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Evaluating arithmetic expressions in configuration files

An interesting question came up in a consultants session this evening - how to take an expression that's held in a configuration file such as (A3+C3)*(A5+E6) and have the program work out the expression in the "conventional" programming sense - i.e. BODMAS (Brackets, O???? Division, Multiplication, Addition, Subtraction).

Now - I learnt how to do this at University and I even did it way back in CGL days when I wrote a complete geometric language - the algotithm is known as a Dijkstra Shunt - but I was blowed if I wanted to try to rewrite it this evening. My visitor had a Visual Basic solution which involves him using a .dll file from Office, but his customers weren't happy having to install office as a pre-requisite to running his software. It was wanted on a web site, so how to do it in PHP?

When you think about it, all the languages we use these days already have the Dijkstra shunt built into them .... we just need to make use of it. In the case of PHP, it's available via the eval function and that's what we ended up using:

a) Take the expression and use a regular expression to extract each of the variables in turn
b) Substitute the variable with the value that it contains (that's a database lookup in my example, since C3 means the third column, 3rd row of a table
c) Use the eval function to do the sum.

Total - a few hundred bytes of code ... (see here) ... and a visitor who left feeling that in an hour I had given him the mechanism to take a complete step and an expensive piece of software out of his system.
(written 2006-05-10, updated 2006-06-05)

 
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