Jezen Thomas

Jezen Thomas Jezen Thomas

I'm a Haskell/Elm programmer, the founder of NBM, co-founder of Comparestack, and a public speaker. Occasionally I write about technology, and currently I am perpetually travelling the world. Find me on Instagram, or on GitHub, or write me an email.

Io Day Two

In the second day of Io language studies, we looked at control structures and how to define custom operators. We also had a deeper look at how message sending works, and how to reflect on an object’s prototype chain.

  1. Things to do
  2. The Fibonacci sequence
  3. Divide by zero
  4. List comprehension
  5. Extending objects
  6. Matrix factory
  7. Reading and writing files
  8. Higher or lower
  9. Thoughts

Things to do

The Fibonacci sequence

A Fibonacci sequence starts with two 1s. Each subsequent number is the sum of the two numbers that came before. Write a program to find the nth Fibonacci number.

I’m absolutely not a maths guy, but I’ve tackled the Fibonacci sequence before on Project Euler and I learned it has a close relationship with the Golden Ratio. Any number in the sequence can be solved using Phi, which to 15 decimal places is 1.618033988749895.

The question originally calls for solutions using loops and recursion, so I may be cheating slightly here. All that needs to be done is to create a method that accepts an integer, and applies the formula fn = Phin / 5½.

Fib := Object clone
Fib Phi := 1.618033988749895
Fib get := method(n, ((Phi ** n) / (5 ** .5)) round)

Divide by zero

How would you change / to return 0 if the denominator is zero?

Overriding default behaviour in Io is surprisingly easy; you can just assign values and methods to operators like any other variable. In this case, we want to retain the original behaviour of the division operator, so we store it in an intermediate variable before assigning our custom implementation.

Number oldDivision := Number getSlot("/")
Number / = method(n, if(n == 0, 0, self oldDivision(n)))

List comprehension

Write a program to add up all of the numbers in a two-dimensional array.

My first thought was to use a nested loop (loopception?), but a less obvious and more elegant solution is to iron out the lists and apply the addition with a couple of chained messages.

Io> list(1,2,list(3,4,5),6,7,list(8,9)) flatten reduce(+)
==> 45

Extending objects

Add a slot called myAverage to a list that computes the average of all the numbers in a list. What happens if there are no numbers in a list?

This is an odd task; sending the slotSummary message to List shows that we already have the message average, which works exactly as you would expect it to.

One potential issue with Io’s core average method however is that it throws an exception if it encounters anything that isn’t a number. Our implementation will only look for numbers and average those. If no numbers are found, it returns 0.

List myAverage := method(
  numbers := self select(isKindOf(Number))
  if (numbers size == 0, return 0)
  numbers sum / numbers size

Matrix factory

Write a prototype for a two-dimensional list. The dim(x, y) method should allocate a list of y lists that are x elements long. The set(x, y, value) method should set a value, and get(x, y) should return that value.

Again, sending slotSummary to List gives us a good starting place. We can use the setSize(n) method to fill a list with (n) nil elements. My approach is to create a list x elements long, and map that list with more lists.

The get and set methods are handled with at() and atPut(). I decrement the indexes because in most applications I can think of, e.g., Chess and Battleships, matrices aren’t zero-based.

Matrix := List clone
Matrix dim := method(x, y, self setSize(x) mapInPlace(list() setSize(y))
Matrix get := method(x, y, self at(x-1) at(y-1))
Matrix set := method(x, y, value, at(x-1) atPut(y-1, value))

Reading and writing files

Write the matrix to a file, and read a matrix from a file.

Working with files in Io is straight-forward and similar to Ruby (and perhaps any other language). Since my Matrix object’s prototype is the List object, I already have a bunch of helper methods for iterating over my matrix. I used simple concatenation to write my matrix as a CSV, and the split method to convert the data back to lists.

Matrix save := method(path,
  file := File with(path) openForUpdating
  self foreach(line,
    file write(line first)
    line rest foreach(value, file write("," .. value))
    file write("\n")
  file close

Matrix load := method(path,
  file := File with(path) openForReading
  file foreachLine(line, self append(line split(",")))
  file close

// Usage
myMatrix save("matrix.csv")
newMatrix := Matrix clone
newMatrix load("matrix.csv")

Higher or lower

Write a program that gives you ten tries to guess a random number from 1 – 100. Give a hint of “hotter” or “colder” after the first guess.

This last challenge is not quite as difficult; it’s essentially an Io adaptation of the guessing game implemented in Ruby from the previous chapter.

"Enter a number…" println
target := (Random value(99) + 1) floor
10 repeat(
  guess := File standardInput readLine asNumber
  if (guess == target, break, "Try again." println)
  if (hasSlot("previousGuess"),
    if ((guess - target) abs < (previousGuess - target) abs,
      "You’re getting warmer…" println,
      "You’re getting colder…" println
  previousGuess := guess

if (guess == target,
  "You win!" println,
  "Better luck next time…" println


After this truly extensive set of exercises, I have a more comprehensive understanding of how Io works. I also have a more comprehensive sense of frustration with Io, since the syntax is confusing and the documentation is sparse. I found that the best documentation is probably the core library referece. As of yet, I’m not sure if Io does anything better or more easily than Ruby does, so I’m unlikely to actually use Io for anything once I’ve finished this chapter.