在C中调整2D数组的大小 [英] Resizing 2D Arrays in C

问题描述

当前，我正在尝试使用此代码段在C中调整2D数组的大小

currently I am trying to resize a 2D Array in C using this code snippet

array = (int**) realloc(array, s * 2 * sizeof(int));

其中s是以行和列为单位的数组大小.但是，当尝试像这样访问数组的新区域时，

Where s is the size of the array in rows and colums. However, when trying to access the new areas of the array like this,

array[3][0] = x;

我只会遇到段错误.阵列的旧区域可以正常工作.我该如何解决这个问题?

I only get a segfault. The old areas of the array work fine. How can I solve this issue?

推荐答案

假设您将array声明为

int **array;

并分配为

array = malloc( sizeof *array * ROWS );
if ( array )
{
  for ( size_t i = 0; i < ROWS; i++ )
    array[i] = malloc( sizeof *array[i] * COLS );
}

您最终想到的结构如下:

The structure you wind up with looks something like:

       +---+        +---+                  +---+
array: |   | -----> |   | array[0] ------> |   | array[0][0]
       +---+        +---+                  +---+
        ...         |   | array[1] ---+    |   | array[0][1]
                    +---+             |    +---+
                     ...              |    |   | array[0][2]
                                      |    +---+
                                      |     ...
                                      |    
                                      |    +---+
                                      +--> |   | array[1][0]
                                           +---+
                                           |   | array[1][1]
                                           +---+
                                           |   | array[1][2]
                                           +---+
                                            ...

如果要增加数组中行的数量，但将列大小保持不变，则可以执行类似的操作

If you want to increase the number of rows in the array but leave the column sizes the same, you'd do something like

int **tmp = realloc( array, sizeof *array * (ROWS + add_rows) );
if ( tmp )
{
  array = tmp;
  for ( size_t i = 0; i < add_rows; i++ )
  {
     array[ROWS + i] = malloc( sizeof *array[ROWS + i] * COLS );
  }
}

如果您希望保持相同的行数但增加每行中的列数量，则可以执行以下操作

If you want to leave the number of rows the same but increase the number of columns in each row, you would do something like

for ( size_t i = 0; i < ROWS; i++ )
{
  int *tmp = realloc( array[i], sizeof *array[i] * (COLS + add_cols) );
  if ( tmp )
  {
    array[i] = tmp;
  }
}

如果要减少数组中的行数，则需要先释放受影响的行:

If you want to reduce the number of rows in the array, you will need to free the affected rows first:

for ( size_t i = 1; i <= del_rows; i++ )
  free( array[ROWS - i] );

int *tmp = realloc( array, ROWS - del_rows );
if ( tmp )
  array = tmp;

如果要减少列数:

for ( size_t i = 0; i < ROWS: i++ )
{
  int *tmp = realloc( array[i], sizeof *array[i] * (COLS - del_cols) );
  if ( tmp )
    array[i] = tmp;
}

从那里，您应该能够找出所需的任何组合.我强烈建议一次只做一维(也就是说，如果您想增加和列的行数，请先行 ，然后执行各列).

From there, you should be able to figure out any combinations you need. I strongly recommend doing only one dimension at a time (that is, if you want to increase the number of rows and columns, do the rows first, then do the columns).

您总是希望将realloc的结果分配给一个临时变量；如果realloc无法满足请求，它将返回NULL，并且如果将其分配回原始变量，则将丢失对先前分配的内存的唯一引用，从而导致内存泄漏.

You always want to assign the result of realloc to a temporary variable; if realloc cannot satisfy the request, it will return NULL, and if you assign it back to the original variable, you will lose your only reference to the memory that was previously allocated, leading to a memory leak.

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