get length of multidimensional array with different length in c ?-CodePudding

I have a multidimensional array and i'm trying to get the length of each column. But it's returning 20 for all of them.

int buttons[16][5] = {{0, 4, 1},{1, 0, 2, 5},{2, 1, 3, 6},{3, 2, 7},{4,0,5,8},{5,1,4,6,9},{6,2,5,7,10},{7,3,6,11},{8,4,9,12},{9,5,8,10,13},{10,6,9,11,14},{11,7,10,15},{12,8,13},{13,9,12,14},{14,10,13,15},{15,11,14}};

sizeof(buttons[0]);

CodePudding user response：

You are declaring an C-style array where each element is an array of 5 int. Therefore, with sizeof(buttons[0]), you are getting the size of 5 ints. Usually, ints have a size of 4 bytes, hence you get 20. Only apparently has buttons[0] 3 elements: in fact, in your code, you initialize only the first 3 elements of buttons[0].

If you want a multidimensional array, where each "column" has different size, you should better use std::vector which can hold array of variable size. Then size() gives you the actual number of element.

Example code:

#include <iostream>
#include <vector>

int main()
{
   std::vector<int> buttons[16] = {{0, 4, 1},{1, 0, 2, 5},{2, 1, 3, 6},{3, 2, 7},{4,0,5,8},{5,1,4,6,9},{6,2,5,7,10},{7,3,6,11},{8,4,9,12},{9,5,8,10,13},{10,6,9,11,14},{11,7,10,15},{12,8,13},{13,9,12,14},{14,10,13,15},{15,11,14}};

   std::cout << buttons[0].size();
}

Even better, you could use a vector of vector:

#include <iostream>
#include <vector>

int main()
{
    std::vector<std::vector<int>> buttons = {{0, 4, 1},{1, 0, 2, 5},{2, 1, 3, 6},{3, 2, 7},{4,0,5,8},{5,1,4,6,9},{6,2,5,7,10},{7,3,6,11},{8,4,9,12},{9,5,8,10,13},{10,6,9,11,14},{11,7,10,15},{12,8,13},{13,9,12,14},{14,10,13,15},{15,11,14}};
    
    std::cout << buttons[0].size();
}

CodePudding user response：

You can't do that with arrays. You can do that with vectors:

vector<vector<int> > buttons = ...;
for (auto const& button : buttons) cout << button.size() << '\n';