如何在 C 中返回具有動態多維 arrays 的結構？

Question

我有以下問題：“如果我嘗試返回一個帶有動態多維數組的結構，我會丟失該數組內的數據。如果我使用指針返回該結構，我會遇到同樣的問題。在某些情況下，我有一些位置在具有預期數據的數組中”-> 如何返回包含動態多維數組的結構？ -> 我的錯誤在哪里？

這是一個顯示問題的示例程序：

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

struct just_a_structure
        {
         double **lf_multi_array;
         float   *f_array;
        };

struct just_a_structure get_struct(int i_length)
    {
     struct just_a_structure return_this_struct;
     return_this_struct.lf_multi_array  = (double**)malloc(i_length*sizeof(double*));
     return_this_struct.f_array         = (float*)malloc(i_length*sizeof(float*));

     double lf_dummy_0[10] = {0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9};
     double lf_dummy_1[10] = {1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9};

     return_this_struct.lf_multi_array[0] = lf_dummy_0;
     return_this_struct.f_array[0]       = 0.00;

     return_this_struct.lf_multi_array[1] = lf_dummy_1;
     return_this_struct.f_array[1]       = 0.01;    
    
     printf("\n output in function \n");
     printf("ma[0][0] %f \n", return_this_struct.lf_multi_array[0][0]);
     printf("ma[0][1] %f \n", return_this_struct.lf_multi_array[0][1]);
     printf("ma[0][2] %f \n", return_this_struct.lf_multi_array[0][2]);
     printf("ma[0][3] %f \n", return_this_struct.lf_multi_array[0][3]);
     printf("ma[0][4] %f \n", return_this_struct.lf_multi_array[0][4]);
     printf("ma[0][5] %f \n", return_this_struct.lf_multi_array[0][5]);
     printf("ma[0][6] %f \n", return_this_struct.lf_multi_array[0][6]);
     printf("ma[0][7] %f \n", return_this_struct.lf_multi_array[0][7]);
     printf("ma[0][8] %f \n", return_this_struct.lf_multi_array[0][8]);
     printf("ma[0][9] %f \n", return_this_struct.lf_multi_array[0][9]);
     printf("a[0] %f \n", return_this_struct.f_array[0]);
     
     printf("\n \n");
     printf("ma[1][0] %f \n", return_this_struct.lf_multi_array[1][0]);
     printf("ma[1][1] %f \n", return_this_struct.lf_multi_array[1][1]);
     printf("ma[1][2] %f \n", return_this_struct.lf_multi_array[1][2]);
     printf("ma[1][3] %f \n", return_this_struct.lf_multi_array[1][3]);
     printf("ma[1][4] %f \n", return_this_struct.lf_multi_array[1][4]);
     printf("ma[1][5] %f \n", return_this_struct.lf_multi_array[1][5]);
     printf("ma[1][6] %f \n", return_this_struct.lf_multi_array[1][6]); 
     printf("ma[1][7] %f \n", return_this_struct.lf_multi_array[1][7]); 
     printf("ma[1][8] %f \n", return_this_struct.lf_multi_array[1][8]); 
     printf("ma[1][9] %f \n", return_this_struct.lf_multi_array[1][9]);
     printf("a[1] %f \n", return_this_struct.f_array[1]);
    
     return return_this_struct;
    }
    
    
int main()
    {    
     int i_length = 500;
     
     struct just_a_structure returned_struct;
    
     returned_struct = get_struct(i_length);
    
     printf("\n output in main \n");
     printf("ma[0][0] %f \n", returned_struct.lf_multi_array[0][0]);
     printf("ma[0][1] %f \n", returned_struct.lf_multi_array[0][1]);
     printf("ma[0][2] %f \n", returned_struct.lf_multi_array[0][2]);
     printf("ma[0][3] %f \n", returned_struct.lf_multi_array[0][3]);
     printf("ma[0][4] %f \n", returned_struct.lf_multi_array[0][4]);
     printf("ma[0][5] %f \n", returned_struct.lf_multi_array[0][5]);
     printf("ma[0][6] %f \n", returned_struct.lf_multi_array[0][6]);
     printf("ma[0][7] %f \n", returned_struct.lf_multi_array[0][7]);
     printf("ma[0][8] %f \n", returned_struct.lf_multi_array[0][8]);
     printf("ma[0][9] %f \n", returned_struct.lf_multi_array[0][9]);
     printf("a[0] %f \n", returned_struct.f_array[0]);
     
     printf("\n \n");
     printf("ma[1][0] %f \n", returned_struct.lf_multi_array[1][0]);
     printf("ma[1][1] %f \n", returned_struct.lf_multi_array[1][1]);
     printf("ma[1][2] %f \n", returned_struct.lf_multi_array[1][2]);
     printf("ma[1][3] %f \n", returned_struct.lf_multi_array[1][3]);
     printf("ma[1][4] %f \n", returned_struct.lf_multi_array[1][4]);
     printf("ma[1][5] %f \n", returned_struct.lf_multi_array[1][5]);
     printf("ma[1][6] %f \n", returned_struct.lf_multi_array[1][6]); 
     printf("ma[1][7] %f \n", returned_struct.lf_multi_array[1][7]); 
     printf("ma[1][8] %f \n", returned_struct.lf_multi_array[1][8]); 
     printf("ma[1][9] %f \n", returned_struct.lf_multi_array[1][9]);
     printf("a[1] %f \n", returned_struct.f_array[1]);

     return 0;
    }

這是程序的output：

 output in function
ma[0][0] 0.000000
ma[0][1] 0.100000
ma[0][2] 0.200000
ma[0][3] 0.300000
ma[0][4] 0.400000
ma[0][5] 0.500000
ma[0][6] 0.600000
ma[0][7] 0.700000
ma[0][8] 0.800000
ma[0][9] 0.900000
a[0] 0.000000


ma[1][0] 1.000000
ma[1][1] 1.100000
ma[1][2] 1.200000
ma[1][3] 1.300000
ma[1][4] 1.400000
ma[1][5] 1.500000
ma[1][6] 1.600000
ma[1][7] 1.700000
ma[1][8] 1.800000
ma[1][9] 1.900000
a[1] 0.010000

 output in main
ma[0][0] 27471811305989270000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[0][1] 27471811305989273000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[0][2] 24361411570075108000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[0][3] 22932960320429427000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[0][4] 0.000000
ma[0][5] 0.000000
ma[0][6] 23754367043167420000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[0][7] 27471811305989273000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[0][8] -519591837366427200000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000 
ma[0][9] 0.000000
a[0] 0.000000


ma[1][0] 21266032917854245000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[1][1] 21427427439475540000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[1][2] 0.000000
ma[1][3] 0.000000
ma[1][4] 0.000000
ma[1][5] 0.000000
ma[1][6] 22334679298120726000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[1][7] -1.#QNAN0
ma[1][8] 23669996832223273000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.000000
ma[1][9] 0.000000
a[1] 0.010000

我試圖實現@Etini 和@Sourav Ghost 提供的解決方案。 但是要知道程序在啟動后會立即崩潰。

     return_this_struct.lf_multi_array[0] = (double*)malloc(10*sizeof(double));
     return_this_struct.lf_multi_array[1] = (double*)malloc(10*sizeof(double));
     return_this_struct.f_array           = (float*)malloc(i_length*sizeof(float*));

     double lf_dummy_0[10] = {0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9};
     double lf_dummy_1[10] = {1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9};
     
     for(int i = 0; i < 10; i++)
        {
         return_this_struct.lf_multi_array[0][i] = lf_dummy_0[i];
        }
     for(int i = 0; i < 10; i++)
        {
         return_this_struct.lf_multi_array[1][i] = lf_dummy_1[i];
        }
        
     return_this_struct.f_array[0]       = 0.00;
     return_this_struct.f_array[1]       = 0.01;    
    ```

Answer 1

在您的代碼中

return_this_struct.lf_multi_array[0] = lf_dummy_0;

和

return_this_struct.lf_multi_array[1] = lf_dummy_1;

both are storing the address of a local variable in the pointer, hence once returned from the called function, these local variables go out of scope and you're accessing invalid memory, creating undefined behaviour.

如果要返回值，請為數組分配 memory，並復制單個元素值。

Answer 2

正如 Sourav Ghost 所說，您需要分配和復制單個元素，這就是我使它適用於第一個數組的方式

 return_this_struct.lf_multi_array[0] = (double*) malloc(10*sizeof(double));
     for(int i = 0; i < 10; i++){
         return_this_struct.lf_multi_array[0][i] = lf_dummy_0[i];
     }

Answer 3

我的 C 生銹了，請耐心等待。

這兩行正在創建一個堆棧 memory，退出 function 時將刪除該堆棧：

double lf_dummy_0[10] = {0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9};
double lf_dummy_1[10] = {1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9};

這兩行並沒有將存儲在lf_dummy_0和lf_dummy_0中的值復制到您的 multi_array 中。

return_this_struct.lf_multi_array[0] = lf_dummy_0;
...
return_this_struct.lf_multi_array[1] = lf_dummy_1;

現在，我們將您的問題分解成幾部分怎么樣？

首先，讓我們編寫一個返回單個值數組的方法，並確保在 function 結束后可以訪問這些值。

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

double *getPointerToArrayWithValues(int capacity){
    double * array = (double *) malloc(sizeof(double) * capacity);    

    double value = 0.0;
    for (int i = 0; i < capacity; ++i){
        array[i] = value;
        value += 0.1;
    }
    
    return array;
}

int main()
{
    int size = 10;
    double * array = getPointerToArrayWithValues(size);
    for (int i = 0; i < size; ++i){
        printf("arra[%d]: %f\n", i, array[i]);
    }
    free(array);
    return 0;
}

您可能會注意到，在我獲取值並顯示它們之后，我正在釋放我們使用的 memory。 我們必須自己清潔。

還注意到我使用的是雙精度而不是浮點數。 原因是它比混合浮點和雙精度更容易使用。

其次，讓我們看看我們是否可以創建一個數組或 arrays aka。 多陣列。

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

double **getPointerToMultiArrayWithValues(int rowCapacity, int columnCapacity){
    // Add space for rows
    double ** multiArray = (double **) malloc(sizeof(double * ) * rowCapacity);
     
    // Add space for columns
    for(int i = 0; i < rowCapacity; ++i){
        multiArray[i] = (double *) malloc(sizeof(double *) * columnCapacity);   
    }
    
    double value = 0.0;
    for (int row = 0; row < rowCapacity; ++row){
        for (int column = 0; column < columnCapacity; ++column){
            multiArray[row][column] = value;
            value += 0.1;
        }
    }
    
    return multiArray;
}

int main()
{
    int numberOfArrayToHold = 10;
    int sizeOfEachArray = 10;
    double ** multiArray = getPointerToMultiArrayWithValues(numberOfArrayToHold, sizeOfEachArray);
    
    for(int i = 0; i < numberOfArrayToHold; ++i){
        for (int j = 0; j < sizeOfEachArray; ++j){
            printf("multiArray[%d][%d]: %f\n", i, j, multiArray[i][j]);
        }
    }

    // Lets be grown up and clean after ourselves
    for (int i = 0; i < numberOfArrayToHold; ++i){
        free(multiArray[i]);
    }
    free(multiArray);

    return 0;
}

現在，您已經解決了如何創建單數組和多數組（在本例中為雙數組），我們可以專注於創建一個包含此類數組的結構：

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

struct ArrayHolder{
  int rows;
  int columns;
  double ** multiArray;
};

double **getPointerToMultiArrayWithValues(int rowCapacity, int columnCapacity){
    // Add space for rows
    double ** multiArray = (double **) malloc(sizeof(double * ) * rowCapacity);
     
    // Add space for columns
    for(int i = 0; i < rowCapacity; ++i){
        multiArray[i] = (double *) malloc(sizeof(double *) * columnCapacity);   
    }
    
    double value = 0.0;
    for (int row = 0; row < rowCapacity; ++row){
        for (int column = 0; column < columnCapacity; ++column){
            multiArray[row][column] = value;
            value += 0.1;
        }
    }
    
    return multiArray;
}

int main()
{
    struct ArrayHolder arrayHolder;
    arrayHolder.rows = 10;
    arrayHolder.columns = 10;
    arrayHolder.multiArray = getPointerToMultiArrayWithValues(arrayHolder.rows, arrayHolder.columns);
    
    for(int i = 0; i < arrayHolder.rows; ++i){
        for (int j = 0; j < arrayHolder.columns; ++j){
            printf("multiArray[%d][%d]: %f\n", i, j,  arrayHolder.multiArray[i][j]);
        }
    }
    
    for (int i = 0; i < arrayHolder.rows; ++i){
        free(arrayHolder.multiArray[i]);
    }
    free( arrayHolder.multiArray);

    return 0;
}

但是，我們希望返回一個結構，不是嗎？ 然后，

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

struct ArrayHolder{
  int rows;
  int columns;
  double ** multiArray;
};

double **getPointerToMultiArrayWithValues(int rowCapacity, int columnCapacity){
    // Add space for rows
    double ** multiArray = (double **) malloc(sizeof(double * ) * rowCapacity);
     
    // Add space for columns
    for(int i = 0; i < rowCapacity; ++i){
        multiArray[i] = (double *) malloc(sizeof(double *) * columnCapacity);   
    }
    
    double value = 0.0;
    for (int row = 0; row < rowCapacity; ++row){
        for (int column = 0; column < columnCapacity; ++column){
            multiArray[row][column] = value;
            value += 0.1;
        }
    }
    
    return multiArray;
}

struct ArrayHolder getStructWithArray(){
    struct ArrayHolder arrayHolder;
    arrayHolder.rows = 10;
    arrayHolder.columns = 10;
    arrayHolder.multiArray = getPointerToMultiArrayWithValues(arrayHolder.rows, arrayHolder.columns);
    return arrayHolder;
}

int main()
{
   
    struct ArrayHolder arrayHolder = getStructWithArray(); 
    for(int i = 0; i < arrayHolder.rows; ++i){
        for (int j = 0; j < arrayHolder.columns; ++j){
            printf("multiArray[%d][%d]: %f\n", i, j,  arrayHolder.multiArray[i][j]);
        }
    }
    
    for (int i = 0; i < arrayHolder.rows; ++i){
        free(arrayHolder.multiArray[i]);
    }
    free( arrayHolder.multiArray);

    return 0;
}

在這種情況下，我們按值返回結構（已復制）

但是，我們也可以通過指針來完成！

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

struct ArrayHolder{
  int rows;
  int columns;
  double ** multiArray;
};

double **getPointerToMultiArrayWithValues(int rowCapacity, int columnCapacity){
    // Add space for rows
    double ** multiArray = (double **) malloc(sizeof(double * ) * rowCapacity);
     
    // Add space for columns
    for(int i = 0; i < rowCapacity; ++i){
        multiArray[i] = (double *) malloc(sizeof(double *) * columnCapacity);   
    }
    
    double value = 0.0;
    for (int row = 0; row < rowCapacity; ++row){
        for (int column = 0; column < columnCapacity; ++column){
            multiArray[row][column] = value;
            value += 0.1;
        }
    }
    
    return multiArray;
}

struct ArrayHolder * getPointerToStructWithArray(){
    struct ArrayHolder* arrayHolder = (struct ArrayHolder *) malloc(sizeof(struct ArrayHolder));
    arrayHolder -> rows = 10;
    arrayHolder -> columns = 10;
    arrayHolder -> multiArray = getPointerToMultiArrayWithValues(arrayHolder -> rows, arrayHolder -> columns);
    return arrayHolder;
}

int main()
{
   
    struct ArrayHolder * arrayHolder = getPointerToStructWithArray(); 
    for(int i = 0; i < arrayHolder -> rows; ++i){
        for (int j = 0; j < arrayHolder -> columns; ++j){
            printf("multiArray[%d][%d]: %f\n", i, j,  arrayHolder -> multiArray[i][j]);
        }
    }
    
    for (int i = 0; i < arrayHolder -> rows; ++i){
        free(arrayHolder -> multiArray[i]);
    }
    free( arrayHolder -> multiArray);
    
    free(arrayHolder);

    return 0;
}

讓我知道這是否對您有所幫助。

Answer 4

在使用循環分配值之前，您必須像這樣分配數組return_this_struct.lf_multi_array[0] = (double*) malloc(10*sizeof(double));