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Lesson 3 - Conditionals and Randomisation

If we want to make some meaningful and interesting musical structures, we need to learn some meaningful and interesting programming structures.

Learning Objectives

  • Know that computer programs can make decisions, and that a simple form of decision is called a conditional.
  • Understand that computer programs can contain random acts.
  • Be able to use comments to explain interesting parts of a program.

Learning Outcomes

All students will be able to:

  • Play a random note.
  • Use an if statement.
  • Place a comment in their code.

Most students will be able to:

  • Play random notes with varying randomness (by supplying a different argument to rand).
  • Write useful comments.
  • Modify code in the if branches to create different execution paths.

Some students will be able to:

  • Use rand to sleep for a random amount of time.
  • Understand that using rand as a test with < allows for different probability distributions other than 50/50.
  • Nest if statements inside other if statements.
  • Understand that comments are about communicating intent.

Lesson Summary

  • Simple use of randomisation.
  • Using conditionals to make decisions.
  • Combining randomisation and conditionals.
  • Comments as a programming tool.


Pupils are first invited to set up and connect their Raspberry Pi hardware. They may then load the Sonic Pi application and find their work from the previous lesson. Through questioning, select students to explain what they did in the previous lesson.

Main Development

  1. Pupils should be should be shown how to add some randomisation to their code. This can be achieved by using the statement rand(10), which returns a random value between 0 and 10 (from 0 up to but not including the number you specify). You can specify other numbers for larger ranges; for instance, rand(20) will return values from 0 to 20. Let’s use this in our program by adding our random number to a note with the + operator:

    3.times do
      play 60 + rand(10)
      sleep 0.5

    Invite the pupils to observe the actual number of the note played in the output window.

  2. Pupils are then asked to form a line as with previous lessons, except this time the line should split into two separate lines (called branches) at one point. Give out the Conditional Statement Cards. The pupil directly before the split should be given the special if card, and the rest of the pupils should be given cards as with previous exercises. The control card should start at the first pupil, and is passed down as each pupil carries out the action of their card. When it gets to the pupil with the if card, they should toss a coin. If it’s heads, the control card should be passed to the first of the two separate lines; otherwise the control card should be passed to the second of the two lines. Once the control card is in one of the separate lines it continues until the end of that line, then the program is terminated. It should be pointed out that the line that didn’t get the control card passed to it was essentially ignored. The if statement is called a conditional and allows for decisions to be made in the program.

  3. Pupils should then be shown how to write an if statement in the Sonic Pi application. Ask them to copy the following code into their worksheet:

    if rand < 0.5
      play 60
      sleep 0.5
      play 62
      play 72
      sleep 0.25
      play 71
      sleep 0.25
      play 70
  4. Once they have copied this code, point out the syntax of the if statement, specifically the words else and end. These are similar to the do and end found in the iteration block discussed in the previous lesson, in that they’re like punctuation. The else separates the two different branches of the if statement.

  5. The first line should also be discussed; this is equivalent to a coin toss in that rand(1) returns a random value between 0 and 1, and we’re testing to see if that random value is less than 0.5. For the advanced pupils, you may wish to point out that changing the 0.5 to different values will affect the probability of which branch is selected. For example, a value of 0.1 would mean that (on average) every 10 runs the first branch be selected only once, and the second branch will be selected 9 times.

  6. The pupils are then invited to press the play button a number of times so that they can hear the different branches being executed, the decision of which branch to execute being random each time.

  7. Pupils are invited to play around with the constructs of this lesson, in addition to everything they’ve learned so far, to design a simple musical program.


Finally, teach the class that the hash symbol # is used to make a comment. Invite them to place comments in their code to explain what is happening. This is not just for other programmers who might read their code; it is also for themselves in the future, when they look back at old code they may have written a long time ago and have forgotten what it does. For example:

# Toss a virtual coin
if rand(1) < 0.5
# if heads, play two ascending notes
  play 60
  sleep 0.5
  play 62
  # if tails, play three descending notes
  play 72
  sleep 0.25
  play 71
  sleep 0.25
  play 70


Students may research electronic-sounding music and think about how they could improve the sound of their own composition.