使用字符串數組使用bsearch時遇到問題

我收到一些令人困惑的行爲，試圖在C中的字符串數組上使用c內建bsearch。下面是代碼。我知道你可以使用內建的strcmp來搜索字符串數組，但是爲了調試的目的，我包含了myStrCmp，因爲我不知道它爲什麼不起作用。使用字符串數組使用bsearch時遇到問題

const char *stateNames[] = {"Alabama", "Alaska", "Arizona", "Arkansas", "California", "Colorado", "Connecticut", "Delaware", "Florida", "Georgia", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky", "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", "Minnesota", "Mississippi", "Missouri", "Montana", "Nebraska", "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York", "North Carolina", "North Dakota", "Ohio", "Oklahoma", "Oregon", "Pennsylvania", "Rhode Island", "South Carolina", "South Dakota", "Tennessee", "Texas", "Utah", "Vermont", "Virginia", "Washington", "Washington DC", "West Virginia", "Wisconsin", "Wyoming"}; 

int myStrCmp(const void *s1, const void *s2) { 
    printf("myStrCmp: s1(%p): %s, s2(%p): %s\n", s1, (char *)s1, s2, (char *)s2); 
    return strcmp(s1, s2); 
} 

int determineState(char *state) { 
    printf("state: %s\n", state); 
    for(int i = 0; i < 51; i++) 
    printf("stateNames[%i](%p): %s\n", i, &(stateNames[i]), stateNames[i]); 

    char *found = (char *) bsearch(state, stateNames, 51, sizeof(char *), myStrCmp); 

    if(found == NULL) 
    return -1; 

    return 0; 
}

這裏是當這個函數被調用來尋找阿拉巴馬州時的一些輸出。

stateNames[0](0x618440): Alabama 
stateNames[1](0x618448): Alaska 
stateNames[2](0x618450): Arizona 
... 
stateNames[24](0x618500): Missouri 
stateNames[25](0x618508): Montana 
stateNames[26](0x618510): Nebraska 
stateNames[27](0x618518): Nevada 
stateNames[28](0x618520): New Hampshire 
stateNames[29](0x618528): New Jersey 
stateNames[30](0x618530): New Mexico 
stateNames[31](0x618538): New York 
stateNames[32](0x618540): North Carolina 
stateNames[33](0x618548): North Dakota 
stateNames[34](0x618550): Ohio 
stateNames[35](0x618558): Oklahoma 
stateNames[36](0x618560): Oregon 
stateNames[37](0x618568): Pennsylvania 
stateNames[38](0x618570): Rhode Island 
stateNames[39](0x618578): South Carolina 
stateNames[40](0x618580): South Dakota 
stateNames[41](0x618588): Tennessee 
stateNames[42](0x618590): Texas 
stateNames[43](0x618598): Utah 
stateNames[44](0x6185a0): Vermont 
stateNames[45](0x6185a8): Virginia 
stateNames[46](0x6185b0): Washington 
stateNames[47](0x6185b8): Washington DC 
stateNames[48](0x6185c0): West Virginia 
stateNames[49](0x6185c8): Wisconsin 
stateNames[50](0x6185d0): Wyoming 
myStrCmp: s1(0x415430): Alabama, s2(0x618508): 
               UA 
myStrCmp: s1(0x415430): Alabama, s2(0x618570): A 
myStrCmp: s1(0x415430): Alabama, s2(0x618540): PUA 
myStrCmp: s1(0x415430): Alabama, s2(0x618528): 1UA 
myStrCmp: s1(0x415430): Alabama, s2(0x618538): GUA 
myStrCmp: s1(0x415430): Alabama, s2(0x618530): <UA

正如你所看到的，在其搜索的過程中通過bsearch到過的地方應該有有效的字符串（如調用bsearch之前剛選中），但輸出，如果你嘗試打印字符*在那位置是垃圾。任何人都能看到我的錯誤？順便說一句，我得到了相同的不良行爲（但沒有得到遵循它作爲密切明顯）當我打電話bsearch與最終的參數設置爲：

(int(*)(const void*, const void*))strcmp

謝謝！

來源

2013-04-05 user2247374

由於您使用的是const char *的數組，因此bsearch()會傳遞給比較函數一個指向這些元素的指針。換句話說，它將在第二個參數中收到const char * const *。

int myStrCmp(const void *s1, const void *s2) { 
    const char *key = s1; 
    const char * const *arg = s2; 
    printf("myStrCmp: s1(%p): %s, s2(%p): %s\n", s1, key, s2, *arg); 
    return strcmp(key, *arg); 
}

來源

2013-04-05 02:37:02 jxh

有趣的是，這工程，我沒有想到會被標準保證。但是，閱讀ISO/IEC 9899：2011，'§7.22.5.1'bsearch'函數'，它會說：_¶3用'compar'指向的比較函數有兩個參數指向指向關鍵對象，而到數組元素，按照這個順序。因此，行爲是確定性的。儘管如此，你不能在'qsort（）'中使用'myStrCmp（）'函數。 – 2013-04-05 05:08:00

@JonathanLeffler：是的，API的定義與'qsort（）'有一點不同，這樣你就可以傳遞一個字符串作爲鍵，但有一個結構數組可以搜索。 – jxh 2013-04-05 05:49:55

您的狀態名稱（或鍵）需要是指向指針的指針。無需在任何地方添加/刪除const。 myStrCmp需要通過一個比較字符串來解引用。下面的代碼做你想要的，我想。請讓我知道如果沒有，謝謝。

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


const char *stateNames[] = {"Alabama", "Alaska", "Arizona", "Arkansas","California", "Colorado", "Connecticut", "Delaware", "Florida","Georgia", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky", "Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", "Minnesota", "Mississippi", "Missouri", "Montana", "Nebraska", "Nevada", "New Hampshire", "New Jersey", "New Mexico", "New York", "North Carolina", "North Dakota", "Ohio", "Oklahoma", "Oregon", "Pennsylvania", "Rhode Island", "South Carolina", "South Dakota", "Tennessee", "Texas", "Utah", "Vermont", "Virginia", "Washington", "Washington DC", "West Virginia", "Wisconsin", "Wyoming"}; 

int myStrCmp(const void *s1, const void *s2) { 
    printf("myStrCmp: s1(%p): %s, s2(%p): %s\n", s1, *(char **)s1, s2, *(char**)s2); 
    return strcmp(*(char **) s1, *(char **) s2); 
} 

int determineState(char *state) { 
    printf("state: %s\n", state); 

    for(int i = 0; i < 51; i++) 
     printf("stateNames[%i](%p): %s\n", i, &(stateNames[i]), stateNames[i]); 

    char **found = (char **) bsearch(&state, stateNames, 51, sizeof(char *), myStrCmp); 

    if(found == NULL){ 
     return -1; 
    } else { 
     printf("Found it!: %s\n", *found); 

    } 

    return 0; 
} 

int main(int argc, const char * argv[]) { 
    determineState("Alabama"); 

}

來源

2017-11-21 18:44:43

使用字符串數組使用bsearch時遇到問題

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