Simplifying function
What you are doing is very basic, and the question really belongs on stackoverflow.com, but it has probably already been answered there, and therefore would be marked as a duplicate.

As I mentioned in the comment, this can be done using simple subtraction of characters when you include ctype.h&gt;, you don't need a switch statement or anything that complcated. The example below should execute faster than the version with the switch statement, and it is a lot less code.

#include &lt;ctype.h&gt;

int getColRow(char input)
{
    int val = 0;
    unsigned char testValue = (unsigned char) input;

    if (isdigit(testValue))
    {
        return testValue - '0';
    }
    else if (isalpha(testValue))
    {
        testValue = tolower(testValue);
        return testValue - 'a';
    }

    fprintf(stderr, "Invalid character %c in getColRow(char input)\n", input);
    return val;
}


Most of the functions / macros in ctype.h take unsigned integers, and it is safer to use unsigned values. The unsigned char should automatically be promoted to an unsigned integer.