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***drdebcol*** · (This post was last modified: 01-07-2011 04:03 AM by drdebcol.)

This is program for mathematical matrix multiplication.
You have everything about that here :
http://en.wikipedia.org/wiki/Matrix_multiplication
And here is the program :

Code:

#include <iostream>

using namespace std;

int main()

{

  unsigned m,n1,n2,p;

  do

  {

  cout<<"Enter height of first matrix !"<<endl;

  cin>>m;

  cout<<"Enter width of first matrix !"<<endl;

  cin>>n1;

  cout<<"Enter height of second matrix !"<<endl;

  cin>>n2;

  cout<<"Enter width of second matrix !"<<endl;

  cin>>p;

  }

  while (n1 != n2);

  //Dynamic allocation of matrix

  //Allocate every one-dmensional array in matrix 

  double **a;

  a=new double*[m];

  for (int i=0;i<m;i++)

    a[i]=new double[n1];

  double **b;

  b=new double*[n2];

  for (int i=0;i<n2;i++)

    b[i]=new double[p];

  double **c;

  c=new double*[m];

  for (int i=0;i<m;i++)

    c[i]=new double[p];

  for (int i=0;i<m;i++)

    for (int j=0;j<p;j++)

      c[i][j]=0;

  cout<<"Enter first matrix (separate numbers with spaces) ! "<<endl;

  for (int i=0;i<m;i++)

    for (int j=0;j<n1;j++)

      cin>>a[i][j];

  cout<<endl;

  cout<<"Enter second matrix (separate numbers with spaces) ! "<<endl;

  for (int i=0;i<n2;i++)

    for (int j=0;j<p;j++)

      cin>>b[i][j];  

      // Here calculation starts

      for (int i=0;i<m;i++)

        for (int j=0;j<p;j++)

          for (int k=0;k<n1;k++)

            c[i][j]+=a[i][k]*b[k][j];

    cout<<"Result is : "<<endl;

    for (int i=0;i<m;i++)

    {

      for (int j=0;j<p;j++)

        cout<<c[i][j]<<" ";

      cout<<endl;  

    }  

    system("pause");

}

***Back_track*** · 05-17-2010, 06:32 AM

Very good work Drdebcol! Nice work with multidimensional arrays

Xupicor · (This post was last modified: 01-06-2011 11:02 PM by Xupicor.)

Note that it is not Standard C++. To get dynamic array you either use non-standard compiler extension (this) or use a standard way of using a pointer and allocating memory by yourself:

Code:

unsigned int n;

cin >> n;

int* array = new int[n];

//do something with it

delete[] array;

You can also forget plain arrays and use a std::vector or any other smart container, as a good C++ programmer would do:

Code:

std::vector<int> array;

array.push_back(15);

array.push_back(16);

array.push_back(17);

// array has 3 elements now, it is flexible and allocates/frees memory by itself

***drdebcol*** · 01-07-2011, 03:54 AM

Oh, yeah that is probably an error. I am going to fix that !
I know that way of declaring arrays can work in all functions except main.
And yeah of course i know how to allocate a dynamic array using new and delete !
Good reply !

CrAzY-KiD · (This post was last modified: 01-09-2011 03:13 AM by CrAzY-KiD.)

that was very confusing and complicated for me....
i m at initial level.... and i can't understand that things... Sad

***drdebcol*** · 01-09-2011, 03:25 AM

(01-09-2011 03:08 AM)CrAzY-KiD Wrote: that was very confusing and complicated for me....
i m at initial level.... and i can't understand that things...

Well mathematically that is not so hard, multiplying matrix is a simple action.
But when you look at the code, there are mostly simple things, except matrix allocation and pointers, which most beginners find hard !

CrAzY-KiD · 01-09-2011, 12:04 PM

but my teacher gave me that question....
i almost solve it but when mulitdimension Multiply matrix in C++ for 3*3 using more loop
can u guide me to make a good program with less loop

***drdebcol*** · 01-09-2011, 09:50 PM

The easiest way to multiply matrix is with 3 for loops (complexity O(n^3)). Every other way is harder to implement, though it has slightly better complexity.
So the easiest way to multiply two matrix-es a and b and get result c is :

Code:

for (int i=0;i<m;i++)

    for (int j=0;j<p;j++)

        for (int k=0;k<n;k++)

            c[i][j]+=a[i][k]*b[k][j];

And sizes are :
a (m*n)
b (n*p)
c (m*p)
where m,n,p are integers that user enters !

CrAzY-KiD · (This post was last modified: 01-10-2011 02:44 AM by CrAzY-KiD.)

for 2*2...!!!
what should i do...!
please write full code...
so that i can understand batter Smile

***drdebcol*** · 01-10-2011, 04:38 AM

Basically you need to understand concept of multiplication !
It doesn't matter if size is 2x2 or 3x3 etc.
But if you which, here is the code for hand multiplication (without loops) for two matrices 2x2 :

Code:

#include <iostream>

using namespace std;

int main()

{

  int a[2][2],b[2][2],c[2][2];

  cout<<"Enter first matrix : "<<endl;

  cin>>a[0][0]>>a[0][1]>>a[1][0]>>a[1][1];

  cout<<"Enter second matrix : "<<endl;

  cin>>b[0][0]>>b[0][1]>>b[1][0]>>b[1][1];

  c[0][0]=a[0][0]*b[0][0]+a[0][1]*b[1][0];

  c[1][0]=a[1][0]*b[0][0]+a[1][1]*b[1][0];

  c[0][1]=a[0][0]*b[0][1]+a[0][1]*b[1][1];

  c[1][1]=a[1][0]*b[0][1]+a[1][1]*b[1][1];

  cout<<"Result is : "<<endl;

  cout<<c[0][0]<<" "<<c[0][1]<<endl;

  cout<<c[1][0]<<" "<<c[1][1]<<endl;

  system("pause");    

}