Question 130241 in Xcode Question posted in
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Xcode – How to calculate number of rows of 2D char array inside a function which takes such an input?

Here is the function:

void printArray(const char arr[][3], int rows, int cols) {
// rows == 3 && cols == 3 is currently a placeholder. I have to confirm whether these are
// actually correct.
    if (rows == 3 && cols == 3 && rows > 0 && cols > 0 && rows <= SIZE && cols <= SIZE) {
        for (int i = 0; i < rows; i++) {
            for (int j = 0; j < cols; j++) {
                cout << arr[i][j];
            }
            cout << 'n';
        }
    }
}

I need to figure out if the rows parameter inputted into the equation is actually correct. To do this, I need to calculate the size of the const char array from within the function.

I have tried adding the following to the if statement:

rows == sizeof(arr)/sizeof(arr[0])
cols == sizeof(arr[0])/sizeof(arr[0][0])

rows == sizeof(arr)/sizeof(arr[0])
cols == sizeof(arr[0])

None of these have worked. Please advise many thanks.

Tags:

2

Answers


  1. 0

    It does not work this way. arr is a pointer type (const char (*)[3]) and you cannot derive a size from it unless you use a function template:

    #include <iostream>

using std::cout;

template <int rows, int cols>
void printArray(const char (&arr)[rows][cols])
{
    static_assert(rows == 3 && cols == 3, "Dimension must be 3x3, mate!");
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            cout << arr[i][j];
        }
        cout << 'n';
    }
}

int main()
{
    char good[3][3] {};
    char bad[2][3] {};
    printArray(good);
    printArray(bad);
}

    Note that the above will automatically infer the dimensions from the array, and create an appropriate function. Additionally, there’s a static assertion here that will fail to compile for anything other than a 3×3 array:

    error: static assertion failed: Dimension must be 3x3, mate!
    8 |     static_assert(rows == 3 && cols == 3, "Dimension must be 3x3, mate!");
      |                   ~~~~~^~~~
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  2. 0

    My advise top using "C" style arrays and move to C++ std::array.

    //You could change your call to arr[3][3] but in general I would advice to switch to using std::array or std::vector in C++.E.g.printArray

#include <array>
#include <iostream>

// array is an object so unlike "C" style arrays you can return them from functions
// without having to use something like char** (which looses all size information)
std::array<std::array<char,3>,3> make_array()
{
    std::array<std::array<char, 3>, 3> values
    { {
        {'a','b','c'},
        {'d','e','f'},
        {'g','h','i'},
    } };

    return values;
}

// pass by const reference, content of array will not be copied and not be modifiable by function
// only will compile for 3x3 array
void show(const std::array<std::array<char, 3>, 3>& values) 
{
    // use range based for loops they cannot go out of bounds
    for (const auto& row : values)
    {
        for (const char value : row)
        {
            std::cout << value;
        }
        std::cout << "n";
    }
}


int main()
{
    auto values = make_array();
    show(values);
    return 0;
}

    Note : the size of the arrays can be templated so your code will work for other array/matrix sizes as well.

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