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Talk:Parsing/RPN to infix conversion: Difference between revisions
Talk:Parsing/RPN to infix conversion (view source)
Revision as of 20:51, 17 December 2011
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::: Sorry I thought what you were talking about is if an operator is commutative? --[[User:Dgamey|Dgamey]] 18:19, 17 December 2011 (UTC)
:::: The RPN code shouldn't try to use commutativity, as that causes problems in situations where there are side-effects. (OK, not in ''this'' example, but in general.) While a few languages allow optimizers to use it (notably [[C]] and [[C++]]), most don't because it causes much confusion; overall, language designers have found it better to be more predictable than that (where side-effects are present at all, of course). –[[User:Dkf|Donal Fellows]] 18:59, 17 December 2011 (UTC)
:::: Hmm? I didn't say anything about commutativity. It has nothing to do with this task. What's of concern is the execution order of operators, that is, given some arbitrary operator *, if <code>a * b * c</code> needs parentheses to be unambiguous. If * is associative (in the strict sense), <code>(a * b) * c = a * (b * c) = a * b * c</code>, and no parens are really required; commutativity is completely independent of this. Subtraction "-" is not commutative, while exclusive-or "xor" is, but neither is associative. It wouldn't have mattered had there not been the (rather pointless) "minimal parens" requirement, but as it is, some clarification is needed as to what constitutes "minimal". Currently for example the Tcl code would convert <code>1 2 3 + +</code> into <code>1 + (2 + 3)</code>, which is fine by me--but I'm normally not the one who demands the output to match some random standard to the last letter. --[[User:Ledrug|Ledrug]] 20:51, 17 December 2011 (UTC)
: Tcl version is now fixed (it would have helped if the original example had been sensitive to such things, but there you go). –[[User:Dkf|Donal Fellows]] 18:59, 17 December 2011 (UTC)
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