Support percentage operator in parser

[titoworlddev]

It just doesn't work right, I have tried to do the calculation of various percentages but it doesn't work right, it gives wrong result.
For example if I put something as simple as:
'8%9'.interpret();
the output is: 8
when it should be: 0.72

If you could fix it please because this gives me a lot of problems and I need to make a calculator that is already ready and that error will not let me finish it.

[fkleon]

Hi @Hekhy, the % symbol denotes the Modulo operator, not the percentage operator.

You need to use decimal notation to perform percentage calculations, see #28 and #50

[titoworlddev]

Well I have had to do my method and every time I press the button with the symbol "%", "/ (100) *" is added, then I apply a format so that "/ (100) *" becomes "% "but only in sight.
It is the only way that it works well, since if you make that when giving "%" the last number is divided by 100 and you get the result (for example 0.25) it gives problems, the only way is that you add it manually but I want to give people that facility of not having to do it and it becomes automatic.
It would be nice if you implemented it, even so thanks, the package is very good.

[fkleon]

Good to hear that you found a workaround. Will consider adding support for this to the parser!

[jvinai]

Any news about adding the percent operator?

[titoworlddev]

Any news about adding the percent operator?

It seems that nothing yet, it has been a long time since I gave my option, that although it is not the best it is how I fix it, if it helps you let me know and I put my code although it is more or less explained above and if not I hope the creator is current early.

[Ruwasoft]

is any update for this issue ?

[GregoryConrad]

Here is my workaround (the key here is the regex with the replaceAllMapped):

extension Expression on String {
  double evaluate(Map<String, double> variables) {
    final cm = ContextModel();
    for (final variable in variables.entries) {
      cm.bindVariableName(variable.key, Number(variable.value));
    }
    final percentsReplaced = replaceAllMapped(
      RegExp(r'(\d+(\.\d*)?)%'),
      (match) => '(${match.group(1)!}/100)',
    );
    return Parser().parse(percentsReplaced).evaluate(EvaluationType.REAL, cm);
  }
}

However, this will break support for modulo unless you have a space before the % and the first number; use with caution.

@fkleon I was taking a look at the factorial support PR (#58) to see how feasible it would be for me to make a PR for basic percentage support (supporting the equivalent complexity of 80% * 100 => 80--a simple substitution of the val% with val/100). However, the keywords map in Lexer proves to be problematic as it maps a single keyword to a single token, which is not always the case (as seen here with modulo and percent). Maybe it would be worth it to switch to a more complex solution/algorithm? I noticed a few hacks in the parser already to make it work, notably the e and ceil conflict amongst at least one more documented in code. Perhaps a CFG could work nicely?
Edit: it looks like https://pub.dev/packages/petitparser would work nicely.

[fkleon]

HI @GregoryConrad - you're right about the shortcomings of the current lexer. I've looked into alternative implementations but never finished them. Busy elsewhere so struggling to find any time to work on this library!

• based on petitparser on branch feature-parser-rewrite. Missing a few more features.
• based on parser_builder custom Experimental parser named 'Parser2' #68 . Feature complete (?) with a completely new approach where a parser is generated from a template. Needed some review.

Coming from a programming background I find the modulo operator to be a more "natural" fit for %, but can understand that people want to use a percentage operator. However it seems obvious to me that you can simply use the decimal representation in your expressions so I haven't prioritised this. However it's a popular request for sure!

[titoworlddev]

@fkleon
You're right about simply using the decimal representation. But the fact of creating this thread, for me, was because I was creating a calculator that is for everyone. This means that there are people who don't like having to think, they just put the % operator and that's it, so that's why I wanted to give it that functionality.

In the end, as I commented some time ago in a comment, my solution was to divide by 100, to get that decimal result and be able to do the same thing I want (for example: 25/100 => 0.25), and then simply when you hit the operator % replacement 25/100 for the same (25/100 => 25%).

I know it's a mess, but if you look at all calculators have this and that's why I said, since they are for everyone there are people who just don't want to think about how to do it.