打印函数中修改的数组内容

Question

In my main() function I have declared an array of type int with the numbers 1 to 10. I then have two other functions of type int* that take this array and its size as parameters, perform some operations, and each returns a pointer to the new array. Where I'm having issues is with a third function that prints the contents of the array .

#include <iostream>

using namespace std;

const int SIZE_OF_ARRAY = 10;

int main() {

    int array[SIZE_OF_ARRAY] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

    int *ptr1 = 0;
    ptr1 = function1(array, SIZE_OF_ARRAY);
    print(array, SIZE_OF_ARRAY);

    cout << endl;

    int *ptr2 = 0;
    ptr2 = function2(array, SIZE_OF_ARRAY);
    print(array, SIZE_OF_ARRAY);

    return 0;
}

void print(int array[], const int SIZE_OF_ARRAY)
{
    for (int i = 0; i < (SIZE_OF_ARRAY * 2); i++)
    {
        cout << array[i] << " ";
    }
}

int* function1(int array[], const int SIZE_OF_ARRAY)
{
    int *ptr = new int[SIZE_OF_ARRAY];

    // Do stuff.

    return ptr;
}

int* function2(int array[], const int SIZE_OF_ARRAY)
{
    int *ptr2 = new int[SIZE_OF_ARRAY * 2];

    // Create new array double in size, and set contents of ptr2 
    // to the contents of array. Then initialize the rest to 0.

    return ptr2;
}

As expected here, the result of calling the print() function twice is something like:

1 2 3 4 5 6 7 8 9 10 465738691 -989855001 1483324368 32767 -1944382035 32767 0 0 1 0
1 2 3 4 5 6 7 8 9 10 465738691 -989855001 1483324368 32767 -1944382035 32767 0 0 1 0

But I want the result to be like this instead:

1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10 0 0 0 0 0 0 0 0 0 0

How can I accomplish this? (Note that for this assignment I'm using C++98). Thanks in advance.

Answer 1

new int[SIZE_OF_ARRAY] allocates memory, but doesn't assign values to the array elements. What you are seeing is what was in that memory when it got allocated for the array. You can change your function2 to assign zeroes to array elements, if that's what you want.

Answer 2

First of all, you want to print different number of elements on the two calls to print , so you should not delegate deciding whether to multiply the size by two to the print , but rather do it on the calling side. Change the print function to only iterate up to SIZE_OF_ARRAY , and change the two places where you call it to:

print(ptr1, SIZE_OF_ARRAY);

and

print(ptr2, SIZE_OF_ARRAY * 2);

correspondingly.

Now, I assume that your second function does assign values to all 20 elements, but if it does not, the ones to which it did not assign values would continue containing garbage. To get around it, just initialize them at the beginning of the second function:

int *ptr2 = new int[SIZE_OF_ARRAY * 2];
for (size_t i = 0; i < SIZE_OF_ARRAY * 2; ++ i) ptr2[i] = 0;

With these two changes you should get the desired behavior.

Also, if you allocate something with new[] , you need to delete it with delete[] , otherwise you get a memory leak. Add these two lines at the end of main :

delete[] ptr1;
delete[] ptr2;

Note, that using delete instead of delete[] would be wrong in this case. If something is allocated as an array, it must be deleted as an array.