c - 二维数组引用整个索引，而不是 C 中的单个单元格

Question

我似乎无法弄清楚我的问题是什么。我正在编写一个 2D 随机游走模拟，我的想法是使用 2D 数组来模拟它所在的网格。但是，在我的代码中，当我尝试引用数组中的特定单元格以增加其值(例如 bin[3][4])时，它会增加整个索引的值，即 bin[1][4],bin[ 2][4]，bin[3][4]等。关于如何解决这个问题或我做错了什么有什么想法吗？谢谢。

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

#define total 10.  /*total iterations*/
#define walk 5  /*total # of steps in a walk*/
#define lambda 1.0 /*step size*/
#define binsize 0.1
/*current chosen values are for bug checking simplicity*/

main()
{
  float range = walk*lambda*2.; /*2 times max range, all positive*/
  int n,prob,i,k,j,placex,placey,bins;
  double valuex,valuey, a; /*value starts at half range so all values are +*/
  bins=(range/binsize);

  int bin[bins][bins];
  for(i=0;i<=bins;i++) /*zero out bin*/
  {
    for(j=0;j<=bins;j++)
    {
      bin[i][j]=0;
    }
  }

  for(k=0;k<total;k++)
  {
    valuex=range/2.;
    valuey=range/2.;

    for(n=1;n<=walk;n++)
    {
      prob= rand(4) % 100+1;
      if(prob<=25)
      {
        valuex=valuex+pow(lambda,n);
      }
      else if(prob>25 && prob<=50)
      {
        valuex=valuex-pow(lambda,n);
      }
      else if(prob>50 && prob<=75)
      {
        valuey=valuey+pow(lambda,n);
      }
      else if(prob>75)
      {
        valuey=valuey-pow(lambda,n);
      }
    }   

    placex=floor(valuex/binsize+0.5);
    placey=floor(valuey/binsize+0.5);

    bin[placex][placey]=++bin[placex][placey];

    printf("%d %d %d\n",placex,placey,bin[placex][placey]); /* for bug checking. it prints the bin numbers where the value should go and then the value of that element.*/
  }

  for(i=0;i<=bins;i++)
  {
    for(j=0;j<=bins;j++)
    {
      a=bin[i][j]/total;
      //printf("%lf %lf %lf\n",i*binsize-range/2.,j*binsize-range/2.,a); /*format for input into IGOR*/
    }
  }
}

Answer 1

想通了。您正在超出数组范围。您的 for 循环会上升到 bin，但数组的索引从 0 到 bin -1。当我在 for 循环中从 <= 中取出 = 时，它就起作用了。我的最终代码如下所示。一般来说，超出数组范围会产生不可预测的行为。

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

#define total 10.  /*total iterations*/
#define walk 5  /*total # of steps in a walk*/
#define lambda 1.0 /*step size*/
#define binsize 0.1
/*current chosen values are for bug checking simplicity*/

main()     {
   float range = walk * lambda * 2.; /*2 times max range, all positive*/
   int n, prob, i, k, j, placex, placey, bins;
   double valuex, valuey, a; /*value starts at half range so all values are +*/

   bins = (range / binsize);

   int bin[bins][bins];

   /*zero out bin*/
   for(i = 0; i < bins; i++) {
      for(j = 0; j < bins; j++) 
         bin[i][j] = 0;
   }

   for(k = 0; k < total; k++) {
      valuex = range / 2. - 1;
      valuey = range / 2. - 1;

      for(n = 1; n <= walk; n++) {
         prob = rand(4) % 100 + 1;
         if(prob<=25)
            valuex += pow(lambda, n);
         else if(prob <= 50)
            valuex -= pow(lambda, n);
         else if(prob <= 75)
            valuey += pow(lambda, n);
         else 
            valuey -= pow(lambda, n);
      }   

      placex = floor(valuex / binsize + 0.5); 
      placey = floor(valuey / binsize + 0.5); 
      bin[placex][placey] += 1;

      printf("%d %d %d\n", placex, placey, bin[placex][placey]); /* for bug checking. it prints the bin numbers where the value should go and then the value of that element.*/
   }

   for(i = 0; i < bins; i++) {
    printf("\n");
      for(j = 0 ;j < bins; j++) {
        if (bin[i][j] == 0)
        printf("  ");
        else
        printf("%d ", bin[i][j]);
         a = bin[i][j] / total;

       }
    }
}

我在底部添加的 for 循环只是输出模式。