c expression evaluator take 2

Question

In reference to the accepted solution in: SO:expression_evaluator

Can anyone provide a version that works with negation as well? things like

((!(0 or !1) and !((0 or 1 or 1) and !1))

need to work as well.

I got it working so that negating the 0's or 1's is fine but I can't get it to work with the negation of whole groups(!'s at beginning of parenthesis)

I tried negating tmp after returning from eval in the *expr == '(' block.. if ! had been seen before calling it, but that didn't work.

I do it: http://stackoverflow.com/questions/1465909/c-expression-evaluator/1472960#1472960 — sambowry, Oct 29 '09 at 06:52

score 6 · Accepted Answer · answered Oct 28 '09 at 23:31

6

The code in that answer is a bit ad-hoc. Parsing is a well-understood field, there's no need for invention. One of the other answers there recommends looking into recursive descent parsing. I second that recommendation.

answered Oct 28 '09 at 23:31

Ned Batchelder

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+1 - there's nothing more to say, except that one should build something like an AST which can then easily be computed. – Lucero Oct 29 '09 at 00:13

score 0 · Answer 2 · answered Oct 29 '09 at 10:11

The algorithm described here can deal with unary operators (operators having one operand), as well as functions.

I once used it as the basis for a complete C expression evaluator with support for symbolic variables for a command-line console on an embedded system. I did not use the accompanying example code, but rather wrote it from scratch to suit my purposes.

c expression evaluator take 2

2 Answers2