How would I differentiate -5-5 from (-5)(-5)

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the title may sound confusing(I don't know a better way of phrasing it)

but I've got a calculator and I realized a big error in it , it cant accept '-' as it belives they are negative numbers: snippet of the space adding regex:

C++

regex qret("ATAN|SIN|ACOS|LOG|ASIN|sqrt|CIR|sin|TAN|!|e|ln|EE|\\^|[^-?0-9a-z]",regex_constants::icase);

name = regex_replace( name, qret, " $& " ) ;

any way to not have -5-5 turn into -5 -5? instead -5 - 5?

What I have tried:

I've tried removing -? but it just disallowed negative numbers and allowed the statement, not very practical.

Posted 18-Jan-22 11:33am

Abdulrahmon Tijani

Updated 19-Jan-22 5:08am

Rick York

v2

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Rick York 18-Jan-22 17:38pm

In C the binding of the minus sign with the value versus considering it an operation is a matter of precedence. That means you have to either implement precedence in your calculator or do something like require spaces to separate the values and operators.

I don't consider this a solution because I can't tell you how to implement precedence.

Luc Pattyn 18-Jan-22 18:14pm

I've never encountered a regex in the implementation of a calculator.
The typical approach is:
1. turn the input into RPN (reverse Polish notation)
2. evaluate what you have using a stack, where you either immediately execute an operator or push it onto the stack depending on precedence.

Obviously step 1 is to be skipped if you are mimicking an RPN calculator (such as HP35).

Abdulrahmon Tijani 18-Jan-22 18:39pm

@lucpattyn i do turn it in to rpn and do the precedences, i just need to give the expression spaces i order to tokenize the string

Luc Pattyn 18-Jan-22 20:11pm

if your tokenizer needs spaces, I'd suggest not to care about monadic minus. Simply threat a monadic minus as any other (dyadic) operator, so let it turn "-5" into "- SP 5" the same way it turns "+5" into "+ SP 5".

During the evaluation when you are expecting an operand and you get a minus sign, you only then must realize it is a monadic minus, not a dyadic minus, and act accordingly.

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Greg Utas · Answer 1 · 2022-01-19T05:08:00

This ambiguity is the reason many calculators have a - key for subtraction and a +/- key for changing a number's sign. If a number is currently displayed and + or - is entered, how do you know whether it should be a binary (addition or subtraction) or unary (change sign) operation? You could expect the user to put parentheses around a number when a unary operation is intended. But that's not very user friendly, because it's too late once a result is already on the display and you then want to change its sign.