Math Expression Evaluation

python parsing math expression evaluation
Ori Shavit picture Ori Shavit · Oct 9, 2009 · Viewed 12.9k times · Source

What is the best way to implement a python program that will take a string and will output its result according to operator precedence (for example: "4+3*5" will output 19). I've googled for ways to solve this problem but they were all too complex, and I'm looking for a (relatively) simple one.

clarification: I need something slightly more advanced than eval() - I want to be able to add other operators (for example a maximum operator - 4$2 = 4) or, also I am more interested in this academically than professionaly - I want to know how to do this.

Answer

Martin B picture Martin B · Oct 9, 2009

If you're "academically interested", you want to learn about how to write a parser with operator precedence.

Simple Top-Down Parsing in Python is a nice article that builds an example parser to do exactly what you want to do: Evaluate mathematical expressions.

I can highly recommend having a go at writing your own first parser -- it's one of those "ah, that's how that works" moments!

Related questions

How do I parse a string to a float or int?

In Python, how can I parse a numeric string like "545.2222" to its corresponding float value, 545.2222? Or parse the string "31" to an integer, 31? I just want to know how to parse a float str to a float, and (separately) an int …

Read More
Why can't Python parse this JSON data?

I have this JSON in a file: { "maps": [ { "id": "blabla", "iscategorical": "0" }, { "id": "blabla", "iscategorical": "0" } ], "masks": [ "id": "valore" ], "om_points": "value", "parameters": [ "id": "valore" ] } I wrote this script to print all of the JSON data: import json from pprint import pprint …

Read More
How to parse data in JSON format?

My project is currently receiving a JSON message in python which I need to get bits of information out of. For the purposes of this, let's set it to some simple JSON in a string: jsonStr = '{"one" : "1", "two" : "2", "three" : "3"}…

Read More