Objective

The objective is to reinforce the class knowledge of numbers and simple sums in Go and to reinforce the skills needed to create and run a Go program.

The pupils are introduced to brackets and shown how to use them to group terms. The pupils are also introduced to the idea of operator precedence.

Creating this program will require the class to use the text editor (either Atom or LiteIDE) to create and save the source code file and the terminal/command line to run their program.

Numbers Reinforcement

This lesson reinforces and extends the concepts introduced in the previous numbers lesson. The lesson extends by introducing brackets to group terms and operator precedence.

The lesson reinforces by asking the pupils to type in another program bolstering the skills and approach needed to create and run a Go program.

Learning outcome

The class should be able to:

  • Start their text editor
  • Create a source code file
  • Save a source code file
  • Build and run their program
  • Understand how to type simple mathematics in Go including expression using brackets.
  • Reason about the result of mathematics in Go including expression using brackets.
  • Understand that the program represents a sequence
  • Understand that the sequence executes in order

Teaching prerequisites

The class needs to have covered.

  • Basic keyboard/typing and mouse skills.
  • The class needs to understand the four basic mathematics operators and how to use them.
  • The class needs to understand numbers and how to read and write them them, from units to thousands and ideally tens of thousands.
  • The class need to understand that mathematics can be chained together. So example 1 + 2 + 3 + 4 + 5 = 15
  • The class should have been introduced to brackets and the laws of association and commutative for addition and multiplication.

Sequences. The program demonstrates a sequence of instructions that are executed in order to print the results to the terminal window.

There is a direct link to mathematics, both in terms of number theory and simple mathematics.

Sums in Go

Notes for slide 2

This is a recap of the important points from the numbers lesson.

The symbols are:

  • addition is a +
  • subtraction is a -
  • multiplication is a *
  • division is a /

The sums would be written as:

    1 3 + 4
    2 6 - 1
    3 5 * 6
    4 10 / 2

in Go.

Operator order

Notes for Slide 3

As per maths, in a sum without any brackets, multiplication occurs before addition or subtraction and division occurs before subtraction or addition. Go follows the same rules.

Therefore the answers to the sums are

3 + 2 * 5 is 3 + (2 * 5) = 13
4 * 4 + 2 is (4 * 4) + 2 = 18
25 - 20 / 4 is 25 - (20 / 4) = 20
30 / 2 + 5 is (30 / 2) + 5 = 20

In the case where division and multiplication in the same expression without brackets the evaluation order is left to right. As in

4 * 5 / 2 is (4 * 5) / 2 = 10

These rules are called the rules of operator precedence. Go have 5 levels of operator precedence. Multiplication and division are some of the operators in the top level. Addition and subtraction are some of the operators in the next level down. We will expand on the other operators and the other three lower levels in time.

All programming languages have the concept of operator precedence. However the number of levels and the number of operators in each level varies with each programming language.

How to use Brackets in Go

Notes for Slide 4

Important

Brackets, ( and ) always come as a pair. It is illegal to have an opening bracket, without a corresponding closing bracket and vice-versa.

However, it is a very common mistake for new programmers to forget to type the brackets. Any emphasis placed on this point will help the children avoid problems in the example program. The pupils should consider this a pattern.

This point is also general. Inverted commas, single quotes, back ticks, square brackets and braces also always come in pairs. Take any of the example programs there are always one opening brace, {, at the top and one closing brace, }, at the bottom.

Brackets are used to change the evaluation order or to make the order explicit. As with maths the result of the expression between the brackets must be calculated first. In this respect Go follows the expected rules of maths.

The answers to the challenge questions are

3 + (2 * 5) = 13
(3 + 2) * 5 = 15
4 * (4 + 2) = 24
(28 - 20) / 4 = 2
30 / (2 + 5 + 3) = 3

The first two answers are different because the use of the brackets forces a different evaluation order in each case.

How to Create and Run the Go Program

Notes for slide 5

The pupils have to use a terminal window to make a new directory, and to run the Go program.

The pupils use a text editor to type in the program, and save it, before it can be run. The pupils are creating the source code file.

A Go source code file should be saved with a .go file extension.

How the pupils open a command prompt or terminal window varies with the operating system. See the Windows, MacOS X, Linux or Raspberry Pi install guides for the instructions

We are going to put each Go program in its own directory. This is the recommended practice for Go programs.

In your terminal you need to change to the location of your Go Workspace. To do this type

On Linux, Raspberry Pi and Mac OS X

cd $GOPATH/src/

On Windows

cd %GOPATH%\src\

Now you need to make a new directory. We need to call this extra-numbers after the program we will write. Then we need to change directory into the new extra-numbers directory.

mkdir extra-numbers
cd extra-numbers

Now you need to start you editor, either Atom or LiteIDE

On Linux, Windows and MacOS X

atom extra-numbers.go

On Raspberry Pi

liteide extra-numbers.go

You must make sure that the source code file is saved as extra-numbers.go in the extra-numbers directory in the Go workspace.

To run the program the pupils need to use the terminal window and type:

go run extra-numbers.go

Where extra-numbers.go is the name of the source code file. The command must be run from the extra-numbers directory.

The pupils will need to repeat this process in each lesson, so they will quickly become used to it.

The extra-numbers Program

Notes for Slide 6

Let’s look at the extra-numbers.go program in more detail. Here it is again.

    1 package main
    2 
    3 import "fmt"
    4 
    5 func main() {
    6    fmt.Println("The extra-numbers program shows you how to add, subtract")
    7    fmt.Println("multiple and divide numbers and how to use brackets.")
    8    fmt.Println("Sums without brackets.")
    9    fmt.Print("3 + 2 * 5 = ")
   10    fmt.Println(3 + 2*5)
   11 
   12    fmt.Print("4 * 4 + 2 = ")
   13    fmt.Println(4*4 + 2)
   14 
   15    fmt.Print("25 - 20 / 4 = ")
   16    fmt.Println(25 - 20/4)
   17 
   18    fmt.Print("30 / 2 + 5 = ")
   19    fmt.Println(30/2 + 5)
   20 
   21    fmt.Println("Sums with brackets.")
   22    fmt.Print("3 + (2 * 5) = ")
   23    fmt.Println(3 + (2 * 5))
   24 
   25    fmt.Print("(3 + 2) * 5 = ")
   26    fmt.Println((3 + 2) * 5)
   27 
   28    fmt.Print("4 * (4 + 2) = ")
   29    fmt.Println(4 * (4 + 2))
   30 
   31    fmt.Print("(28 - 20) / 4 = ")
   32    fmt.Println((28 - 20) / 4)
   33 
   34    fmt.Print("30 / (2 + 5 + 3) = ")
   35    fmt.Println(30 / (2 + 5 + 3))
   36 }
Fig-1. The extra-numbers.go code

The first thing to notice is that the extra-numbers program is very similar to the previous numbers program.

Both programs have the same structure. Both porgrams start with the same five lines.

package main

import "fmt"

func main() {

As before these lines declare the main package, line 1, and a main function on line 5. The program also uses the fmt package, short for format. The fmt package provides the Print and Println functions that the program uses to display the sums and the answers in the terminal window when the program runs.

You can see an example of how the Println function is used in Lines 6, 7 and 8.

fmt.Println("The numbers program shows you how to add, subtract")
fmt.Println("multiple and divide integer numbers.")

This lines produce the following lines in the output.

The extra-numbers program shows you how to add, subtract
multiple and divide numbers and how to use brackets.
Sums without brackets.

This is exactly the same behaviour as you saw in the numbers program. Whatever appears between the inverted commas appears verbatim in the output. The only difference is the content of the lines that are printed.

The remaining lines all follow the same pattern of a Print line followed by a Println line. For example Lines 9 and 10 are

fmt.Print("3 + 2 * 5 = ")
fmt.Println(3 + 2*5)

In this pattern the first Print line prints the sum that will be calculated. The second Println line prints the result of the sum.

Important

Notice the lack of inverted commas in the Println line. This tells Go not to print the line verbatim. Instead Go evaluates the result of the sum, more correctly called the expression, before printing the result.

So taking the example above the line

fmt.Println(3 + 2*5)

prints the result of 3 + 2 * 5 which is 13 and not 3 + 2 * 5.

The remainder of the program follows the same pattern, changing the sum to be printed and then evaluated with each pair.

Plenary

Notes for slide 7

The answer to 15 / 2 is 7. So why 7, why not 7.5? Or why not 7 remainder 1?

We have only introduced integer maths at this stage. So the pupils have no idea how Go manages real numbers i.e. numbers that can include a fractional part. The pupils will be introduced to how Go manages these numbers in upper KS2. The pupils will need to have been introduced to fractions and decimal fractions before this idea can be introduced in programming. The answer cannot be 7.5 because the sum is an integer division sum. Neither number involved contains a decimal point so cannot be a real number.

The answer cannot be 7 remainder 1 because integer division throws away the remainder. The result of the sum is therefore a single number, 7, and not two numbers a 7 and a 1. So the answer cannot be 7 remainder 1 because the remainder is never stored.

We could prompt just the remainder if we used the modulus operator, %, and typed

15 % 7

To give the answer 1. NB: Obviously this has nothing to do with percentages, despite the presence of the percentage sign.

The pupils can prove this by adding these two lines at the end of the program like so:

    1 package main
    2 
    3 import "fmt"
    4 
    5 func main() {
    6    fmt.Println("The extra-numbers program shows you how to add, subtract")
    7    fmt.Println("multiple and divide numbers and how to use brackets.")
    8    fmt.Println("Sums without brackets.")
    9    fmt.Print("3 + 2 * 5 = ")
   10    fmt.Println(3 + 2*5)
   11 
   12    fmt.Print("4 * 4 + 2 = ")
   13    fmt.Println(4*4 + 2)
   14 
   15    fmt.Print("25 - 20 / 4 = ")
   16    fmt.Println(25 - 20/4)
   17 
   18    fmt.Print("30 / 2 + 5 = ")
   19    fmt.Println(30/2 + 5)
   20 
   21    fmt.Println("Sums with brackets.")
   22    fmt.Print("3 + (2 * 5) = ")
   23    fmt.Println(3 + (2 * 5))
   24 
   25    fmt.Print("(3 + 2) * 5 = ")
   26    fmt.Println((3 + 2) * 5)
   27 
   28    fmt.Print("4 * (4 + 2) = ")
   29    fmt.Println(4 * (4 + 2))
   30 
   31    fmt.Print("(28 - 20) / 4 = ")
   32    fmt.Println((28 - 20) / 4)
   33 
   34    fmt.Print("30 / (2 + 5 + 3) = ")
   35    fmt.Println(30 / (2 + 5 + 3))
   36 
   37    fmt.Print("15 / 2 = ")
   38    fmt.Println(15 / 2
   39 }
Fig-2. The extra-numbers.go code showing 15 divided by 2

Lines 37 and 38 are the new lines. These lines must be added before the closing brace, the }, that terminates the program.

The pattern the pupils are tryign to spot is this:

fmt.Print( The sum in inverted commas )
fmt.Println( The sum NOT in inverted commas )

The first line in the pattern prints the sum to the terminal verbatim. It does not evaluate the expression. The second line contains the expression and that expression is evaluated.

Like mathematics, patterns are fundamental to computer science. Being able to spot, reuse and adapt patterns is a key part of programming. We will be introducing more patterns with each lesson.

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