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Blocks of code and lambdas

This activity has the following desired goals:

  • Getting to know about blocks of code (A).
  • Learning about the Unit value and type (A).
  • Learning how commands return the Unit value (A, M).
  • Learning about blocks of code that take an input (A, M).
  • Learning about lambdas (A, M).
  • Using repeatFor and lambdas to work with numbers (M, T).

Step 1

Type in the following code and run it:

val x1 = 3 * 10 + 2
println(x1)

val x1b = {
    val a = 10
    val b = a * 3
    b + 2
}
println(x1b)
// println(a)

Q1a. What’s the difference in the calculation done for computing x1 and x1b?

Q1b. In the calculation done for x1b, do you see how the curly brackets (which create a block of code), allow you to:

  • use multiple steps in the calculation?
  • introduce named values to help with the calculation?

Q1c What’s the result of a block of code? If it has three instructions, one after the other, which produce values v1, v2, and v3, what’s the value of the whole block?

Q1d Do you see how the value of a block of code is the value of the last instruction in it?


Step 2

Type in the following code and run it:

val x2 = { 
    clear()
    forward(100)
    right(90)
}

println(x2)

Q2a. What’s the value of x2?


Step 3

Type in the following code and run it in Worksheet mode via Shift+Enter:

val x3 = right(90)

Q3a. What’s the value of x3? What’s its type?


Step 4

Type in the following code and run it in Worksheet mode via Shift+Enter:

val x4 = forward(100)

Q4a. What’s the value of x4? What’s its type?

Q4b. Do you see how (all) commands return the value (), whose type is Unit?
The unit value () is meant to signify no information - and this makes sense based on what you have learned earlier - that commands carry out actions but do not return any information, as opposed to functions, which take in information and return new information.


Step 5

Type in the following code and run it (via Ctrl+Enter again):

val x5 = { a: Int =>
    val b = a * 3
    b + 2
}

println(x5(10))

Q5a. Do you see how the block of code above takes an input, and does a calculation based on this input?


Step 6

Type in the following code and run it:

def x6(a: Int) = {
    val b = a * 3
    b + 2
}

println(x6(10))

Q6a. What’s the similarity between x5 and x6?

Q6b. What’s the difference between x5 and x6?

Notice that in Step 5, first an un-named block of code that takes an input is created, and then it is given the name x5 via the val keyword instruction. In Step 6, a named block of code that takes an input is directly created via the def keyword instruction – with the name x6?

The named block of code above cannot exist without its name. But the un-named block of code can. To convince yourself of this, type in the following code and run it via Shift+Enter:

(
    { a: Int =>
        val b = a * 3
        b + 2
    }
)(10)

Do you see how the un-named block of code that takes an input is used above without giving it an intermediate name.

A named block of code that takes an input is called a user-defined command or function, while an un-named block of code that takes an input is called a lambda or an anonymous command or function.


Step 7

Type in the following code and run it:

clearOutput()

val ab = rangeTo(3, 10, 2)

def printInt(n: Int) {
    println(n)
}

repeatFor(ab)(printInt)

repeatFor(ab) { n =>
    println(n)
}

Q7a. Describe what the above code is doing.

Q7b. Do you see that the second input to repeatFor is a block of code that takes one input? Why does repeatFor want such an input?

Q7c. Can you identify the named code block in the code above? Can you identify the un-named code block?

Q7d. What’s the benefit of a named block of code (user-defined command or function)? What’s the drawback?

Q7e. What’s the benefit of an un-named block of code (anonymous command or function)? What’s the drawback?


Exercise

Write a program to print out all the positive integers below 20 that are divisible by 3.


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