使用C语言和OpenMP实现pagerank算法

以下是使用C语言和OpenMP实现PageRank算法的代码示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <omp.h>

#define N 5 // 网页总数

int links[N][N] = {
    {0, 1, 1, 0, 0},
    {0, 0, 1, 1, 0},
    {0, 0, 0, 1, 1},
    {0, 0, 0, 0, 1},
    {1, 0, 0, 0, 0}
};

int out_degree[N];

float pagerank[N];
float new_pagerank[N];

float damping_factor = 0.85;
int max_iterations = 100;
float convergence_threshold = 0.0001;

void calculate_out_degree() {
    for (int i = 0; i < N; i++) {
        out_degree[i] = 0;
        for (int j = 0; j < N; j++) {
            if (links[i][j] == 1) {
                out_degree[i]++;
            }
        }
    }
}

void initialize_pagerank() {
    for (int i = 0; i < N; i++) {
        pagerank[i] = 1.0 / N;
    }
}

void calculate_pagerank() {
    float diff = 1.0;
    int iter = 0;
    while (iter < max_iterations && diff > convergence_threshold) {
        #pragma omp parallel for
        for (int i = 0; i < N; i++) {
            new_pagerank[i] = 0.0;
            for (int j = 0; j < N; j++) {
                if (links[j][i] == 1) {
                    new_pagerank[i] += pagerank[j] / out_degree[j];
                }
            }
            new_pagerank[i] = damping_factor * new_pagerank[i] + (1 - damping_factor) / N;
        }

        diff = 0.0;
        for (int i = 0; i < N; i++) {
            diff += fabs(new_pagerank[i] - pagerank[i]);
        }

        memcpy(pagerank, new_pagerank, sizeof(new_pagerank));

        iter++;
    }
}

void print_pagerank() {
    for (int i = 0; i < N; i++) {
        printf("Page %d: %f\n", i, pagerank[i]);
    }
}

int main() {
    calculate_out_degree();
    initialize_pagerank();
    calculate_pagerank();
    print_pagerank();
    return 0;
}

在这个示例中，我们使用一个5个网页的小例子进行演示。首先，我们定义网页链接关系矩阵links，然后计算每个网页的出度，并将PageRank向量pagerank初始化为1/N。接下来，使用OpenMP中的并行循环迭代计算PageRank值，直到收敛。最后，输出每个网页的PageRank值。