Question

我试图在C中实现哈希表来存储英文单词。所以我在互联网上搜索一些最好的非加密哈希函数。其中一些是 Murmurhash，Seahash，xxHash ，但它们似乎都很难实现。所以我搜索了一些较简单的，然后我发现了 DJB2，sdbm，输了。在实现sdbm时我得到了这个

try.c:12:18: error: using the result of an assignment as a condition without 
parentheses [-Werror,-Wparentheses]
    while (c = *str++)
           ~~^~~~~~~~
try.c:12:18: note: place parentheses around the assignment to silence this 
warning
    while (c = *str++)
             ^
           (         )
try.c:12:18: note: use '==' to turn this assignment into an equality 
comparison
    while (c = *str++)
             ^
             ==
try.c:26:31: error: passing 'char *' to parameter of type 'unsigned char *' 
converts between pointers to integer types with
  different sign [-Werror,-Wpointer-sign]
unsigned long hash = sdbm(argv[1]);
                          ^~~~~~~
2 errors generated.

我的代码是

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

static unsigned long
sdbm(str)
unsigned char *str;
{
    unsigned long hash = 0;
    int c;

    while (c = *str++)
        hash = c + (hash << 6) + (hash << 16) - hash;

    return hash;
}

int main(int argc,char *argv[])
{
if(argc!=2)
{
    printf("Enter the second command line argument\n");
    return 1;
}

unsigned long hash = sdbm(argv[1]);
printf("The returned hashcode is %lu\n", hash);
}

如果您还可以帮我使用Murmurhash，Seahash或xxHash，请执行此操作。

Answer 1

Oups！带有简单参数列表的此函数定义是一个过时的功能：

static unsigned long
sdbm(str)
unsigned char *str;
{

标准方式是（至少从80年代后期开始）是使用原型定义：

static unsigned long
sdbm(unsigned char *str)
{

现在出现错误：

while ((c = *str++))
    ...

括号告诉编译器您测试了赋值的结果值。

unsigned long hash = sdbm((unsigned char *) argv[1]);

只强制转换为预期的指针类型。

在C中实现非加密哈希函数

1 个答案: