create a Dynamic std::array of std::array

Question

My task requires me to create an array of arrays to store some data , where the number of row is fixed and the columns are to be decided at run-time.

If I was using a simple int array then this would've been a simple task but because I have to use std::array , I am lost .

My attempt so far

#include<iostream>
#include<array>
using std::array;
int main(){
  array<array<int,1>*,3> x;
  for(size_t i=0;i<3;i++)
      {
        x[i][0]=array<int,3>;
      }
}

which leads to the error

array1.cpp:12:29: error: expected '(' for function-style cast or type construction x[i][0]=array; ~~~~~~~~~~~~^ 1 error generated.

when using clang++

I have read that an std::array is equivalent to a normal array , I know i can use vectors but since i know how many rows i have from the beginning , I feel it to be a waste to use vectors , while I would love the added functionality of std::array when compared to a traditional array. How do I implement this .

You have an array of array pointers, so you would need to assign an array pointer, not an array. You could have an array of arrays. — Raymond Chen
– Raymond Chen, Commented Dec 10, 2018 at 16:22
You can't do this with just std::array, it is always set at compile time. — Galik
– Galik, Commented Dec 10, 2018 at 16:22
There is no such thing as "dynamic std::array". You can use std::vector instead. — n. m. could be an AI
– n. m. could be an AI, Commented Dec 10, 2018 at 16:23
If std::array is equivalent to a normal (compile time) array (int a[5];) then std::vector is equivalent to a normal dynamic array (int* a = new int[5];). — Galik
– Galik, Commented Dec 10, 2018 at 16:24

Yakk - Adam Nevraumont · Accepted Answer · 2018-12-10 16:22:20Z

5

std::array<std::vector<int>,3> is the type you want.

std::vector is a dynamicly sized array.

int main(){
  std::array<std::vector<int>,3> x;
  for(std::size_t i=0;i<3;i++)
  {
    x[i]=std::vector<int>(22);
  }
}

this creates a 3 "major" element array of 22 "minor" size.

Note that column-major and row-major (which is first and which is second) is a matter of convention. So std::vector<std::array<3,int>> is another equally valid interpretation of the requirements.

If you are banned from using std::vector, you will have to figure out an alternative solution, possibly rolling your own. I'd advise against unique_ptr<int[]> (or worse, raw pointers) as they don't store the size of the element.

A std::array< std::array< cannnot be dynamic in either dimension. std::array is fixed size.

answered Dec 10, 2018 at 16:22

Yakk - Adam Nevraumont

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1 Comment

Enerccio Over a year ago

std::vector uses heap allocation and random time

Maxim Egorushkin · Accepted Answer · 2018-12-10 16:32:40Z

0

My task requires me to create an array of arrays to store some data , where the number of row is fixed and the columns are to be decided at run-time.

It sounds like your task requires using a matrix class. Like boost::numeric::ublas::matrix.

The benefit of using a matrix class is that internally it uses one dynamic array to minimize storage overhead.

If you are coding this as an exercise create your own matrix class. Otherwise use an existing one.

answered Dec 10, 2018 at 16:32

Maxim Egorushkin

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