matrix multiplication

http://www.yudd97.com/archives/148

void Matmul(LLX[MAXN][MAXN],LL Y[MAXN][MAXN])  

{

    LL t[MAXN][MAXN]={0};

    for(int i=0;i<N;i++)

        for(int k=0;k<N;k++)

            if(X[i][k])

            for(int j=0;j<N;j++)

                t[i][j]=(t[i][j]+X[i][k]*Y[k][j]);

    for(int i=0;i<N;i++)

        for(int j=0;j<N;j++)  

            X[i][j]=t[i][j];

}

HDOJ 4920 Matrixmultiplication

Problem Description

Given two matrices A and B of size n×n, find the product of them.
bobo hates big integers. So you are only asked to find the result modulo 3.

 Input

The input consists of several tests. For each tests:
The first line contains n (1≤n≤800). Each of the following n lines contain n integers — the description of the matrix A. The j-th integer in the i-th line equals Aij. The next n lines describe the matrix B in similar format (0≤Aij,Bij≤109).

 Output

For each tests:
Print n lines. Each of them contain n integers — the matrix A×B in similar format.

const int MAXN = 810;    const int Modn = 3;    int N;    int X[MAXN][MAXN];    int Y[MAXN][MAXN];    int t[MAXN][MAXN];      int main()    {        while(scanf("%d",&N)!=EOF){            for(int i=0;i<N;i++){                 for(int j=0;j<N;j++){                    scanf("%d",&X[i][j]);                     X[i][j]%=3;                 }            }            for(int i=0;i<N;i++){                 for(int j=0;j<N;j++){                    scanf("%d",&Y[i][j]);                     Y[i][j]%=3;                     t[i][j]=0;                 }            }            for(int i=0;i<N;i++){                 for(int k=0;k<N;k++)                     if(X[i][k])                         for(intj=0;j<N;j++)                            t[i][j]=(t[i][j]+X[i][k]*Y[k][j]%3)%3;            }            for(int i=0;i<N;i++){                 printf("%d",t[i][0]);                 for(int j=1;j<N;j++){                     printf("%d",t[i][j]);                 }                 printf("\n");            }        }        return 0; }

单独考察矩阵相乘的题目并不多见，普遍都是结合矩阵快速幂，高斯消元等矩阵算法。进行这些算法之前我们必须要掌握矩阵相乘算法

我们需掌握简单的相乘规则，才能学习后面的一些矩阵算法。同时，为了以后学习算法以及做题的需要，我们也要记得矩阵相乘时的算法复杂度及优化细节。