[閒聊] generating exponential(beta) distribution

[PanJC ((#‵Д′)f〒﹌﹌﹌﹌﹌ꄩ]

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#define PI 3.141592653589793

void genExp(double *arr, int n, double para);
double mean(double *arr, int n);
double variance(double *arr, int n, double sum);

int main(){
        srand(10);
        double *rv;
        int n;
        printf("Input the sample size: ");
        scanf("%d",&n);
        rv = (double *) calloc(n,sizeof(double));
        genExp(rv, n, 2.0);

        double m1, v1;
        m1 = mean(rv,n);
        v1 = variance(rv,n,m1);
        printf("mean: %lf\n",m1);
        printf("variance: %.4lf",v1);
        free(rv);
}

void genExp(double *arr, int n, double para){
        int i;
        double u1;
        for( i = 0 ; i < n ; i++ ){
                u1 =  (double) rand() / RAND_MAX;
                arr[i] = -para*log(u1);
        }
}

double mean(double *arr, int n){
        int i;
        double sum=0.0;
        for( i = 0 ; i < n ; i++ ){
                sum += arr[i];
        }
        return(sum/n);
}

double variance(double *arr, int n, double sum){
        int i;
        double tmp=0.0;
        for( i = 0 ; i < n ; i++ ){
                tmp += pow(arr[i],2);
        }
        return(tmp/n - pow(sum,2));
}

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