Question

just to give you background. We have a school project where we need to write our own compiler in C. My task is to write a lexical analysis. So far so good but I am having some difficulties with escape sequences.

When I find an escape sequence and the escape sequence is correct I have it saved in a string which looks like this \xAF otherwise it is lexical error.

My problem is how do I convert the string containing only escape sequence to one char? So I can add it to “buffer” containing the rest of the string.

I had an idea about a massive table containing only escape sequences and then comparing it one by one but it does not seem elegant.

Answer 1

you can use the following code, call xString2char with your string.

char x2char(const char c)
{
    if (c >= '0' && c <= '9')
        return c - '0';
    if (c >= 'a' && c <= 'f')
        return c - 'a';
    if (c >= 'A' && c <= 'F')
        return c - 'A';
    //if we got here it's an error - handle it as you like...
}

char xString2char(const char* buf)
{
    char ans;
    ans = x2char(buf[2]);
    ans <<= 4;
    ans += x2char(buf[3]);
    return ans;
}

This should work, just add the error checking & handling (in case you didn’t already validate them in your code)