分配内存的二维数组中的函数C [英] Allocate memory 2d array in function C

问题描述

如何为二维数组中的功能分配动态内存？ 我想是这样的：

How to allocate dynamic memory for 2d array in function ? I tried this way:

int main()
{
  int m=4,n=3;
  int** arr;
  allocate_mem(&arr,n,m);
}


void allocate_mem(int*** arr,int n, int m)
{
  *arr=(int**)malloc(n*sizeof(int*));
  for(int i=0;i<n;i++)
    *arr[i]=(int*)malloc(m*sizeof(int));
} 

不过，这是行不通的。 有人可以摆正自己的错误？

But it doesn't work. Someone can to put right my mistakes ?

推荐答案

您code是错误的，在 *常用3 [I] =（INT *）malloc的（M *的sizeof（INT）） ; ，因为 precedence 的 [] 运营商高于 * 尊重运营商因此在EX pression *改编[I] ，第改编[I] 计算则 * 应用。凡为你需要的是相反的。

Your code is wrong at *arr[i]=(int*)malloc(m*sizeof(int)); because precedence of [] operator higher then * deference operator so in expression *arr[i], first arr[i] evaluates then * applied. Where as your need is reverse.

不要像（* ARR）[我] 覆盖运营商precedence。

Do like (*arr)[i] to override operator precedence.

void allocate_mem(int*** arr, int n, int m)
{
  *arr = (int**)malloc(n*sizeof(int*));
  for(int i=0; i<n; i++)
    (*arr)[i] = (int*)malloc(m*sizeof(int));
} 

要理解上面的内存分配code阅读：<一href="http://stackoverflow.com/questions/19240540/dynamically-allocating-array-explain/19240932#19240932">Dynamically分配数组解释。

To understand above memory allocation code read: Dynamically allocating array explain.

重要应始终释放动态分配的内存明确一个你工作，完成它。
为了释放由上述函数分配的内存：

Important you should always deallocate dynamically allocated memory explicitly one you work done with it.
To free memory allocated by above function:

void deallocate_mem(int*** arr, int n){
    for (int i = 0; i < n; i++)
        free((*arr)[i]);
    free(*arr); 
}

此外，第二个办法：<一href="http://stackoverflow.com/questions/17477234/difference-b-w-allocating-memory-to-2d-array-in-1-go-or-row-by-row">Allocate连续的内存
一个更好的办法可以是在分配内存继续内存单的malloc（）函数调用如下：

Additionally, second way: Allocate contiguous memory
A better way can be to allocate memory in continue memory with single malloc() function call as below:

int* allocate_mem(int*** arr, int n, int m)
{
  *arr = (int**)malloc(n * sizeof(int*));
  int *arr_data = malloc( n * m * sizeof(int));
  for(int i=0; i<n; i++)
     (*arr)[i] = arr_data + i * m ;
  return arr_data; //free point
} 

要释放内存：

void deallocate_mem(int*** arr, int* arr_data){
    free(arr_data);
    free(*arr);
}

所以，你注意到第二个方法的malloc被称为只有两次，所以在释放code免费只调用从两次代替调用一个循环 - 所以这个技术应该更好地

So do you notice in second technique malloc is called only two times and so in deallocation code free is called only from two times in stead calling in a loop - So this technique should be better.

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