# Expression trees

File: expression_trees.js (click for a live demo)

``````const math = require('../../index')

// Filter an expression tree
console.log('Filter all symbol nodes "x" in the expression "x^2 + x/4 + 3*y"')
const node = math.parse('x^2 + x/4 + 3*y')
const filtered = node.filter(function (node) {
return node.isSymbolNode && node.name === 'x'
})
// returns an array with two entries: two SymbolNodes 'x'

filtered.forEach(function (node) {
console.log(node.type, node.toString())
})
// outputs:
//  SymbolNode x
//  SymbolNode x

// Traverse an expression tree
console.log()
console.log('Traverse the expression tree of expression "3 * x + 2"')
const node1 = math.parse('3 * x + 2')
node1.traverse(function (node, path, parent) {
switch (node.type) {
case 'OperatorNode':
console.log(node.type, node.op)
break
case 'ConstantNode':
console.log(node.type, node.value)
break
case 'SymbolNode':
console.log(node.type, node.name)
break
default: console.log(node.type)
}
})
// outputs:
//   OperatorNode +
//   OperatorNode *
//   ConstantNode 3
//   SymbolNode x
//   ConstantNode 2

// transform an expression tree
console.log()
console.log('Replace all symbol nodes "x" in expression "x^2 + 5*x" with a constant 3')
const node2 = math.parse('x^2 + 5*x')
const transformed = node2.transform(function (node, path, parent) {
if (node.isSymbolNode && node.name === 'x') {
return new math.expression.node.ConstantNode(3)
} else {
return node
}
})
console.log(transformed.toString())
// outputs: '3 ^ 2 + 5 * 3'

``````