定义使用malloc一个二维数组，并修改它 [英] defining a 2D array with malloc and modifying it

问题描述

如何定义使用malloc一个二维数组？ （可以说10X20）。

How do i define a 2D array using malloc ? (lets say 10X20).

第二，我可以增加行列的数量，而无需创建一个新增加的数组和复制所有数据呢？

second, can i increase number of rows or cols without creating a new increased array and copying all data to it?

比如说，我怎么分配内存以使阵列将是10X30或15x20？

for instance, how do i allocate memory so the array will be 10x30 or 15x20?

感谢！

推荐答案

10X30：

int(*array)[30] = malloc((sizeof *array) * 10);

15x20：

int(*array)[20] = malloc((sizeof *array) * 15);

调整到20x25：

int(*array2)[25] = realloc(array, (sizeof *array2) * 20);

的外部尺寸（10，15，20）可以在运行时确定的，因为它不需要如由编译指数计算的一部分。内部尺寸（30，20，25）的需要在编译时是已知的。希望它帮助。

The outer dimension (10, 15, 20) can be determined at runtime, because it's not need as part of index calculations by the compiler. The inner dimension (30, 20, 25) needs to be known at compile time. Hope it helps.

请注意，不像数组的指针的解决方案，这一项可以作为一个单独的内存块处理，因为它在记忆像一个真正的声明数组单块分配的一切：

Note that unlike the array-of-pointer solutions, this one can be handled as a single block of memory, because it allocates everything in a single chunk of memory like a real declared array:

memcpy(somewhere, array2, sizeof(int) * 20 * 25); // (sizeof *array2) * 20

这最终取决于你的用例，虽然。

It ultimately depends on your usecase, though.

由于一些人认识到通过索引操作所采取的行动困难阵列，让我们什么锵给了我们在以下$ C指数前pression $ C

Since some people have difficulties understanding the actions taken by an index operation to array, let's what Clang gives us for an index expression in the following code

int main() {
  int(*array)[10] = malloc((sizeof *array) * 5);
  array[4][9] = 0;

  int(*array1)[10][5] = malloc((sizeof *array1) * 20);
  array1[19][9][4] = 0;
}

这是一个不错的编译器，它可以打印其AST以易于阅读的方式

It's a nice compiler, which can print its AST in an easily readable manner

  // array[4][9] = 0;
  (BinaryOperator 0xba62cc0 <line:5:3, col:17> 'int' '='
    (ArraySubscriptExpr 0xba62c80 <col:3, col:13> 'int'
      (ImplicitCastExpr 0xba62c60 <col:3, col:10> 'int *' <ArrayToPointerDecay>
        (ArraySubscriptExpr 0xba62c20 <col:3, col:10> 'int [10]'
          (DeclRefExpr 0xba62bdc <col:3> 'int (*)[10]' Var='array' 0xba62a00)
          (IntegerLiteral 0xba62c00 <col:9> 'int' 4)))
      (IntegerLiteral 0xba62c40 <col:12> 'int' 9))
    (IntegerLiteral 0xba62ca0 <col:17> 'int' 0))

  // array1[19][9][4] = 0;
  (BinaryOperator 0xba630b8 <line:8:3, col:22> 'int' '='
    (ArraySubscriptExpr 0xba63078 <col:3, col:18> 'int'
      (ImplicitCastExpr 0xba63058 <col:3, col:15> 'int *' <ArrayToPointerDecay>
        (ArraySubscriptExpr 0xba63018 <col:3, col:15> 'int [5]'
          (ImplicitCastExpr 0xba62ff8 <col:3, col:12> 'int (*)[5]' <ArrayToPointerDecay>
            (ArraySubscriptExpr 0xba62fa0 <col:3, col:12> 'int [10][5]'
              (DeclRefExpr 0xba62f5c <col:3> 'int (*)[10][5]' Var='array1' 0xba62db0)
              (IntegerLiteral 0xba62f80 <col:10> 'int' 19)))
          (IntegerLiteral 0xba62fc0 <col:14> 'int' 9)))
      (IntegerLiteral 0xba63038 <col:17> 'int' 4))
    (IntegerLiteral 0xba63098 <col:22> 'int' 0)))

请注意，每个数组下标ex pression需要一个指针，加索引的值，并产生寻址元件。如果该子元件是一个数组，它被衰减到一个指针到它的第一个元素。这实在不是比一个声明数组

Note that each array subscript expression takes a pointer, adds the value of the index, and yields the addressed element. If that subelement is an array, it is decayed to a pointer to its first element. This is really not actually different than the steps done for a declared array

int main() {
  int array[5][10] = { };
  array[4][9] = 1;
}

产生非常相似的AST，只有最内侧前pression首先被衰减到一个指向它的第一个元素

Yields a very similar AST, with just the most inner expression first being decayed to a pointer to its first element

  // array[4][9] = 1;
  (BinaryOperator 0xbf9f7e8 <line:5:3, col:17> 'int' '='
    (ArraySubscriptExpr 0xbf9f7a8 <col:3, col:13> 'int'
      (ImplicitCastExpr 0xbf9f788 <col:3, col:10> 'int *' <ArrayToPointerDecay>
        (ArraySubscriptExpr 0xbf9f748 <col:3, col:10> 'int [10]'
          (ImplicitCastExpr 0xbf9f728 <col:3> 'int (*)[10]' <ArrayToPointerDecay>
            (DeclRefExpr 0xbf9f6cc <col:3> 'int [5][10]' Var='array' 0xbfa81f0))
          (IntegerLiteral 0xbf9f6f0 <col:9> 'int' 4)))
      (IntegerLiteral 0xbf9f768 <col:12> 'int' 9))
    (IntegerLiteral 0xbf9f7c8 <col:17> 'int' 1)))

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