Array initialization in C++ (not strings)

Question

I'm playing around with arrays in C++. I defined a 2d array called matrix and am going to extract the negative values and assign it to the array called array.

Is there a way to initialize an array to zero quickly rather than enumerating all the elements? I looked through other postings and lines such as: int array[10] = {} or int array[10] = {0} do not work on my compiler. I get the error message error: variable-sized object ‘array’ may not be initialized if I try using those statements.

My text book said that all arrays are initialized to zero when declared, but I tested this on my compiler and this was not true; I had to force it to zero by using a for-loop. Is there a correct way of doing this?

Oh by the way, I have a mac and use g++ to compile. When I do man g++ it says its a symbolic link to llvm-gcc compiler.

#include<iostream>

const int NROWS = 4, NCOLS = 5;
int matrix[][NCOLS] = {    16,  22,  99,  4, 18, 
                         -258,   4, 101,  5, 98, 
                          105,   6,  15,  2, 45, 
                           33,  88,  72, 16, 3};

int main()
{
    int SIZE = 10;
    int array[SIZE];
    int count=0;

    // Values of array before initalized    
    for(int i = 0; i < SIZE; i++)
    {
        std::cout << array[i] << " ";
    }    
    std::cout << std::endl;

    //Initialize array to zero
    for(int i = 0; i < SIZE; i++)
    {
        array[i]=0;
        std::cout << array[i] << " ";
    }    
    std::cout << std::endl;

    // Extract negative numbers and assign to array
    for(int i = 0; i < NROWS; i++)
    {
        for(int j = 0; j < NCOLS; j++)
        {
            printf("matrix[%i,%i]=%5i\n",i,j,matrix[i][j]);

            if(matrix[i][j] < 0)
            {
                array[count] = matrix[i][j];
                printf("\tarray[%d]= %4d",count, matrix[i][j]);
                printf("\tcount=%d\n", count);
                count++;                
            }
        }

    }

    // Values of array
    for(int i = 0; i < SIZE; i++)
    {
        std::cout << array[i] << " ";
    }

    std::cout << std::endl;

    return 0;
}

Answer 1

I'm taking a guess here.

int array[10] = {0};

is perfectly legal and should work on any compiler, but I think you tried

int SIZE = 10;
int array[SIZE] = {0};

which is entirely different, and not legal. Array bounds must be constants, not variables.

Some compilers accept variable bounds, but that doesn't make it legal.

Answer 2

Change int SIZE = 10; to const int SIZE=10; or enum{SIZE=10};, and your {} based initialization should work.

You have accidentally used a gcc extension allowing for variable sized arrays.

Answer 3

What you need to do is up at the top where you have:

int array[SIZE];

replace it with:

int array[SIZE] = {};

if you were trying to do:

array[SIZE] = {}; 

later on, it wouldn't work the same. in that case, it would fail (accessing [10] when there's only [0]-[9]).

You could also use a static array.

See:

How to initialize all members of an array to the same value?

Or if you want to use the STL Array type,, you can look at:

http://www.cplusplus.com/reference/array/array/?kw=array

you might even need to just make the SIZE var const.

Answer 4

This should work, and will zero-initialize the 9 remaining elements.

int array[10] = {0};

See: Array initialization in C++

Answer 5

If array size is variable-dependent you must: loop through or

int array[x];

memset(array, 0, x);

If array size is hardcoded you can:

int array[10] = {0};