warning: array subscript has type char [-Wchar-subscripts]

[richardpd]

I get this warning when trying to compile a code snippet from the net with GCC GNU compiler.

: warning: array subscript has type    char    [-Wchar-subscripts]
     arr[0] = (int)tolower(strMove[0]) - 'a';//letter here
     ^
: warning: array subscript has type    char    [-Wchar-subscripts]
     arr[1] = (int)tolower(strMove[1]) - '0';
     ^
: warning: array subscript has type    char    [-Wchar-subscripts]
     arr[2] = (int)tolower(strMove[2]) - 'a';//letter here
     ^
: warning: array subscript has type    char    [-Wchar-subscripts]
     arr[3] = (int)tolower(strMove[3]) - '0';
     ^
Compilation finished successfully.

I roughly think it has something to do with signed and unsigned char conflict? I don't understand it though and have tried to fix it but without success.

int GetMove(int arr[]) {

    int count = 0;
    int i,j;
    int validInputArr[ROW_SIZE] = {0,1,2,3,4,5,6,7};

    String strMove;
    String strResign = "resign";

    printf("Enter coordinates in the form g4e4\n");
    printf("Enter resign to resign.\n");

    gets(strMove);

    if(StringCompare(strMove, strResign) == 1) {
        return RESIGN;
    } else if (strlen(strMove) != 4 ) {
        return INVALID;
    }

    //Convert the chars to ints, which gives ascii numbers. So then convert to
    //the decimal value by subtracting the base.
    arr[0] = (int)tolower(strMove[0]) - 'a';//letter here
    arr[1] = (int)tolower(strMove[1]) - '0';
    arr[2] = (int)tolower(strMove[2]) - 'a';//letter here
    arr[3] = (int)tolower(strMove[3]) - '0';

    for(i=0;i<4;i++) {
        for(j=0;j<ROW_SIZE;j++) {
            if(arr[i] == validInputArr[j]) {
                count++;
            }
        }
    }
    if(count != 4) {
        return INVALID;
    }else {
        printf("Move piece from %i:%i to %i:%i\n", arr[0], arr[1], arr[2], arr[3]);
        return VALID;
    }

    return 1;
}//End GetMove

I am grateful for help to fix it,many thanks.

[SirPrattlepod]

What's "String"?

[richardpd]

OK-so I said this isn't my code but here is the relevant typedef:
typedef char String[STRING_SIZE];
Does that help?

[tabstop]

It would be incredible for the compiler to think that 0 was a char constant. I might start looking to see whether you perhaps have a #define that has run amok -- maybe use the -E flag to get what's there after the preprocessor has gone through and look at those lines in the processed output.

[Matticus]

A little bit - it would be a lot more helpful if you could post a simplified yet complete, compilable program that produces the warning messages you've received (i.e. include a basic "main" and all necessary typedefs and includes).