[英]I'm not sure why my code in C is giving me a segmentation fault at free, any ideas?
所以這是我的代碼,一直運行到免費(右); 更像是它完成合並排序然后有錯誤,任何解決方案?
#include <stdio.h>
#include <stdlib.h>
void bubble_sort(int *l, int len) {
// Iterate through the list
for (int i = 0; i < len; i++) {
// Iterate through the list
for (int j = 0; j < len - 1; j++) {
// If the current element is greater than the next element, swap them
if (l[j] > l[j + 1]) {
// Swap the elements
int temp = l[j];
l[j] = l[j + 1];
l[j + 1] = temp;
// Print the list
for (int k = 0; k < len; k++) {
printf("%d ", l[k]);
}
printf("\n");
}
}
}
}
void selection_sort(int *l, int len) {
// Iterate through the list
for (int i = 0; i < len; i++) {
// Set the minimum index to the current index
int min_index = i;
// Iterate through the list
for (int j = i + 1; j < len; j++) {
// If the current element is less than the minimum element, set the minimum index to the current index
if (l[j] < l[min_index]) {
min_index = j;
}
}
// Swap the elements
int temp = l[i];
l[i] = l[min_index];
l[min_index] = temp;
// Print the list
for (int k = 0; k < len; k++) {
printf("%d ", l[k]);
}
printf("\n");
}
}
void insertion_sort(int *l, int len) {
// Iterate through the list
for (int i = 1; i < len; i++) {
// Set the current index to the current index
int j = i;
// While the current index is greater than 0 and the previous element is greater than the current element, swap them
while (j > 0 && l[j - 1] > l[j]) {
// Swap the elements
int temp = l[j - 1];
l[j - 1] = l[j];
l[j] = temp;
// Decrement the current index
j--;
}
// Print the list
for (int k = 0; k < len; k++) {
printf("%d ", l[k]);
}
printf("\n");
}
}
void merge(int *left, int left_len, int *right, int right_len) {
// Create a new list
int *result = malloc((left_len + right_len) * sizeof(int));
// Set the left index to 0 and the right index to 0
int i = 0;
int j = 0;
// While the left index is less than the length of the left list and the right index is less than the length of the right list
while (i < left_len && j < right_len) {
// If the left element is less than or equal to the right element, append the left element to the result list and increment the left index
if (left[i] <= right[j]) {
result[i + j] = left[i];
i++;
}
// Else, append the right element to the result list and increment the right index
else {
result[i + j] = right[j];
j++;
}
}
// Append the remaining elements in the left list to the result list
for (int k = i; k < left_len; k++) {
result[k + j] = left[k];
}
// Append the remaining elements in the right list to the result list
for (int k = j; k < right_len; k++) {
result[k + i] = right[k];
}
// Print the result list
for (int k = 0; k < left_len + right_len; k++) {
printf("%d ", result[k]);
}
printf("\n");
// Copy the result list to the original list
for (int k = 0; k < left_len + right_len; k++) {
left[k] = result[k];
}
// Free the result list
free(result);
}
void merge_sort(int *l, int len) {
// If the list is empty or has one element, return the list
if (len <= 1) {
return;
}
// Set the middle index to the length of the list divided by 2
int mid = len / 2;
// Set the left list to the first half of the list
int *left = malloc(mid * sizeof(int));
for (int i = 0; i < mid; i++) {
left[i] = l[i];
}
// Set the right list to the second half of the list
int *right = malloc((len - mid) * sizeof(int));
for (int i = mid; i < len; i++) {
right[i - mid] = l[i];
}
// Sort the left list
merge_sort(left, mid);
// Sort the right list
merge_sort(right, len - mid);
// Merge the left list and the right list
merge(left, mid, right, len - mid);
// Free the left list and the right list
free(left);
free(right); //Error ln 142, in picture below
}
int binary_search(int *l, int len, int target) {
// Set the low index to 0 and the high index to the length of the list minus 1
int low = 0;
int high = len - 1;
// While the low index is less than or equal to the high index
while (low <= high) {
// Set the middle index to the sum of the low index and the high index divided by 2
int mid = (low + high) / 2;
// If the middle element is equal to the target, return the middle index
if (l[mid] == target) {
return mid;
}
// Else if the middle element is less than the target, set the low index to the middle index plus 1
else if (l[mid] < target) {
low = mid + 1;
}
// Else, set the high index to the middle index minus 1
else {
high = mid - 1;
}
}
// If the target is not found, return -1
return -1;
}
int main() {
// Create a list
int l[] = {17, 36, 3, 10, 29, 42, 34, 8};
int len = sizeof(l) / sizeof(l[0]);
// Print the list
printf("Bubble Sort:\n");
// Sort the list using bubble sort
bubble_sort(l, len);
// Print the list
printf("Selection Sort:\n");
// Sort the list using selection sort
selection_sort(l, len);
// Print the list
printf("Insertion Sort:\n");
// Sort the list using insertion sort
insertion_sort(l, len);
// Print the list
printf("Merge Sort:\n");
// Sort the list using merge sort
merge_sort(l, len);
// Print the list
printf("Binary Search:\n");
// Search for the target in the list using binary search
printf("%d\n", binary_search(l, len, 42));
return 0;
}
於是我把python的代碼改寫成C,在GDB調試給我截圖報錯。
我試圖編輯 function 本身來糾正 memory 問題,但它不起作用,所以我又回到了這里,希望有人有更多的見解。
段錯誤在merge_sort()
merge()
中觸發。 其他一切都無關緊要。
在merge_sort()
中,您將輸入數組l
的一半復制到新分配的數組left
中,另一半復制到另一個新分配的數組right
中。 然后遞歸merge_sort()
這兩半就可以了。 合並兩半merge()
在您錯誤地假設左和右 arrays 是連續分配的地方被調用:
for (int k = 0; k < left_len + right_len; k++) {
left[k] = result[k];
}
最小的修復是使假設有效:
void merge_sort(int *l, int len) {
if (len <= 1) {
return;
}
int mid = len / 2;
int *left = malloc(len * sizeof(int));
for (int i = 0; i < mid; i++) {
left[i] = l[i];
}
int *right = left + mid;
for (int i = mid; i < len; i++) {
right[i - mid] = l[i];
}
merge_sort(left, mid);
merge_sort(right, len - mid);
merge(left, mid, right, len - mid);
free(left);
}
更好的解決方案是:
main()
,而不是在您的排序算法中執行該任務。 這允許merge_sort()
對輸入數組進行操作( merge()
仍然使用臨時數組)。merge()
的right
數組指針參數。 該文檔表明左右 arrays 是同一數組的一部分。merge()
和merge_sort()
接口,使長度參數位於數組參數之前,這樣您就可以記錄它們之間的關系。merge_sort()
中分配一次合並所需的臨時空間,並將其傳遞給merge_sort2()
和merge2()
。 這樣你只有O(n)
空間開銷而不是O(n*log(n))
。 值得指出的是, malloc()
可能需要 kernel 上下文切換,這反過來又是merge()
+ merge_sort()
實現中最昂貴的操作。 對malloc()
執行 1 而不是 n*log(n) 調用可能是運行時中的一個重要(常數)因素。 然而,共享臨時空間是有代價的,因為您將無法再並行執行其他非重疊合並排序。size_t
而不是int
。 sizeof() 特別返回一個size_t
值,並且對於大於INTMAX
的大小,轉換為(帶符號的) int
會出現問題。memcpy()
而不是顯式循環。 memcpy()
經過高度優化,並簡潔地表達了意圖。sizeof()
。 如果您更改變量的類型,前者是健壯的,而如果您沒有對類型使用typedef
,則后者需要更改代碼。print()
function 這樣您就不需要排序函數本身中的調試打印語句。#include <string.h>
#include <stdio.h>
#include <stdlib.h>
void merge(size_t left_len, size_t right_len, int l[left_len + right_len]) {
int *result = malloc((left_len + right_len) * sizeof(*l));
int i = 0;
int j = 0;
while (i < left_len && j < right_len) {
if (l[i] <= l[left_len + j]) {
result[i + j] = l[i];
i++;
} else {
result[i + j] = l[left_len + j];
j++;
}
}
memcpy(result + i + j, l + i, (left_len - i) * sizeof(*l));
memcpy(result + left_len + j, l + left_len + j, (right_len - j) * sizeof(*l));
memcpy(l, result, (left_len + right_len) * sizeof(*l));
free(result);
}
void merge_sort(size_t len, int l[len]) {
if (len < 2) return;
int mid = len / 2;
merge_sort(mid, l);
merge_sort(len - mid, l + mid);
merge(mid, len - mid, l);
}
void print(size_t len, int a[len]) {
for(size_t i = 0; i < len; i++) {
printf("%d%s", a[i], i + 1 < len ? ", " : "\n");
}
}
int main() {
int l[] = {17, 36, 3, 10, 29, 42, 34, 8};
size_t len = sizeof(l) / sizeof(*l);
int l2[len];
memcpy(l2, l, sizeof(l));
merge_sort(len, l2);
print(len, l2);
}
它返回:
3, 8, 10, 17, 29, 34, 36, 42
valgrind 很高興:
ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)
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