使用数组乘法矩阵？

Question

This is an assignment I have been struggling with. Basically I have the core idea and the code in working order. But the problem is that I did it in a way different from the instructions.

So, to multiply the matrices I asked for the dimensions of the array beforehand (rows,columns). Then I would ask again for the values of the of the array.

But would what I would like to do is simply enter the values of my array and automatically find the dimensions of the array by the number of integers that are input. But I am not sure how to do this because I thought my instructor said something about not being able to set arrays to variable values or something like that.

//what I'd like to be able to do
Enter the first matrix:
1 2 3
4 5 6

Enter the second matrix:
5 6
7 8
9 0

// what I am currently doing
#include<iostream>
using namespace std;
int main()
{
int l,m,z,n;
int matrixA[10][10];
int matrixB[10][10];
int matrixC[10][10];

cout<<"enter the dimension of the first matrix"<<endl;
cin>>l>>m;
cout<<"enter the dimension of the second matrix"<<endl;
cin>>z>>n;
if(m!=z||z!=m){
cout<<"error in the multiblication enter new dimensions"<<endl;
cout<<"enter the dimension of the first matrix"<<endl;
cin>>l>>m;
cout<<"enter the dimension of the second matrix"<<endl;
cin>>z>>n;
}

else{
cout<<"enter the first matrix"<<endl;
for(int i=0;i<l;i++){
for(int j=0;j<m;j++){
     cin>>matrixA[i][j];
     }
     }
cout<<"enter the second matrix"<<endl;
for(int i=0;i<z;i++){
for(int j=0;j<n;j++){
    cin>>matrixB[i][j];
}
}
for(int i=0;i<l;i++){
for(int j=0;j<n;j++){
        matrixC[i][j]=0;
        for(int k=0;k<m;k++){
matrixC[i][j]=matrixC[i][j]+(matrixA[i][k] * matrixB[k][j]);
}
}
}

cout<<"your matrix is"<<endl;
for(int i=0;i<l;i++){
for(int j=0;j<n;j++){
cout<<matrixC[i][j]<<" ";
}
cout<<endl;
}
}
//system("pause");
return 0;
}

Answer 1

You cannot declare an array with a runtime dimension (known as a variable-length array in C, where they are allowed), the dimension must be known at compile time. The solution is to either use a standard container like std::vector

std::vector<std::vector<int>> matrix(M, std::vector<int>(N)); // M x N 

or use dynamic arrays,

int** matrix = new int*[M]; // allocate pointers for rows
for(size_t i = 0; i < N; ++i)
    matrix[i] = new int[N]; // allocate each row

then don't forget to delete them at the end of your program

for(size_t i = 0; i < N; ++i)
    delete[] matrix[i];
delete[] matrix;

You should prefer the std::vector approach, as you don't have to deal with memory allocation etc.

Answer 2

From your lines of code, it seems that you want to ask the user to enter the correct values of column of matrix 1 and row of matrix 2. what you have used is an if statement which will only run a single time. if your value of "m" and "z" are not equal at the very first input , then you would never be able to enter the else part of your code where you have declared rest of your code to enter and multiply the matrices.

In order to check whether the values of "m" and "z" are equal or not, you can use a while loop or a do while loop.

while(m!=z)
{cout<<"enter the value m and z";`
cin>>m>>z;
}

besides this , it would be good if you use l,m,z,n as static members to save the memory.

if you want to declare a matrix of your desired dimensions, then you can do it like this.

firstly, you ask the user to enter the dimension of the matrix . then after when correct dimension has been entered, then you should be able to make a matrix of your required dimensions. Here is what i am trying to say:

cout<<"enter the values of l and m";
cin>>l>>m;
cout<<"enter the values of z and n";
cin>>z>>n;
int array1[l][m];
int array2[z][n];

Thus , you can enter the dimensions of your desired matrix easily.