of elements in array:"; 
    std::cin >> n;
    std::cout << "use random values?"; 
    std::cin >> useRandom;

    std::vector<int> arr(n);
    init_array(useRandom, arr);

    std::copy(arr.begin(), arr.end(), std::ostream_iterator<int>(std::cout, " "));

    return 0;
}

void init_array(bool useRandom, std::vector<int>& vec)
{
    ::srand(time(0)); //initialize random;
    int n = 0;
    std::transform(vec.begin(), vec.end(), vec.begin(), [&](int i)
    {
        if (useRandom)
        {
            i = rand() % 100;
        }
        else
        {
            std::cout << "arr[" << n++ << "]:  ";
            std::cin >> i;
        }
        return i;
    });
}

P0W · Accepted Answer · 2013-11-10 19:04:41Z

2

Your code is asking for a number at the end because of last cin>>n

fix else part in init_array as:

 else
        for (i = 0; i<n; i++)
            { //Notice braces
             cout << "arr[" << i << "]: ";
             cin >> arr[i];
            }

Also fix:

for (i = 0; i < n; i++) //display array from index 0 to n-1
    cout << " " << arr[i];

answered Nov 10, 2013 at 19:04

P0W

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