C程序在创建数组并写入.txt文件时会花费很多时间

Question

I'm trying to create a random array of integers from 0-31 for a 256 x 256 array. The code works as I can do it for smaller arrays really well. However, when trying to create the 256x256 array and write it out to a .txt file it is taking forever to run. I'm new to C so any advice on how to fix this would be appreciated. thank you

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

#define MaxC 256
#define MaxL 256

 int main(){

 FILE *fp;
 fp = fopen("arrays256.txt","w+");
     int c[257] = {0};
     int i=0,j=0,r;
     int n[MaxC][MaxL] = {{0}};

     //int* n = (int*)malloc(MaxC * MaxL * sizeof(int));
     srand(time(NULL));

     for(i = 0; i < MaxC; i++) {

       for(j = 0; j < MaxL; j++) {

         do {
            r=(rand()%32);
         } while( c[r] > 258);
         ++c[r];
         n[i][j]=r;
         printf("%3d",n[i][j]);
         fprintf(fp,"%3d",n[i][j]);
       }

       printf("\n");
       fprintf(fp,"\n");

     }
     //free(n);
     fclose(fp);
     return 0;

 }

Answer 1

Consider the effects of:

do {
    r=(rand()%32);
} while( c[r] > 258);
++c[r];

This code is executed MaxC*MaxL times, due to the loops on i and j , which is 256•256 = 65,536 times. Each time, some r in the range 0 to 31 is chosen. The while loop excludes an r from being chosen if c[r] exceeds 258. This causes various c[r] to be incremented, with r ranging from 0 to 31.

Consider what happens when the loop has been executed 259•32 = 8,288 times. Then each of the 32 c[r] must have been incremented to 259. At that point, there is no c[r] left that does not satisfy c[r] > 258 , so the while loop continues forever.