C中鏈接列表的快速排序

Question

好的，在使用gdb和valgrind失敗之后，我謙虛地向您提出了我的問題。 我們被要求使用列表中的第一個元素作為樞紐，以c語言實現quicksort的版本（以保持與本學期早些時候完成的簡單haskell實現的可比性）。 提供了LIST實現（制作，打印和結構定義），其余的工作由我們決定。 我（感到驚訝）感到異常，但是valgrind出現了大量錯誤以及堆棧溢出，因此也許有些新鮮的眼睛可以對我有所幫助。

我的代碼：

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

typedef struct CELL *LIST;
struct CELL {
    int element;
    LIST next;
};


LIST MakeList();
void PrintList(LIST);
LIST qSort(LIST);
int listLength(LIST);
LIST combine(LIST, LIST, LIST);


int main(){
   LIST list;
    printf ("enter a several numbers separated by spaces or returns and ended by Ctrl-D\n");
    list = MakeList();
    PrintList(qSort(list));
   return 0;
}

LIST qSort(LIST list){
   LIST current, pivot = list, temp = NULL;//use first element as pivot, start comparison at list->next
   LIST little, big, littleHead, bigHead;
   little = (LIST) malloc(sizeof(struct CELL));
   little->element = 0;
   little->next = NULL;
   littleHead = little;
   big = (LIST) malloc(sizeof(struct CELL));
   big->element = 0;
   big->next = NULL;
   bigHead = big;

   if(listLength(list) <= 1){//base case
      return list;
   }
//remove pivot by setting current to list->next
   current = list->next;

   do{
      if(current->element <= pivot->element){
         little->element = current->element;
         little->next = (LIST) malloc(sizeof(struct CELL));
         little = little->next;
         little->next = NULL;
      }
      else{
         big->element = current->element;
         big->next = (LIST) malloc(sizeof(struct CELL));
         big = big->next;
         big->next = NULL;
      }

      current = current->next;
   }while(current != NULL);

   littleHead = qSort(littleHead);
   bigHead = qSort(bigHead);

   return combine(littleHead, bigHead, pivot);
}

int listLength(LIST list){
   int length = 0;
   LIST current = list;
   if(NULL==list){
      return length;
   }
   else{
      while(current != NULL){
         current = current->next;
         length++;
      }
   }
   return length;
}



LIST combine(LIST little, LIST big, LIST pivot){
   LIST temp = little;
   while(temp->next != NULL){
      temp = temp->next;
   }
   temp->next = pivot;
   pivot->next = big;
   return little;
}

LIST MakeList()
{
    int x;
    LIST pNewCell;

    if (scanf("\%d", &x) == EOF) return NULL;
    else {
        pNewCell = (LIST) malloc(sizeof(struct CELL));
        pNewCell->next = MakeList();
        pNewCell->element = x;
        return pNewCell;
    }
}

void PrintList(LIST list)
{
    while (list != NULL) {
        printf("\%d\n", list->element);
        list = list->next;
    }
}

和valgrind輸出

==20391== Conditional jump or move depends on uninitialised value(s)
==20391==    at 0x804855D: qSort (2100assignment4.c:45)
==20391==    by 0x80485E0: qSort (2100assignment4.c:61)
==20391==    by 0x80484BD: main (2100assignment4.c:22)
==20391==  Uninitialised value was created by a heap allocation
==20391==    at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==20391==    by 0x8048574: qSort (2100assignment4.c:47)
==20391==    by 0x80484BD: main (2100assignment4.c:22)
==20391==
==20391== Conditional jump or move depends on uninitialised value(s)
==20391==    at 0x804855D: qSort (2100assignment4.c:45)
==20391==    by 0x80485E0: qSort (2100assignment4.c:61)
==20391==    by 0x80485E0: qSort (2100assignment4.c:61)
==20391==    by 0x80484BD: main (2100assignment4.c:22)
==20391==  Uninitialised value was created by a heap allocation
==20391==    at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==20391==    by 0x8048574: qSort (2100assignment4.c:47)
==20391==    by 0x80484BD: main (2100assignment4.c:22)
==20391==
==20391== Conditional jump or move depends on uninitialised value(s)
==20391==    at 0x804855D: qSort (2100assignment4.c:45)
==20391==    by 0x80485E0: qSort (2100assignment4.c:61)
==20391==    by 0x80485E0: qSort (2100assignment4.c:61)
==20391==    by 0x80485E0: qSort (2100assignment4.c:61)
==20391==    by 0x80484BD: main (2100assignment4.c:22)
==20391==  Uninitialised value was created by a heap allocation
==20391==    at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==20391==    by 0x8048574: qSort (2100assignment4.c:47)
==20391==    by 0x80485E0: qSort (2100assignment4.c:61)
==20391==    by 0x80484BD: main (2100assignment4.c:22)
==20391==
==20391== Stack overflow in thread 1: can't grow stack to 0xbe297ff4
==20391==
==20391== Process terminating with default action of signal 11 (SIGSEGV)
==20391==  Access not within mapped region at address 0xBE297FF4
==20391==    at 0x402BE35: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==20391==  If you believe this happened as a result of a stack
==20391==  overflow in your program's main thread (unlikely but
==20391==  possible), you can try to increase the size of the
==20391==  main thread stack using the --main-stacksize= flag.
==20391==  The main thread stack size used in this run was 8388608.
==20391== Stack overflow in thread 1: can't grow stack to 0xbe297fe4
==20391==
==20391== Process terminating with default action of signal 11 (SIGSEGV)
==20391==  Access not within mapped region at address 0xBE297FE4
==20391==    at 0x4025430: _vgnU_freeres (in /usr/lib/valgrind/vgpreload_core-x86-linux.so)
==20391==  If you believe this happened as a result of a stack
==20391==  overflow in your program's main thread (unlikely but
==20391==  possible), you can try to increase the size of the
==20391==  main thread stack using the --main-stacksize= flag.
==20391==  The main thread stack size used in this run was 8388608.
==20391==
==20391== HEAP SUMMARY:
==20391==     in use at exit: 4,190,952 bytes in 523,869 blocks
==20391==   total heap usage: 523,869 allocs, 0 frees, 4,190,952 bytes allocated
==20391==
==20391== LEAK SUMMARY:
==20391==    definitely lost: 0 bytes in 0 blocks
==20391==    indirectly lost: 0 bytes in 0 blocks
==20391==      possibly lost: 0 bytes in 0 blocks
==20391==    still reachable: 4,190,952 bytes in 523,869 blocks
==20391==         suppressed: 0 bytes in 0 blocks
==20391== Rerun with --leak-check=full to see details of leaked memory
==20391==
==20391== For counts of detected and suppressed errors, rerun with: -v
==20391== ERROR SUMMARY: 261929 errors from 3 contexts (suppressed: 0 from 0)

Answer 1

首先要解決的一些問題：

if (scanf("\%d", &x) == EOF) return NULL;

不需要\\。 檢查！= 1可能會更容易，因為您期望一個值。

這導致您的MakeList無限遞歸（至少在我的機器上）。

在您的qsort函數中，小列表和大列表始終在末尾有一個虛擬條目-通過此代碼完成

little->next = (LIST) malloc(sizeof(struct CELL));
little = little->next;
little->next = NULL;

這意味着，當您拆分列表時，最終得到的條目總是比開始時要多，因此它永遠都不會結束。 還要注意，這些新條目未設置其元素值，這可能是您收到未初始化警告的地方。

您應該重新考慮如何存儲子列表，也許以NULL開頭，以指示為空，盡管這會使添加新值變得更加困難。

Answer 2

您可以按以下方式編輯清單列表功能。

您將觀察到一些變化。

1. scanf通過刪除\\是正確的（請參閱《黑暗的答案》）
2. scanf讀取時，如果您輸入任何不兼容的輸入（例如字母）而不是Ctrl + D，這將確保讀取停止。

LIST MakeList()
{
    int x;
    LIST pNewCell;

    while(scanf("%d", &x))
    {
        pNewCell = (LIST) malloc(sizeof(struct CELL));
        pNewCell->next = MakeList();
        pNewCell->element = x;
        return pNewCell;
    }
    return NULL;
}