How to show matrix in c++?

Answer 1

#include <iostream>
using namespace std;

int main() {

int r,c;
cin >> r;
cin >> c;

int (*matrix)[c] = (int(*)[c]) new int[r*c]; // r rows c columns

for(int i = 0; i < r; i++)
for(int j = 0; j < c; j++)
*((int*)(matrix+i)+j) = i*10 + j;

for(int i = 0; i < r; i++)
{
cout << endl;
for(int j = 0; j < c; j++)
cout << *((int*)(matrix+i)+j) << " ";
}

delete [] matrix;

return 0;
}

Answer 2

What a HOT debate guys!

Answer 3

Thank you Andrea and NeutronStar. I've got some value out of your analyses.

Answer 4

First of all, it depends to the dementionality of your matrix. But for a simple "m * n" matrix you need to declare a 2D array and after which two for loop to access to every single element.

int arr[2][3] = { fill with some integer };

for (int i = 0; i < 2; ++i)
for (int j = 0; j < 3; ++j)
cout << arr[i][j]; // print out elements

There's another elegant way to do so, using STL vector.

std::vector<int> v(2, 75);// fill with 75
std::vector<std::vector<int> > v2d(3, v);

for (size_t i = 0; i < v2d.size(); ++i)
for (size_t j = 0; j < v2d[i].size(); ++j)
cout << v2d[i][j];

Answer 5

Hi Andrea. Great snippet, but you need to add 1 into the right hand side of condition.

https://code.sololearn.com/c9OB8knZHGE3/?ref=app

Answer 6

Thank you Andrea.

Answer 7

You can also do this with only one loop and one pointer:

int matrix[30][50];
//fill your matrix

int *p = matrix[0];
while(p < &matrix[29][49])
cout << *p++;

Answer 8

yes because i forgot =

Answer 9

try p<= without +1

Answer 10

also with only one line of code

for(int *p = matrix[0]; p <= &matrix[1][2] ; p++) cout << *p;

Answer 11

😉😉😉

Answer 12

Pointer version

#include <iostream>
using namespace std;

/* This code demonstrate implementation of 2D array as array of array */

int main() {

int const rows = 4;
int const columns = 5;

// allocate memory for rows
int** m = new int*[rows];

// for each row allocate memory for columns

for(int r =0; r<rows; r++)
{
*(m+r) = new int[columns];
}

// fill array with values

for(int r =0; r<rows; r++)
{
for(int c=0; c<columns; c++)
{
*(*(m+r)+c) = r+c;
}
}

// display the stored values
for(int r =0; r<rows; r++)
{
for(int c=0; c<columns; c++)
{
cout << *(*(m+r)+c) << " ";
}

cout << endl;
}

// for each row free the memory allocated for columns

for(int r = 0; r<rows;r++)
{
delete [] m[r];
}

// now free memory allocated for rows
delete [] m;

return 0;
}

Answer 13

too much lines and things NeutronStar...learn about pointer-to-array

Answer 14

#include <iostream>
using namespace std;

int main() {

int (*matrix)[10] = (int(*)[10]) new int[5*10]; // 5 rows 10 columns

for(int i = 0; i < 5; i++)
for(int j = 0; j < 10; j++)
matrix[i][j] = i*10 + j;

for(int i = 0; i < 5; i++)
{
cout << endl;
for(int j = 0; j < 10; j++)
cout << matrix[i][j] << " ";
}

delete [] matrix;

return 0;
}

Answer 15

why use array of array? greater is the matrix greater is memory wasted to store all pointers to each rows

Answer 16

https://code.sololearn.com/cbhq0oVaRpW1/?ref=app

Answer 17

I know that arrays decay into pointers and so one...I think that implement a matrix like a pointer to pointer is not the optimized way to implement a matrix.

Answer 18

No? This app has a purpose: teach something! I know...learn how to optimize a code may be useless if you create a program to add two number having available 2Gb of RAM. But try to create something more important with few kbytes in a embedded system like Arduino and so on.

Then with your method you need:
1)more memory to store all pointers
2)much time to dereference them
And think with 3d, 4d, 5d arrays...
You dereference and dereference and...

I get an andress (matrix[0], tensor[0], 5d[0]) then I add at this address some values...then I dereference only one time. At the end, indipendently of array's dimension. Without waste memory for array of array of matrix of array of pointers and so on...

Answer 19

My
*((int*)(matrix+i)+j)
is faster than your
*(*(matrix+i)+j)

because your matrix is a pointer to pointer
my matrix is a pointer to array

you dereference your matrix then you "move" it
i "move" my matrix

Answer 20

The last way would be to define a class for Matrix as well, which will allow you to do something like this easily.

Matrix a;
//Define a matrix object...
cin>>a;
// Input the matrix like you do with any other integer, character, etc...
cout<<2*a;
//Print scalar multiples of a matrix as easily as this...
Matrix b;
cin>>b;
cout<<a+b<<"\n"<<a*b<<endl;
//Print sum and products of matrices easily...