Skip to content

Basic Examples

Simple programs demonstrating fundamental Onion features.

Hello World

The simplest Onion program:

File: Hello.on 

println("Hello")

Run: 

onion Hello.on

Output: 

Hello

Arrays

Working with arrays:

File: Array.on 

val array: String[] = new String[3]
array[0] = "A"
array[1] = "B"
array[2] = "C"

for var i: Int = 0; i < array.length; i = i + 1 {
  println(array[i])
}

Topics: - Array declaration and initialization - Index-based access - For loop with .length property

String Concatenation

String operations and list iteration:

File: StringCat.on 

val list: List[String] = ["a", "b", "c", "d", "e", "f", "g"];
for var i: Int = 0; i < list.size; i = i + 1 {
  System::out.println("list[" + i + "] = " + list[i]);
}

Topics: - Array literals - String concatenation with + - .size property - System output

User Input

Reading user input:

File: ReadLine.on 

val name: String = IO::readln("What's your name? ")
println("Hello, " + name + "!")

Run: 

onion ReadLine.on

Interaction: 

What's your name? Alice
Hello, Alice!

Topics: - IO::readln() for reading input - String concatenation - Interactive programs

Dynamic Lists

Using ArrayList with the << append operator:

File: List.on 

val list: List[String] = [];

list << "a";
list << "b";
list << "c";
list << "d";

for var i: Int = 0; i < list.size; i = i + 1 {
  System::out.println(list[i]);
}

Topics: - Importing Java classes - ArrayList creation - << append operator - Index-based access on collections

Foreach Loop

Enhanced for loop with collections:

File: Foreach.on 

import { java.util.ArrayList; }

val list: ArrayList[String] = new ArrayList[String]
list << "HELLO";
list << "WORLD";
list << "ONION";

foreach object :String in list {
  println(object.toLowerCase())
}

Output: 

hello
world
onion

Topics: - foreach loop syntax - Type annotations in loops - Method calls on loop variables

Pattern Matching

Using the select statement:

File: Select.on 

val value: Int = (Math::random() * 10) as Int

select value {
  case 0, 1, 2, 3:
    println("Low: " + value)
  case 4, 5, 6:
    println("Medium: " + value)
  case 7, 8, 9:
    println("High: " + value)
  else:
    println("Out of range: " + value)
}

Topics: - select statement - Multiple values per case - else default clause - Type casting with $ - Math::random() function

Complete Example: Array Processing

Combining multiple concepts:

import { java.util.ArrayList; }

// Create numbers list
val numbers: java.util.List[Int] = [10, 20, 30, 40, 50]

// Filter even numbers
val evens: ArrayList[Integer] = new ArrayList[Integer]
foreach num :Int in numbers {
  if num % 2 == 0 {
    evens << num
  }
}

// Print results
println("Even numbers:")
foreach even :Object in evens {
  println((even as Int))
}

Output: 

Even numbers:
10
20
30
40
50

Complete Example: Simple Calculator

val x: Int = 10
val y: Int = 3

println("Addition: " + (x + y))
println("Subtraction: " + (x - y))
println("Multiplication: " + (x * y))
println("Division: " + (x / y))
println("Modulo: " + (x % y))

Output: 

Addition: 13
Subtraction: 7
Multiplication: 30
Division: 3
Modulo: 1

CSV Processing

Parse CSV data and compute a simple aggregate:

File: CsvProcessor.on 

import {
  java.lang.Integer as JInt;
}

class CsvProcessor {
public:
  static def main(args: String[]): String {
    val input = "name,score\nAlice,85\nBob,92\nCharlie,78\nDiana,95\n"
    val rows = Csv::parseWithHeader(input)

    var total: Int = 0
    var count: Int = 0
    for var i: Int = 0; i < rows.size(); i = i + 1 {
      val row = rows.get(i)
      val scoreText = row.get("score")
      if (scoreText != null) {
        total = total + JInt::parseInt(scoreText)
        count = count + 1
      }
    }

    var average: Int = 0
    if (count > 0) {
      average = total / count
    }
    return "Processed " + count.toString() + " rows, average score = " + average.toString()
  }
}

Topics: - CSV parsing with Csv::parseWithHeader - Iterating over a List<Map<String, String>> - Parsing strings to integers with JInt::parseInt - Null-safe field access

Regex Log Parsing

Extract numeric values from text with regular expressions:

File: RegexLogParser.on 

import {
  java.lang.Integer as JInt;
}

class RegexLogParser {
public:
  static def main(args: String[]): String {
    val logs = Colls::mutableListOf(
      "[INFO] request took 45ms",
      "[WARN] request took 120ms",
      "[INFO] request took 30ms"
    )

    val pattern = "\\d+ms"
    var total: Int = 0
    var count: Int = 0

    for var i: Int = 0; i < logs.size(); i = i + 1 {
      val line = logs.get(i)
      val match = Regex::findFirst(line, pattern)
      if (match.length() > 0) {
        val msText = match.substring(0, match.length() - 2)
        total = total + JInt::parseInt(msText)
        count = count + 1
      }
    }

    var average: Int = 0
    if (count > 0) {
      average = total / count
    }
    return "Average response time: " + average.toString() + "ms"
  }
}

Topics: - Regex::findFirst for pattern matching - String slicing with substring - Aggregating values from a list

Next Steps