Get ASCII value from yytext

Question

This maybe a silly question but I'm stuck on this.

I'm doing an assignment on Compiler. I have to match a character literal. It's defined as any single character enclosed with ' ', with the exception of '\t','\n','\a' etc. In my lex file, I have written a pattern to match it. When the pattern matches the input, it stores it in yytext. Now yytext in a char pointer. It catches the value as '\t'. (4 individual chars). But I have save this character's ascii value in my Symbol table. I'm struggling with this.

I have this following char pointer.

 char *yytext = new char[5];
 *(yytext + 0) = '\'';
 *(yytext + 1) = '\\';
 *(yytext + 2) = 't';
 *(yytext + 3) = '\'';
 *(yytext + 4) = '\0';

 cout << yytext << endl; // prints '\t'

Now what I want to do is get the '\t' ascii character in a single char. Something like:

char ch = '\t';

How can I do this? I might be able to do this by a brute-force approach, but is there any simple and straightforward way to achieve this?

_Something like: `char ch = '\t';`_ That's how you do it. But I don't see what the rest of the code has to do with that. — 001, May 03 '18 at 20:31
and `*(ch + 0) = '\'';` can be replaced with simple and more readable form `ch[0] = '\''`. — haccks, May 03 '18 at 20:32
It maybe something else. Like '\a' or '\n'. How can I know in advance what characters this char pointer holds? I only have this char pointer at my disposal. — Robur_131, May 03 '18 at 20:34
Are you asking how to access the third `char` in your array? — Drew Dormann, May 03 '18 at 20:35
The third char in my array is 't'. But I want the whole '\t' or ascii 9. — Robur_131, May 03 '18 at 20:37
@KillzoneKid, Thank you. But I have match it with several escape characters. Is there any alternate way without checking each case with a conditional? — Robur_131, May 03 '18 at 20:46
This sounds like a question about [tag:yacc]. Maybe you should reword the question in reference to that. — 001, May 03 '18 at 20:49
@KillzoneKid, I've tried your solution. But it doesn't seem to work. — Robur_131, May 03 '18 at 20:54

score 1 · Accepted Answer · answered May 03 '18 at 20:54

But how do I parse it from the char pointer in an easy way ?

Use map with key being the string and the value being corresponding char:

#include <iostream>
#include <string>
#include <unordered_map>

int main()
{
    std::unordered_map<std::string, char> convert = { {"'\t'", '\t'},{ "'\n'", '\n' },{ "'\r'", '\r' } };

    const char *ch1 = "'\t'";
    const char *ch2 = "'\n'";

    std::cout << 1 << convert[ch1] << 2 << convert[ch2] << 3 << std::endl;

    return 0;
}

Working demo: https://ideone.com/rsSKXL

1   2
3

Get ASCII value from yytext

1 Answers1