我是C新手,我正在学习来自coursera的算法,在这里我正在尝试实现三向快速排序,我理解我遇到了一个错误的内存错误,它发生在第一次排序数组之后。
我在这里附加我的代码和coursera讲师使用的java代码的页面,任何改变错误的建议将不胜感激。
#include <stdio.h>
void swap(int *a, int *b)
{
int temp = *a;
*a = *b;
*b = temp;
}
void quicksort(int arr[],int low, int high)
{
if(high<=low)
return;
int lt = low;
int gt = high;
int pivot = low;
int i = low;
while(i<=gt)
{
if(arr[i]==arr[pivot])
i++;
else if(arr[i]<arr[pivot])
{
swap(&arr[i],&arr[lt]);
i++;
lt++;
}
else
{
swap(&arr[i],&arr[gt]);
gt++;
}
}
quicksort(arr,low,lt-1);
quicksort(arr,gt+1,high);
}
void printArray(int arr[], int size)
{
int i;
for (i=0; i < size; i++)
printf("%d ", arr[i]);
printf("\n");
}
int main()
{
int arr[] = {5,1,2,8,7,5,5,6,5,4,5,3,9,10};
int n = sizeof(arr)/sizeof(arr[0]);
quicksort(arr, 0, n-1);
printf("Sorted array: ");
printArray(arr,n);
return 0;
}
而且我附上了coursera讲师的java代码Java code的图片
我遇到了错误的内存错误
你从Java代码中做了一个错误
else
{
swap(&arr[i],&arr[gt]);
gt++;
}
一定是
else
{
swap(&arr[i],&arr[gt]);
gt--;
}
在更正,编译和执行之后:
pi@raspberrypi:/tmp $ gcc -g -pedantic -Wextra so.c
pi@raspberrypi:/tmp $ ./a.out
Sorted array: 1 5 2 5 3 4 5 5 5 7 6 8 9 10
在valgrind下执行:
pi@raspberrypi:/tmp $ valgrind ./a.out
==16601== Memcheck, a memory error detector
==16601== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==16601== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==16601== Command: ./a.out
==16601==
Sorted array: 1 5 2 5 3 4 5 5 5 7 6 8 9 10
==16601==
==16601== HEAP SUMMARY:
==16601== in use at exit: 0 bytes in 0 blocks
==16601== total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated
==16601==
==16601== All heap blocks were freed -- no leaks are possible
==16601==
==16601== For counts of detected and suppressed errors, rerun with: -v
==16601== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)
所以现在没有无效的内存访问...但排序不起作用,这是因为你没有遵循关于v / pivot的Java代码
你必须用int pivot = low;
替换int pivot = arr[low];
,当然还有arr[pivot]
的其他地方pivot
之后 :
pi@raspberrypi:/tmp $ gcc -g -pedantic -Wextra so.c
pi@raspberrypi:/tmp $ ./a.out
Sorted array: 1 2 3 4 5 5 5 5 5 6 7 8 9 10
在valgrind下:
pi@raspberrypi:/tmp $ valgrind ./a.out
==16833== Memcheck, a memory error detector
==16833== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==16833== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==16833== Command: ./a.out
==16833==
Sorted array: 1 2 3 4 5 5 5 5 5 6 7 8 9 10
==16833==
==16833== HEAP SUMMARY:
==16833== in use at exit: 0 bytes in 0 blocks
==16833== total heap usage: 1 allocs, 1 frees, 1,024 bytes allocated
==16833==
==16833== All heap blocks were freed -- no leaks are possible
==16833==
==16833== For counts of detected and suppressed errors, rerun with: -v
==16833== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)