write interpreter for file format (C++ Arduino)

Question

So I have a .txt file (Excellon) which I want to interpret.

Example file:

M48
FMAT,2
ICI,OFF
METRIC,TZ,000.000
T1C1.016
%
G90
M71
T1
X36551Y-569519
X17780Y-589280

When I scan the file I seperate the statement (e.g. METRIC) and save this in a string. After this I want to execute code based on the value of this string.

What would be the best practice to execute commands on statement detection.

if(String == "METRIC")
{
  execute code;
}
else if (String == "M48")
{
  execute code;
}
etc.

Or something like this:

switch(String)
{
  case: "M48"
    execute code;
    break;
  case: "METRIC"
    execute code;
    break;
  etc.
}

Or are both of these methods wrong and should I use a different method?

I found this: Switch or if statements in writing an interpreter in java they are talking about using a map should I also try this? If so could you provide a simple example because I don't really understand this method.

Well, only one of these alternatives is valid C++, so there's clearly a preferable one between them. Beyond that, it's really dependant on many external factors, and there is not really a true one-size-fits-all optimal solution. — , Oct 04 '18 at 15:52
You could convert the strings to integer tokens when you read in the file. — Galik, Oct 04 '18 at 16:08
1) The reason for them not being valid C++ (as Frank commented) is that you cannot `switch` by strings (https://stackoverflow.com/questions/650162/why-the-switch-statement-cannot-be-applied-on-strings). 2) The `case` label in C++ looks like `case "M48":` instead of `case: "M48"`. — Algirdas Preidžius, Oct 04 '18 at 16:10
If you have some complex grammar - you can try [Antrl](https://tomassetti.me/antlr-mega-tutorial/) i.e. compiler or compilers. For something more simpler you can try [boost spirit](https://theboostcpplibraries.com/boost.spirit) — Victor Gubin, Oct 04 '18 at 16:32
Also there is a classic Dragon book [Compilers: Principles, Techniques, and Tools](https://en.wikipedia.org/wiki/Compilers:_Principles,_Techniques,_and_Tools) you'd better read before write your interpreter manually. — Victor Gubin, Oct 04 '18 at 16:36

score 0 · Answer 1 · answered Oct 04 '18 at 16:13

The proper answer will depend on many factors, but reading between the lines of your post, I am 98% sure that what you want is a simple tokenizer to enum:

enum class Token {
  AAA,
  BBB,
  CCC
};

// Trivially implementable as a if() {} else if {} sequence,
// or as a trie search if you want to get fancy.
Token token_from_string(const std::string& str);

// and in the code.
Token tok = token_from_string(String);
switch(tok) {
  case Token::AAA:
  break;
  case Token::BBB:
  break;
  case Token::CCC:
  break;
}

And then, a good practice is to tokenize the string as soon as it comes out of the stream, and then operate on the token itself.

I'm not so experienced in programming, so could you please explain how tokenizing is a better practice than just use the string in a switch case. Because I have difficulty understanding how I am supposed to implement your code. — user6480540, Oct 04 '18 at 18:05
@user6480540, well, you can't use strings in switch statements, so that's kind of a non starter. — , Oct 04 '18 at 18:06

score 0 · Answer 2 · answered Oct 05 '18 at 11:06

Q: What would be the best practice to execute commands on statement detection.

You want to change control flow, when a certain string is found. A switch is saying "pick one of commands based on this variables value". You could also use if/else.

Q: If so could you provide a simple example because I don't really understand this method.

The Excellon file format isn't far away from CNC g-code.

This is an example for a switch from an EXCELLON to GCODE converter.

The trick would be to modify the output method generateFile, to not generate the G-code file using fprint's, but call your commands instead (probably move, lift, wait, etc.).

You could also start with a g-code parser and modify it to handle the excellon format.

write interpreter for file format (C++ Arduino)

2 Answers2