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    <p>Well, you need to determine if a given operator is binary/unary during the <em>parsing</em> stage. Once you do that, when you create the RPN, you can tag the operators as taking 2 or 1 arguments.</p> <p>You could try using the <a href="http://en.wikipedia.org/wiki/Shunting_yard_algorithm" rel="noreferrer">Shunting Yard</a> algorithm to do the parsing (and creation of RPN at the same time), which was designed to work with unary operators too.</p> <p>In any case, if all you care about is unary + or -, you could just insert a 0 with brackets when you see a + or - that appears 'unexpectedly'.</p> <p>For instance</p> <p><code>+1 + +2 - -3 - -4</code></p> <p>You should be able to make a pass through it and convert to</p> <p><code>(0+1) + (0+2) - (0-3) - (0-4)</code> </p> <p>Now you don't need to worry about the unary + or - and can probably forget about tagging the operators with the number of arguments they take.</p>
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    1. COYes, I had implemented the shunting yard algoritm but it, as you say, requires that you have already identified the - and the + as either unary +, - or the binary variations. I ended up using some changed parsing code to beter decide what I am dealing with. I do not want to change the actual expression because I need to write back the parsed expression again. Still, I have some troubles deciding when to write back unary operators to the RPN output. I'll have another look at the algorythm.
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    2. CO@Jaapjan. RPN evaluation needs to know how many arguments an operator takes. The proper way to do this would be with a grammar which can tell when + is unary and when it is binary etc. btw, why does it matter if you include zeroes in the written back expression? Since you have it tagged as self-learning, it seems like that should be irrelevant.
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    3. COIt is self-learning because I am trying to learn how to read expressions in my source code using a program. If my program writes back 'cleaned' expressions then I prefer not to have extra zero's in them. Unfortunately it also needs to deal with variable number of parameters to functions. Hence trying to figure out how others deal with this situation. I presume for example -1--1 would become 1 neg 1 neg minus in this instance. But I do not find any clear way to deal with nested expressions with variable length functions in them... mmm.
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