Confused by the behavior of this simple function

Question

This function is extremely simple, or at least it should be. Some background: I'm building a chess program, and this function runs to assign promote values to pawns which have reached the end of the table. This is just the function to read promotion requests; the detection and actual promotion are one fxn higher. Here's the code:

    char promote (int player)
{

    char req, out;
    req = '0'; //shouldn't be necessary, and should be overwritten by scanf
    out = '0'; //just in case, should be overwritten
    printf("What piece would you like to promote to? (Q/N/R/B) \n");
    scanf("%c", &req);

    if (req == 'q' || req == 'Q')
    {
        out = 'q';
    }
    else if (req == 'r' || req == 'R')
    {
        out = 'r';
    }
    else if (req == 'n' || req == 'N')
    {
        out = 'n';
    }
    else if (req == 'b' || req == 'B')
    {
        out = 'i';
    }
    else if (req != 'q' && req != 'Q' && req != 'r' && req != 'R' && req != 'n' && req != 'N' && req != 'b' && req != 'B')
    {
        printf("req: %c, out: %c \n", req, out); //to test variable values
        printf("That character is invalid. Please enter another.\n");
        return promote(player);
    } //a simple else should suffice here. I put in the else if as a possible fix via redundancy (didn't work)

    if(player == 2)
    {
        out = toupper(out); //designates team by case
    }

    printf("Returning %c\n", out); //test out value again
    return out;
}

What SHOULD happen: Fxn scans for a single-character input, assigns it to req, checks req against eight possible choices. If it fits in one of the four bins, it assigns one of four values to out; if not, it prints a simple error message and returns its own call, at which point it scans, etc.

What DOES happen: When called, the scanf reads a \n (automatically), and immediately spits back the error message and loops upon itself without waiting for input. In the second loop, it works fine, with one caveat: when I enter an invalid character, it returns the proper error message, and upon calling itself again, goes through one loop of reading a \n before working again. Suggests the function itself, and not the error message, is to blame.

The output:

You can now promote your pawn. What piece would you like to promote to? (Q/N/R/B)
req:
, out: 0
That character is invalid. Please enter another.
What piece would you like to promote to? (Q/N/R/B)
q
Returning q

Any ideas? I've tried a dozen different things trying to fix this, and none of 'em have worked. Probably really simple like all my big mistakes are. Thanks for any help!

Answer 1

Looks like scanf is reading a newline, probably from a previous input. To fix this, you can simply loop scanf until you get something that isn't whitespace:

do {
    scanf("%c", &req);
} while(isspace(req));

As an aside, you can use tolower to convert a character to lowercase, which should simplify your req testing.