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c++arraysmultidimensional-arraydynamic-memory-allocation
Hira Tanveer
Hira Tanveer

Reputation: 365

How to convert 2D array to 1D in C++?

So for example I have a 2D array A={{1,2,3}, {2,3},{5}}; and I want to get all the rows existing in the array A. I have the length of array stored in variable "lenA", here lenA=3. Also I have an Array B which has the length of each subarray in A. Say B={3,2,1} in this case. In reference to my example array A, how to I dynamically get 3 subarrays from one 2D array i.e. A? So as a result I would have something like:

A1={1,2,3}
A2={2,3}
A3={5}

Upvotes: 0

Views: 623

Answers (5)

Marek R
Marek R

Reputation: 37512

std::ranges in C++20 or ranges library (C++14 compliant) does it in clean way:

    double A[][4] {
        { 1, 2, 3, 4}, 
        { 5, 6, 7, 0}, 
        { 8, 9, 0, 0},
        {10, 0, 0, 0}
    };
    for (auto x : std::ranges::views::join(A))
        std::cout << x << '\n';

https://godbolt.org/z/hPP7e9

Upvotes: 0

Alireza
Alireza

Reputation: 1

you can find the verification of the below code at http://cpp.sh/57pi2

A1D represent the 1-Dimensional array which you was looking for,

#include <iostream>

int main()
{
    const int lenA = 4;
    double A2D[][lenA]={{1,2,3,4},{5,6,7},{8,9},{10}};
    double *A1D;
    int B[lenA] = {4,3,2,1};

    int num_of_elements = 0; 
    for (int i=0; i < lenA; i++)
        num_of_elements += B[i];
    
    A1D = new double[num_of_elements]   ; 
    for (int i=0, k=0; i<lenA; i++) 
        for (int j=0; j<B[i]; j++)
            A1D[k++] = A2D[i][j];

    for (int j=0; j<num_of_elements; j++)
        std::cout << A1D[j] << std::endl;
}

Upvotes: 0

theOtherMichael
theOtherMichael

Reputation: 229

You can't dynamically generate new identifiers in C++. The closest you can get is using the preprocessor to generate your names, and by definition, that's done before compilation.

If you already have a fixed number of named array pointers, you could assign those dynamically. But any solution that must accept an arbitrary number of rows at runtime will require that you use something like an array index.

for (int i = 0; i < lenA; i++)
{
    // Do something with the row at A[i]
}

Upvotes: 1

ytlu
ytlu

Reputation: 412

My try converting A(4x4) to triangle *a[4]:

#include <iostream>
#include <algorithm>
int main()
{
   constexpr int lenA = 4;
   double A[lenA][lenA]={{1,2,3,4}, {5,6, 7, 0}, {8, 9,0,0},{10,0,0,0}};
   double *a[lenA];
   int B[lenA] = {4, 3, 2, 1};

   for (int i=0; i < lenA; i++)
    {
       a[i] = new double [ B[i] ];
       std::copy_n(A[i], B[i], a[i]);
    }
   for (int i=0; i<lenA; i++) {
       std::cout << "row " << i << ": ";
       for (int j=0; j<B[i]; j++) {
           std::cout << a[i][j] << ".  ";
           }
       std::cout <<std::endl;
      }
   return 0;
}

result: 

$ ./a.exe
 row 0: 1.  2.  3.  4.
 row 1: 5.  6.  7.
 row 2: 8.  9.
 row 3: 10.

Upvotes: 0

Tarik
Tarik

Reputation: 11209

Pseudo code

for i in 0 to lenA-1
    for j in 0 to lenB[i]-1
        A[i][j] whatever...

Upvotes: 0

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