problem with creating lexical analysis

hello everyone i could use some help right now i'm desperate and now where to go
i have been asked to create a lexical analyzer with c++ as a project and i have no idea how to do it and i was told to make the program read from txt file and categories every lexemes as identifiers or special sympol or comments etc this is an example they gave me and i'm supposed to continue it

Code:

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#include <iostream>
#include <fstream>
#include <string>
using namespace std;
void main ()
{
int counter = 0;
string word;
char letter;
char  letters[]={'a' , 'b' , 'c' , 'd'  ,'e' , 'f' , 'g' , 'h' , 'i'  'j' ,  'k' , 'l' , 'm'  ,'n' , 'o' , 'p' , 'q' , 'r' , 's' , 't' , 'u' ,  'v'  ,'w' , 'x' , 'y' , 'z' ,'A' , 'B' , 'C' , 'D'  ,'E' , 'F' , 'G' ,  'H' ,  'I' ,J' , 'K' , 'L' , 'M'  ,'N' , 'O' , 'P' , 'Q' , 'R' , 'S' ,  'T' ,  'U' , 'V'  ,'W' , 'X' , 'Y' ,Z }
string RW []={"void" , "main", "iostream" , "string" }
char digits[]= {'1' ,'2' , '3' , '4' , '5' , '6' , '7' , '8' , '9' ,}
ifstream myfile;
myfile.open("source.txt");

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please i really need help

Quote:

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Originally Posted by dexter00

(...) and categories every lexemes as identifiers or special sympol or comments etc this is an example they gave me and i'm supposed to continue it

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Do you need to identify these by line, or individual character, or...? If you could give a more detailed description of your assignment, it would really help. Posting an example of what source.txt might contain would also be most helpful.

The digits array seems to be missing zero at the end. And what you call lexeme is just a space delimited sequence of characters?

really thx for replying
about the source.txt is just the name of the file u read from
and sorry about the 0 digit
i need to get the output as this example

Code: lexeme                           token type                           sum                           Identifier                           =                           Assignment operator                           i                           Identifier                           +                           Addition operator                           2                           Number                           ; void                           Statement delimiter reserved word
like i write on txt file and the lexical defines what kind lexeme it is and give error massage if it's not defined by user

I think I get what the desired output is (except for where void comes into the picture), but what exactly is inside the text file? Is it C++ code, one line, multiple lines? Clear instructions should be the starting point for any such effort. If you have a written assignment, then just paste that here.

There are several possible approaches to implementing this program, so you need to take a step back and come up with a plan (perhaps after re-reading your notes, books or what you can find online).

For example, you might decide to implement a state machine of some sort: you start by reading a character, then based on the current state and value of that character, you decide if a complete lexeme has been formed and so this is the start of a new one, or if you should continue reading the next character to determine the current lexeme. Once it has been determined that a complete lexeme has been formed, you can then store it somewhere along with its category (e.g., in a std::pair). If so, you need to decide on the states, e.g., currently reading an identifier, or a number, etc. For you to do this, you need to be clear to what are these categories.