How to create 2D array matrix using basic loop

Question

Currently I'm trying to create a sparse matrix with this declaration:

typedef int Data;

struct Element 
{
    int i;
    int j;
    Data value;
};

I'm allowing user to input number of row they want:

int size;

cout << "Enter the number of element: ";
cin >> size;

// Function to construct sparse matrix
constructSparseMatrix(size,3);

My try for constructSparseMatrix function:

void constructSparseMatrix(int row, int col) {
    int** arr = new int*[row];
    for(int i = 1; i <= row; i++) {
        arr[i] = new int[col];
        cout << arr[row][col];
    }

}

But when I input any number then the application output nothing but stop working. My expected output look something like this if I input 2:

Insert (1 ,2, 3)
Insert (4, 5, 6)

Can someone point me in the correct direction? Maybe this is easy but I'm kinda new to C++ so please bear with my ignorance.

Element has coordinates and a data thing. Because it has coordinates already, a 2D-array isn´t necessary at all. You could store all elements in a normal 1D std::vectorand search it through if you need to find a specific element. Or, for better performance, a nested 2D vector. But you don´t need new at all, and if you have trouble with it, just avoid it. — deviantfan
– deviantfan, Commented Jul 21, 2015 at 17:54
Why are you making a 2d array if you want it be sparse? Those two ideas are mutually exclusive... — Jashaszun
– Jashaszun, Commented Jul 21, 2015 at 17:54
...and another thing: Array indices start at 0. for(int i = 1; i <= row; i++) => for(int i = 0; i < row; i++) — deviantfan
– deviantfan, Commented Jul 21, 2015 at 17:55
Is it acceptable to make use of the STL? Specifically std::vector<...> and std::pair<>? — Matt
– Matt, Commented Jul 21, 2015 at 18:00
@deviantfan How can I apply your suggestion without using vectorbut only with a normal loop? Can you post an answer? — Pewds
– Pewds, Commented Jul 21, 2015 at 18:33

sh03 · Accepted Answer · 2015-07-21 18:37:10Z

1

The segmentation fault you are receiving is caused by this line:

cout << arr[row][col];

In that context, row and col are the dimensions of the matrix, therefore the elements go from 0 to row - 1 and from 0 to col - 1. When you access arr[row][col] you accessing elements out of bounds.

Not to mention that your for loop also goes out of bounds:

for(int i = 1; i <= row; i++) {

and will start from 1 instead of 0.

A correct function would look more like:

int** constructSparseMatrix(int row, int col) {
    int** arr = new int*[row];
    for(int i = 0; i < row; i++) {
        arr[i] = new int[col];
        for (int k = 0; k < col; k++)
            arr[i][k] = /* initialize element (i, k) */;
    }
    return arr;
}

Live demo

Finally, I'd recommend a good book on C++. It's clear that you are missing a lot of the basics of C++. new and delete are highly advanced topics. I'd recommend starting with std::array, std::vector and std::map instead.

Sparse matrices are often not implemented with an actual matrix (array) of NxM elements; that is used for dense matrices (where most of the elements are different from 0).

A possible implementation is with an std::map<std::pair<int, int>, element>, where you map 2D indeces to a specific element. For other possible implementations look at this page.

An easier alternative it to use Boost.uBLAS which provides various kind of matrices.

edited Jul 21, 2015 at 18:37

answered Jul 21, 2015 at 18:25

sh03

76.5k39 gold badges177 silver badges282 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Pewds Over a year ago

Is there any way I can achieve the result using a normal loop?

sh03 Over a year ago

@Pewds What's a normal loop?

Pewds Over a year ago

Thank you for your answer, if not using new as your suggestion, is there any other way? Normal loop I mention here is like loop inside a loop to populate the value of row and col for later usage without using any advance feature.

Collectives™ on Stack Overflow

How to create 2D array matrix using basic loop

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related