尝试在循环中访问数组元素会导致分段错误，为什么? [英] Trying to access array element in loop causes segmentation fault, why?

问题描述

我正在尝试创建一个二维数组，该数组将1或0随机分配给每个坐标.直到到达坐标[20] [3]为止，它都可以正常工作.之后，它仅抛出分段错误11".

我绝对不知道如何或为什么.尤其是因为我可以创建一个200 * 200的矩阵，但它仍然仅在坐标[200] [3]时出现相同的问题.因此，总是会在最后一个x坐标中的第三个y坐标发生错误.

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

int main() {

  int x, y, i, j ;
  x = 20;
  y = 20;

  int grid [x][y];

  for ( i = 0; i <= x; i++) {
    for ( j = 0; j <= y; j++) {

      grid[i][j] = rand() % 2 ;

      printf("grid [%d][%d]: %d\n", i, j, grid[i][j]);

    }
  }
  return 0;
}

解决方案

C对数组使用基于0的索引.因此，对于定义为

的数组

int grid [x][y]

循环

 for ( i = 0; i <= x; i++) 
   for ( j = 0; j <= y; j++)

如果一对一. (请注意<=部分).

详细说明，对于维数为p的数组，有效索引为0至p-1(含).

您应该将循环条件更改为i < x和j < y以保持界限.访问超出范围的内存会导致未定义的行为.

说，

• int main()至少应为int main(void)，以符合托管环境的C标准.
• 此处无需将grid设置为VLA.如果尺寸已知，更好的方法是使用编译时常量(#define)生成数组尺寸.

I am trying to create a two dimensional array, which has either 1 or 0 randomly assigned to each coordinate. It works just fine until it gets to the coordinates [20][3]. After that it just throws out "segmentation fault 11".

I am absolutely clueless how or why. Especially since I can create a matrix with 200 * 200 for instance but it still gets the same Problem only at the coordinates [200][3]. So it is somehow always the third y coordinate in the last x coordinate where the error occurs.

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

int main() {

  int x, y, i, j ;
  x = 20;
  y = 20;

  int grid [x][y];

  for ( i = 0; i <= x; i++) {
    for ( j = 0; j <= y; j++) {

      grid[i][j] = rand() % 2 ;

      printf("grid [%d][%d]: %d\n", i, j, grid[i][j]);

    }
  }
  return 0;
}

解决方案

C uses 0-based indexing for arrays. So, for an array defined as

int grid [x][y]

looping for

 for ( i = 0; i <= x; i++) 
   for ( j = 0; j <= y; j++)

if off-by-one. (Note the <= part).

to elaborate, for an array of dimension p, the valid indexes are 0 to p-1, inclusive.

You should change your loop conditions as i < x and j < y to stay withing the bounds. Accessing out of bound memory causes undefined behavior.

That said,

• int main() should be int main(void), at least, to conform to C standards for hosted environments.
• There is no need to make grid as VLA here. If the dimensions are already known, better approach is to use a compile-time constant (#define) to generate the array dimensions.

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