Arithmetic Parser

Question

I am building a parser on top of the TDArithmeticParser.m of ParseKit Tests. I extended the TDArithmeticParserTest.m with the failing test:

- (void)testMath {
    s = @"10+(2*3)-15";
    result = [p parse:s];
    TDEquals((double)1.0, result); // result == 0.0
}

The problem is that I don't understand why the grammar is not working with this test. The corresponding BNF-grammar of the arithmetic parser is:

expr           = term (plusTerm | minusTerm)*;
term           = factor (timesFactor | divFactor)*;
plusTerm       = '+' term;
minusTerm      = '-' term;
factor         = phrase exponentFactor | phrase;
timesFactor    = '*' factor;
divFactor      = '/' factor;
exponentFactor = '^' factor;
phrase         = '(' expr ')' | Number;

I would be very thankful for any ideas that helps me identifying the problem.

Thx for the hint that the result is missing, Martin. Result is 0.0 what is the result of a nil casted to a double. I updated the code above. — Muscovado, Nov 13 '13 at 09:47

Todd Ditchendorf · Accepted Answer · 2013-11-13T00:30:42.107

3

Developer of ParseKit here.

First, note: TDArithmeticParser is just some (not-terribly-robust) sample code included in the test bundle for ParseKit. It is not part of ParseKit itself.

Looks like this is a bug/deficiency in the TDArithmeticParser class where -15 is recognized as negative fifteen, not subtract fifteen.

If you add whitespace, this issue is resolved:

s = @"10+(2*3)- 15";

The opposite behavior is also possible with a small change, but then a stand-alone -15 would not be recognized as valid input (which may or may not be a problem for you).

Both could be supported simultaneously with some changes to the Grammar and the Assembler callbacks.

edited Nov 13 '13 at 00:30

answered Nov 12 '13 at 22:00

Todd Ditchendorf

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But is it really a problem of the TDArithmeticParser? Sadly for me changing the text that I want to parse is not much of a good option. I search for a better solution then to do some kind of "pre-parsing" to prevent this problem to occur. The BNF-grammar looks fine to me and the code corresponds to the grammar… Does the parser needs a further setup? I am just using what comes out of [PKParser parser] like the TDArithmeticParser does… – Muscovado Nov 13 '13 at 10:12

Arithmetic Parser

1 Answers1