垃圾值

Question

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

void* lsearch(void* key,void* base,int size,int elemSize){ 
    int i = 0; 
    for(i = 0; i < size; i++){ 
    void* elemAddr = ((char*)base + (i * elemSize)); 
    if(memcmp(key,elemAddr,elemSize)==0){ 
      return elemAddr; 
    } 
    } 
    return NULL; 
} 

int main(){ 
    char* a[] = {"Hello","Good","Morning","Ladies"}; 
    int size = sizeof(a)/sizeof(a[0]); 
    char* key = "Good"; 
    char* search = (char*)lsearch(&key,&a,size,sizeof(char*)); 
    if(search == NULL){ 
    printf("\n key value not found!! \n"); 
    return -1; 
    } 
    printf("\n search : %s \n",search); 
    return 0; 
} 

OUTPUT：

search : �@ 

我試圖做琴絃一個lsearch在一個數組...匹配字符串，但我得到打印的垃圾值。爲什麼會這樣..

Answer 1

修正：

有幾件事情是錯誤的，這裏有最顯着的問題：在void*

• 算術：非法，
• 一個some_type array[] = {...};的長度sizeof(array)（沒有花哨的部門），
• 不恰當的選擇接口：當你剛剛閱讀時，考慮使用const參數。另外，確保參數的類型與您的參數一致（例如base是const char* []而不是char*）。最後，要查找字符串數組中的鍵，您需要一個鍵，數組的句柄和數組的長度，就這些了。

Run It Online

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

const char* lsearch(const char* key, const char* base[], const size_t size){ 
    size_t i = 0; 
    const size_t max_len = strlen(key); // strlen(), that's how you compute the length of a string (you might add `+1` to include `\0` if you want) 

    for(i = 0; i < size; i++){ 
    const char* elemAddr = base[i]; // Arithmetic on a void* is illegal in both C and C++: https://stackoverflow.com/a/3524270/865719 
    if(memcmp(key, elemAddr, max_len)==0) { // @TODO use strncmp(), more appropriate, and safer. In particular, if strlen(elemAddr) < strlen(key) 
      return elemAddr; 
    } 
    } 
    return NULL; 
} 

int main() { 
    // init 
    const char* a[] = {"Hello","Good","Morning","Ladies"}; 
    const size_t size = sizeof(a); // size of the array -- i.e. element count 

    // search 
    const char* key = "Morning"; 
    const char* search = lsearch(key, a, size); 

    // results 
    if(search == NULL) { 
    printf("\n key value not found!! \n"); 
    return -1; 
    } 

    printf("\n search : %s \n",search); 
    return 0; 
} 

正如指出的Jongware，更好的辦法是使用字符串比較功能。在我看來，最安全的辦法是

int strncmp(const char *s1, const char *s2, size_t n); 

而且你會用它作爲這樣的：

// safer. in particular, if (and when) strlen(elemAddr) < strlen(key) 
if(strncmp(key, elemAddr, max_len) == 0) { 
     return elemAddr; 
} 

Answer 2

這將做到這一點：

printf("\n search : %s \n",*(char**)search); 

你試圖從右側位置讀取，但是編譯器沒有千牛流如何訪問它