C语言作业问题：separate函数if语句未执行且代码无法编译

Hey there, let's break down the problems in your code and get it working properly. I see a few key issues that are causing compilation failures and unexpected behavior:

1. Wrong Operator (Immediate Compilation Error) #

You're using the equality comparator == instead of the assignment operator = when trying to copy values into your even/odd arrays. That's why your if blocks aren't doing anything useful, and it's triggering compiler warnings or errors:

// ❌ This is a comparison, not an assignment—does nothing useful
if ((arr[j]%2) == 0) arr1[j] == arr[j];
// ✅ Correct assignment to copy values into the new array
if ((arr[j]%2) == 0) arr1[idx1++] = arr[j];

The same mistake applies to the odd array filling code—fix those first to resolve the immediate compilation issues.

2. Pointer Passing Logic Flaw #

C uses pass-by-value, so when you assign arr1 = calloc(...) inside your separate function, you're only modifying the local copy of the pointer, not the newArr1/newArr2 variables in main. To update the pointers in the main function, you need to pass pointers to pointers (double pointers) instead.

3. Incorrect Array Indexing #

Using the original array's index j to fill your new even/odd arrays is wrong. For example, if the 0th and 2nd elements are even, your even array should store them at indices 0 and 1—not 0 and 2. You need separate counters to track the current position in each new array.

4. Minor Header & Memory Cleanup Issues #

• system("pause") requires <stdlib.h>; without it, you'll get a compiler warning.
• <malloc.h> isn't a standard C header—use <stdlib.h> instead (it includes calloc).
• Don't forget to free dynamically allocated memory to avoid memory leaks.

Fixed Full Code #

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

// Updated to use double pointers for arr1/arr2 to modify main's pointers
void separate(int* arr, int n, int* size1, int* size2, int** arr1, int** arr2);

int main() {
    int size1 = 0, size2 = 0;
    int* newArr1 = NULL;
    int* newArr2 = NULL;
    int arr[] = {6, 57, 14, 21, 11, 3, 22, 42, 9, 15};

    printf("The array before change:\n");
    for (int i = 0; i < 10; i++) {
        printf(" %d", arr[i]);
    }
    printf("\n");

    // Pass addresses of the pointers to modify them in the separate function
    separate(arr, 10, &size1, &size2, &newArr1, &newArr2);

    printf("The even array is:\n");
    for (int i = 0; i < size1; i++) {
        printf(" %d", newArr1[i]);
    }
    printf("\n");

    printf("The odd array is:\n");
    for (int i = 0; i < size2; i++) {
        printf(" %d", newArr2[i]);
    }
    printf("\n");

    // Clean up dynamically allocated memory to avoid leaks
    free(newArr1);
    free(newArr2);

    system("pause");
    return 0;
}

void separate(int* arr, int n, int* size1, int* size2, int** arr1, int** arr2) {
    int i, idx1 = 0, idx2 = 0;

    // Reset sizes before counting to avoid unexpected values
    *size1 = 0;
    *size2 = 0;
    for (i = 0; i < n; i++) {
        if (arr[i] % 2 == 0) {
            (*size1)++;
        } else {
            (*size2)++;
        }
    }
    printf("size1: %d size2: %d\n", *size1, *size2);

    // Allocate memory and assign directly to the pointers from main
    *arr1 = (int*)calloc(*size1, sizeof(int));
    *arr2 = (int*)calloc(*size2, sizeof(int));

    // Fill even array with correct indexing using a dedicated counter
    for (i = 0; i < n; i++) {
        if (arr[i] % 2 == 0) {
            (*arr1)[idx1++] = arr[i];
        }
    }

    // Fill odd array with correct indexing using a dedicated counter
    for (i = 0; i < n; i++) {
        if (arr[i] % 2 != 0) {
            (*arr2)[idx2++] = arr[i];
        }
    }
}

Key Fixes Explained #

• Double Pointers: By changing int* arr1 to int** arr1, we can modify the actual pointer variables in main instead of just local copies. The *arr1 = calloc(...) line updates the pointer in main directly.
• Assignment Operator & Index Counters: Replaced == with = to actually copy values into the new arrays, and added idx1/idx2 counters to keep track of where to place each element in the even/odd arrays.
• Memory Management: Added free() calls in main to clean up the dynamically allocated memory, preventing leaks.
• Header Fixes: Swapped <malloc.h> for standard <stdlib.h> and ensured all required headers are included.

内容的提问来源于stack exchange，提问作者David Bu