for循环中的赋值问题

Question

我的代码中有一个愚蠢的错误。我将不胜感激任何帮助。我知道这是特定于语言 (C) 的（因为我对 Python 和 JS 有一些经验）。我试图将源数组分成两部分。当我尝试将源数组的当前索引的值分配给另一个数组时，它工作正常但只能到 for 循环结束。循环完成后，这些数组的值完全不同。请在下面查看我的代码：

#include <stdio.h>
//#include <cs50.h>

int merge_sort(int unsorted_array[], int size_of_arr);

int main(void)
{
    int unar[] = {9, 8, 7, 6, 5, 4, 3, 2, 1};     // unsorted array
    int len_of_arr = sizeof(unar) / sizeof(int);  // finding lenght of array
    merge_sort(unar, len_of_arr);                 
}

int merge_sort(int unsorted_array[], int size_of_arr)
{
    // dividing
    int ls_len; // lenght of left side of an array
    int rs_len; // lenght of right side of an array

    if (size_of_arr % 2 == 0) // if lenght of an array is even then lenght of each half equals: lenght of arr / 2
    {
        ls_len = size_of_arr / 2;
        rs_len = size_of_arr / 2;
    }
    else                      // else lenght of arr / 2 and right half++
    {
        ls_len = size_of_arr / 2;
        rs_len = size_of_arr / 2 + 1;
    }

    int unsorted_ls[ls_len]; // unsorted_array_length // 2
    int unsorted_rs[rs_len]; // if array_len is odd ? array_len / 2 + 1 : array_len / 2
    printf("left half len: %i\nRight half len: %i\n", ls_len, rs_len);

    for (int i = 0; i < size_of_arr; i++)
    {
        if (i < ls_len)
        {
            unsorted_ls[i] = unsorted_array[i];
            printf("Current value of original array: %i\n", unsorted_array[i]);
            printf("Currently assigned value of left half: %i\n\n", unsorted_ls[i]);       // current
        } else
        {
            unsorted_rs[i] = unsorted_array[i];
            printf("Current value of original array: %i\n", unsorted_rs[i]);
            printf("Currently assigned value of right half: %i\n\n", unsorted_array[i]);
        }
    }

    printf("Left half outside of for loop: ");
    for (int i = 0; i < ls_len; i++)
    {
        printf("%i", unsorted_ls[i]);
    }
    printf("\n");

    printf("Right half outside of for loop: ");
    for (int i = 0; i < rs_len; i++)
    {
        printf("%i", unsorted_rs[i]); 
    } 
    printf("\n"); 

    // sorting                                                       // not implemented yet
    // int sorted_left_side[] = merge_sort(unsorted_left_half); 
    // int sorted_right_side[] = merge_sort(unsorted_right_half); 
    
    // merging                                                       // not implemented yet
    // // for (int i = 0; )

    // return sorted_array;
    return 0;
}

Answer 1

这个赋值语句

unsorted_rs[i] = unsorted_array[i];

调用未定义的行为，因为当索引 i 的值大于或等于 ls_len.[=17 时，试图访问数组 unsorted_rs 之外的内存=]

你应该写

unsorted_rs[i - ls_len] = unsorted_array[i];

注意那个而不是这个 if-else 语句

if (size_of_arr % 2 == 0) // if lenght of an array is even then lenght of each half equals: lenght of arr / 2
{
    ls_len = size_of_arr / 2;
    rs_len = size_of_arr / 2;
}
else                      // else lenght of arr / 2 and right half++
{
    ls_len = size_of_arr / 2;
    rs_len = size_of_arr / 2 + 1;
}

你可以写

ls_len = size_of_arr / 2;
rs_len = size_of_arr - ls_len;

for循环中的赋值问题

Problem with value assignment inside a for loop

c

arrays

for-loop

memory-access

cs50