apainintheneck / simple-math-parser

A math expression parser built in Java.

Simple Math Parser

This a simple math expression parser built in Java. If you want to try it out for yourself, build and run CalcApp.java in the mathTree package. It will open a console program where you can enter a math expression in plain text.

How MathTree Class Works

Essentially, the MathTree data structure takes a string and builds a parsing tree. If it fails because the input string is an invalid math statement, it deletes the tree and returns false.

1. Scanning The String

This breaks up the string into tokens which can be used to build the tree later on.

First Pass

The first pass breaks up the string based upon delimiters, in this case whitespace characters, and special characters, in this case operators and parenthesis. This step is executed by strScanner in MathTree. It returns a list of strings.

5+5 * 85 --> {"5", "+", "5", "*", "85"}

Second Pass

The second pass takes the list of strings and handles special cases. It discerns the difference between subtraction and negation symbols. It also applies implicit multiplication with parenthesis.

5--5 --> {"5", "-", "-5"}

5- 5 --> {"5", "-", "5"}

5 -5 --> {"5", "-5"}

5(6) --> {"5", "*", "(", "6", ")"}

5+ (6) --> {"5", "+", "(", "6", ")"}

(7+6) 4 --> {"(", "7", "+", "6", ")", "*", "4"}

2. Building The Tree

The tree is built according to math precedence rules. Expressions are solved from left to right unless the operation on the right has higher precedence.

Math Expression Nodes

The operator nodes have links to a left and right node. Number nodes, Int and Dec, do not have links to other nodes. The nodes have built in recursive operations for calculate(), toString(), clone(), and checkTree().

Expression.calculate(): Calculates result of all the operations below this node and returns a Number object.

Expression.toString(): Returns the string value of the entire tree below this node.

Expression.clone(): Returns a clone of the entire tree below this node.

Expression.checkTree(): Returns a bool based upon if the tree has been constructed correctly.

Inserting Nodes

Nodes are always inserted in the right side of the tree because math expression are always evaluated left to right. Operators are compared starting from the top of the tree with other operator nodes to find the right spot based upon precedence. Number nodes are added to the rightmost bottom part of the tree.