How to deal with this warning at 15 and 18."warning: array subscript has type ‘char’ [-Wchar-subscripts]"

Question

the problem is this warning at 15 and 18 warning: array subscript has type ‘char’ [-Wchar-subscripts]

deal with Sample Input: They are students. aeiou Sample Output: Thy r stdnts.

#include <cstdio>
#include <cstring>
const int MAXN = 10005;
char str1[MAXN], str2[MAXN];
bool HashTable[128] = {false};
//use HashTable to record the item need to minus

int main()
{
    fgets(str1, sizeof(str1), stdin);
    fgets(str2, sizeof(str2), stdin);
    int len1 = strlen(str1);
    int len2 = strlen(str2);
    for (int i = 0; i < len2; i++) {
        HashTable[str2[i]] = true;
    }
    for (int i = 0; i < len1; i++) {
        if (HashTable[str1[i]] == false) {
            printf("%c", str1[i]);
        }
    }
    return 0;
}

I can run it, but the warning I have no idea.

here

Maybe the compile fears the char can be negative... Try with unsigned chars — Alain Merigot, Jun 08 '19 at 09:01
It seems to me the warning is not useful here since even a signed `char` can cover the index range of the array. — ikegami, Jun 08 '19 at 09:02
There is an independent problem, however. `str1[i]` and `str2[i]` could be outside of the bounds of `HashTable`. — ikegami, Jun 08 '19 at 09:04
The warnings are a bit aggressive. If you're confident your code is correct and just want to bypass the warnings, you can use `HashTable[(int)str2[i]]` and `HashTable[(int)str1[i]]`. — Tom Karzes, Jun 08 '19 at 09:05
From GCC documentation: _"This is a common cause of error, as programmers often forget that this type is signed on some machines."_ — the busybee, Jun 08 '19 at 09:57
If somebody enters an emoji character, your program has an out-of-bounds array index. — Raymond Chen, Jun 08 '19 at 10:00
casting char to int should suppress the warning `HashTable[(int)str2[i]] = true;` — Shubham, Jun 08 '19 at 10:01

Petr Skocik · Answer 1 · 2019-06-08T10:08:48.303

Casting the char to int or promoting it, e.g., with the unary plus would rid you of the warning.

Note that if the user can enter anything they want (and they usually can) you're risking an out-of-bound access as you could get either a value larger than 127 if char is unsigned or a negative value if char is signed.

The safest strategy would be to expand the array to size 256 and cast the char to unsigned char before using it as an index.

How to deal with this warning at 15 and 18."warning: array subscript has type ‘char’ [-Wchar-subscripts]"

1 Answers1