代码报array subscript is of type 'char'警告，寻求解决方法

你的代码可正常运行，但触发了4处array subscript is of type 'char' [-Wchar-subscripts]编译警告，具体位置如下：

A32.c:42:13: warning: array subscript is of type 'char' [-Wchar-subscripts]
table[pattern[i]] = m - 1 - i;
^~~~~~~~~~~

A32.c:55:18: warning: array subscript is of type 'char' [-Wchar-subscripts]
i += table[text[i]];
^~~~~~~~
A32.c:74:13: warning: array subscript is of type 'char' [-Wchar-subscripts]
table[pattern[i]] = m - 1 - i;
^~~~~~~~~~~
A32.c:88:22: warning: array subscript is of type 'char' [-Wchar-subscripts]
i += table[text[i]];

警告原因 #

C语言中char类型默认是有符号的（取决于编译器），当字符的ASCII值大于127时，会被解析为负数。数组下标要求是非负整数，负数下标会导致未定义行为，因此编译器触发警告提醒风险。

通用修改方案 #

将所有作为数组下标的char值强制转换为unsigned char，确保下标始终处于0-255的合法范围（匹配你定义的256长度的shift table）。

需要修改的4处代码行：

• table[pattern[i]] → table[(unsigned char)pattern[i]]
• table[text[i]] → table[(unsigned char)text[i]]
• table[pattern[i]] → table[(unsigned char)pattern[i]]
• table[text[i]] → table[(unsigned char)text[i]]

修改后的完整代码 #

int horspool_search(char* text, char* pattern) {
   int n = strlen(text);
   int m = strlen(pattern);
   int count = 0;
   int shifts = 0;

   // Create a shift table
   int table[256];
   for (int i = 0; i < 256; i++) {
       table[i] = m;
   }
   for (int i = 0; i < m - 1; i++) {
       table[(unsigned char)pattern[i]] = m - 1 - i;
   }

   int i = m - 1;
   while (i < n) {
       int j = 0;
       while (j < m && text[i - j] == pattern[m - 1 - j]) {
           j++;
       }
       shifts++;
       if (j == m) {
           count++;
       }
       i += table[(unsigned char)text[i]];
   }
   printf("Horspool's algorithm shifts: %d\n", shifts);
   return count;
}

// Function to implement the Boyer-Moore algorithm
int boyer_moore_search(char* text, char* pattern) {
   int n = strlen(text);
   int m = strlen(pattern);
   int count = 0;
   int shifts = 0;

   // Create a shift table
   int table[256];
   for (int i = 0; i < 256; i++) {
       table[i] = m;
   }
   for (int i = 0; i < m - 1; i++) {
       table[(unsigned char)pattern[i]] = m - 1 - i;
   }

   int i = m - 1;
   while (i < n) {
       int j = 0;
       while (j < m && text[i - j] == pattern[m - 1 - j]) {
           j++;
       }
       shifts++;
       if (j == m) {
           count++;
           i += m;
       } else {
           i += table[(unsigned char)text[i]];
       }
   }
   printf("Boyer-Moore algorithm shifts: %d\n", shifts);
   return count;
}

内容的提问来源于stack exchange，提问作者David