Talk:Parsing/RPN to infix conversion: Difference between revisions

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Revision as of 05:49, 17 December 2011

Other examples

The Ruby Quiz article mentions the following two additional examples:

    $ ruby postfix_to_infix.rb '56 34 213.7 + * 678 -'
    56 * (34 + 213.7) - 678
    $ ruby postfix_to_infix.rb '1 56 35 + 16 9 - / +'
    1 + (56 + 35) / (16 - 9)

--Paddy3118 20:19, 3 December 2011 (UTC)

Examples Incorrect

Several examples (i.e. Python, and TCL) don't deal with associativity correctly - the code doesn't even refer to the attribute - it's defined and not used. I'm not sure of the Ruby code. The following "1 2 + 3 4 + ^ 5 6 + ^" should produce "( ( 1 + 2 ) ^ ( 3 + 4 ) ) ^ ( 5 + 6 )". --Dgamey 04:03, 17 December 2011 (UTC)

It's actually a matter of precedence rather than associativity. RPN per se doesn't need to worry about if an op is left or right associated, but for infix notation brackets are needed for ops at same precedence level. To wit: 1 2 3 + - is 1 - (2 + 3), not the same as 1 - 2 + 3. Generally the easiest way is wrap the expression with parens whenever you pop a binary op off the stack, which will be unsightly, but correct: 1 2 3 4 5 + + + + becomes (1 + (2 + (3 + (4 + 5)))) --Ledrug 05:49, 17 December 2011 (UTC)

@@ Line 8: / Line 8: @@
 == Examples Incorrect ==
 Several examples (i.e. Python, and TCL) don't deal with associativity correctly - the code doesn't even refer to the attribute - it's defined and not used.  I'm not sure of the Ruby code.  The following "1 2 + 3 4 + ^ 5 6 + ^" should produce "( ( 1 + 2 ) ^ ( 3 + 4 ) ) ^ ( 5 + 6 )".  --[[User:Dgamey|Dgamey]] 04:03, 17 December 2011 (UTC)
+: It's actually a matter of precedence rather than associativity.  RPN per se doesn't need to worry about if an op is left or right associated, but for infix notation brackets are needed for ops at same precedence level. To wit: <code>1 2 3 + -</code> is <code>1 - (2 + 3)</code>, not the same as <code>1 - 2 + 3</code>. Generally the easiest way is wrap the expression with parens whenever you pop a binary op off the stack, which will be unsightly, but correct: <code>1 2 3 4 5 + + + +</code> becomes <code>(1 + (2 + (3 + (4 + 5))))</code> --[[User:Ledrug|Ledrug]] 05:49, 17 December 2011 (UTC)