Expr Evaluation Tool Guide

The eval tool allows you to evaluate expressions using the powerful expr language. This guide demonstrates various examples to showcase its capabilities.

Basic Operations

Arithmetic Operations

eval("10 + 5")         // Result: 15
eval("10 - 5")         // Result: 5
eval("10 * 5")         // Result: 50
eval("10 / 5")         // Result: 2
eval("10 % 3")         // Result: 1
eval("2 ^ 3")          // Result: 8 (exponentiation)

String Operations

eval("'Hello' + ' ' + 'World'")  // Result: "Hello World"
eval("len('Hello')")             // Result: 5

Boolean Operations

eval("true and false")           // Result: false
eval("true or false")            // Result: true
eval("!true")                    // Result: false
eval("10 > 5")                   // Result: true
eval("10 == 10")                 // Result: true
eval("'a' in ['a', 'b', 'c']")   // Result: true

Working with Arrays

Array Creation and Access

eval("[1, 2, 3, 4, 5]")          // Result: [1, 2, 3, 4, 5]
eval("[1, 2, 3][1]")             // Result: 2 (zero-based indexing)

Array Functions

eval("len([1, 2, 3, 4, 5])")     // Result: 5
eval("map([1, 2, 3], # * 2)")    // Result: [2, 4, 6]
eval("filter([1, 2, 3, 4], # > 2)") // Result: [3, 4]
eval("all([1, 2, 3], # > 0)")    // Result: true
eval("any([1, 2, 3], # > 2)")    // Result: true
eval("sum([1, 2, 3, 4])")        // Result: 10

Sorting Arrays

eval("sort([3, 1, 4, 2])")       // Result: [1, 2, 3, 4]

Working with Maps/Objects

Map Creation and Access

eval("{a: 1, b: 2, c: 3}")       // Result: {"a": 1, "b": 2, "c": 3}
eval("{a: 1, b: 2}.a")           // Result: 1

Complex Examples

Filtering and Transforming Data

users = [{"Name": "John", "Age": 30}, {"Name": "Ivan", "Age": 51}, {"Name": "Eve", "Age": 15}]
eval("filter(users, .Age >= 18)")  
// Result: [{"Name": "John", "Age": 30}, {"Name": "Ivan", "Age": 51}]

eval("map(users, {name: .Name, isAdult: .Age >= 18})")
// Result: [{"name": "John", "isAdult": true}, {"name": "Ivan", "isAdult": true}, {"name": "Eve", "isAdult": false}]

Sorting Complex Data

users = [{"Name": "John", "Age": 30}, {"Name": "Ivan", "Age": 51}, {"Name": "Eve", "Age": 15}]
eval("sortBy(users, .Age, 'desc')")
// Result: [{"Name": "Ivan", "Age": 51}, {"Name": "John", "Age": 30}, {"Name": "Eve", "Age": 15}]

eval("sortBy(users, .Name)")
// Result: [{"Name": "Eve", "Age": 15}, {"Name": "Ivan", "Age": 51}, {"Name": "John", "Age": 30}]

Aggregation Functions

products = [{"Name": "Apple", "Price": 0.5, "Quantity": 10}, {"Name": "Banana", "Price": 0.3, "Quantity": 15}, {"Name": "Orange", "Price": 0.6, "Quantity": 8}]
eval("sum(map(products, .Price * .Quantity))")
// Result: 13.3 (total value of inventory)

eval("reduce(products, 0, acc + .Price * .Quantity)")
// Result: 13.3 (same calculation using reduce)

Conditional Logic

users = [{"Name": "John", "Age": 30}, {"Name": "Ivan", "Age": 51}, {"Name": "Eve", "Age": 15}]
eval("map(users, {name: .Name, status: .Age < 18 ? 'minor' : (.Age >= 65 ? 'senior' : 'adult')})")
// Result: [{"name": "John", "status": "adult"}, {"name": "Ivan", "status": "adult"}, {"name": "Eve", "status": "minor"}]

String Manipulation

eval("upper('hello')")           // Result: "HELLO"
eval("lower('HELLO')")           // Result: "hello"
eval("trim(' hello ')")          // Result: "hello"
eval("split('a,b,c', ',')")      // Result: ["a", "b", "c"]

Date Functions

eval("now()")                    // Result: current timestamp
eval("now().Year")               // Result: current year
eval("now().Format('2006-01-02')") // Result: current date formatted as YYYY-MM-DD

Advanced Features

Function Composition

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
eval("sum(filter(numbers, # % 2 == 0))")
// Result: 30 (sum of even numbers)

Custom Environment Variables

You can provide custom variables in the environment:

// With env: {"x": 10, "y": 20}
eval("x + y")                    // Result: 30

Regular Expressions

eval("match('hello world', 'hello')")  // Result: true
eval("replace('hello world', 'hello', 'hi')") // Result: "hi world"

Tips for Using the Eval Tool

1. Use single quotes for strings within the expression
2. Use dot notation to access object properties
3. Use # as a placeholder in map/filter functions
4. Remember that array indexing is zero-based
5. For complex expressions, break them down into smaller parts

This guide covers many of the powerful features of the expr language. For more details and advanced usage, refer to the official documentation.