# Basic Expression Parsing

**URL:** <https://forum.kirupa.com/t/basic-expression-parsing/255520>\
**Category:** open source\
**Created:** [March 24, 2008, 10:58am UTC](https://forum.kirupa.com/t/basic-expression-parsing/255520 "2008-03-24T10:58:39Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![h00d](https://avatars.discourse-cdn.com/v4/letter/h/22d042/32.png) [@h00d](https://forum.kirupa.com/u/h00d)\
**Post date:** [March 24, 2008, 10:58am UTC](https://forum.kirupa.com/t/basic-expression-parsing/255520/1 "2008-03-24T10:58:39Z")

</div>

I am working on an article which covers how algorithms translate into code. The two algorithms I’m using are a variation of [Dijkstra’s “Shunting Yard Algorithm” for the postfix to infix conversion, and the [URL=“http://en.wikipedia.org/wiki/Reverse\_Polish\_notation#The\_postfix\_algorithm”]postfix evaluation](http://en.wikipedia.org/wiki/Shunting_yard_algorithm#The_algorithm_in_detail). This is also a good exercise for people new to data structures. A little stack-based arithmetic to play with too.

The source is in a state for people to play with and I’ve managed to keep the code pretty readable.

The code is attached, but here is the usage.

```auto

trace( new ExpressionParser().parse( "(1+(100/10))*3" ) ); // 33

```

The features are basic (to say the least):

- addition, subtraction, multiplication, division
- grouping
- multi-digit operands

If I (or you) were to take this any further I’d consider:

- negative operands from input (ie. -3+4)
- exponents
- basic math functions (ie. cos(), sin(), round(), floor(), whatever())

(attaching didn’t work … I don’t know why)  
[http://iamavila.com/dontdelete/expression\_parser.zip](http://iamavila.com/dontdelete/expression_parser.zip)

- I included the Polygonal data structures … more than was necessary, sorry I’m lazy.