在 C 中调整二维数组的大小 [英] Resizing 2D Arrays in C

问题描述

目前我正在尝试使用此代码片段在 C 中调整二维数组的大小

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;

我只得到一个段错误.阵列的旧区域工作正常.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 );
}

你最终得到的结构看起来像:

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]
                                           +---+
                                            ...

如果您想增加数组中行的数量但保持列大小不变，您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 );
  }
}

如果你想让行数保持不变，但增加每行的列数，你可以这样做

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;
  }
}

如果你想减少数组中的行数，你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，您应该能够找出您需要的任何组合.我强烈建议一次只做一个维度(也就是说，如果你想增加行数和列数，先做行>，然后做列).

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).

您总是想将 realloc 的结果分配给一个临时变量；如果realloc不能满足请求，它会返回NULL，如果你把它赋值回原来的变量，你将失去对之前分配的内存的唯一引用，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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