Objective

To extend the pupil’s knowledge of if selection statements by introducing the closely related if else statement.

The lesson also introduces the concepts of boolean, negation and not equals to.

The class will learn the pattern for an if else statement in Go.

The class will practice using an if else statement and see the effect with the timesquestion program.

Creating these 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.

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 that a boolean variable can only represent the values true and false.
  • Understand that the result of a comparison is a boolean value.
  • Understand that Go has a boolean type, bool.
  • Understand that a variable of type bool can be used to store the result of a comparison
  • Understand the idea of negation i.e. true is the opposite of false
  • Understand the negation and not equals to operator
  • Understand that selection can be used with strings
  • Understand the if else patterns
  • Understand that the program represents a sequence
  • Understand that the selection can change the sequence order by causing parts of the program to be skipped over

Teaching prerequisites

The pupils must have encountered the comparison operators, <, > and = in mathematics Comparison is fundamental to selection to the pupils must be already familiar with the idea from mathematics.

In addition to this the pupils need to have covered Lessons 1-8 in this series.

Selection. The program demonstrates how to use selection and the effect of using selection statements. The selection statements change the sequence order by causing some parts to be skipped over. The program will no longer execute every line in the program.

Variables. Variables and selection are used together. The condition part of the selection will compare the value of a variable to the value of another variable or to a fixed value.

Input and Output. The program asks the user to input the current temperature and displays if the temperate is hotter or colder than other cities around the world.

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

Maths. The example program,timesquestion will ask the pupils to solve multiplications from the 1-12 times tables.

Comparing Numbers

Notes for Slide 2

The symbols mean

==equals to
<less than
<=less than or equal to
>greater than
>=greater than or equal to

Note that double equals sign, == is used for equals to. A single equals sign is already used in Go for variable assignment.

A computer program can only answer true or false to a question. This means that the programmer has to ask a very precise question.

The if statement

if temperature = 100
    fmt.Println("The water is boiling.")
fmt.Println("The water is getting hotter.")

has three mistakes in it. The first two mistakes are the missing opening and closing braces, the { and } that should surround the line

fmt.Println("The water is boiling.")

The last mistake is more subtle. The if statement does not contain a valid condition, it contains a variable assignment. The single equals sign, = in

if temperature = 100

should be replaced with a double equals sign, ==, to test for equivalence.

The corrected if statement would therefore be

if temperature == 100 {
    fmt.Println("The water is boiling.")
}
fmt.Println("The water is getting hotter.")

These are both very common mistakes that the pupils will make when learning.

What is the Result of a Comparison

Notes for Slide 3

The expression

12 <= 34

asks the question is 12 less than or equal to 34.

The answer can only be yes or no, or more correctly true or false. The concept of true and false, or yes and no, or one and zero so common in computing that it has its own type.

The type is known as a boolean type, named after the 19th century mathematician George Boole. Who first described how addition could be described using sets. This has a realisation in computer science as boolean algebra.

Go expresses the boolean type as a type called bool. Variables of type bool can be created using the same variable declaration pattern as say other variables. Likewise values can be assigned to variables of type bool using the variable assignment pattern.

As an example consider

    1 var answer bool
    2 answer = 12 <= 34
    3 if answer == true {
    4     fmt.Println("12 is less than or equal to 34")
    5 }

The result of the comparison of 12 <= 34 is a boolean value and is assigned to the variable answer in line 2. The answer variable is declared as a bool type in line 1.

Notes

The if test can be collapsed into this

    1 var answer bool
    2 answer = 12 <= 34
    3 if answer {
    4     fmt.Println("12 is less than or equal to 34")
    5 }

The equivalence test of answer against the value true is unnecessary. The if statement will test the condition. If the condition happens to be a boolean variable then the test against true or false is implicit in the meaning of the if statement itself. There is no need to write the test explicitly.

But why would you want to do this? Sometimes it is necessary, or just simpler, to calculate the value of a condition and store the result in a boolean variable for later use. Examples of this approach would be used in repetition, which we will show in later lessons, or to test the conditions necessary to exit the program early.

The Opposite of True is?

Notes for Slide 4

The challenge answers are:

False is the opposite of true True is the opposite of false

In this case we are talking about the logical opposite. This is negation. The symbol Go used to logically negate a boolean variable is !

For example

    1 var answer bool
    2 var negatedAnswer bool
    3 answer = true
    4 negatedAnswer = !answer

This can also be used in if statements. Like this

    1 var locked bool
    2 locked = IsTheDoorLocked()
    3 if !locked {
    4     fmt.Println("The door is open")
    5 }

If the programmer does not have a function called IsTheDoorUnlocked() available to them then the only way to test if the door is open is to test if the door is not locked. To do this we need to use negation.

Not Equals

Notes for Slide 5

The pupils can tell if two numbers are different by using a combination of an equals test and negation. The question is not asking if the numbers are larger or smaller relative to each other only that they are different.

If you want to tell if the numbers first-number and second-number are different then you can first test them for equality, using a == operator.

If the numbers are different the result of the equality test is false.

But we want to test for difference. So when the numbers are different we want the answer to be true. This can be achieved if we negate the answer of the equality test i.e.

    1 !(first_number == second_number)

The brackets are required because we want to negate the answer of the equality operator, the == so we need this to happen first. The negation operator, ! has a higher precedence then the == operator.

There is a better way. Go provides a ‘not equals to’ operator, typed !=. It is more usual to use the ‘not equals to’ operator, the != instead of the bracketed expression.

If the number on the left of the != operator is different from the number on the right the result is true. Otherwise it is false.

Comparing Strings

Notes for Slide 6

String comparison uses the same set of comparison operators as numbers. The meaning of each operator, <, <=, ==, !=, >=, > is the same as for numbers.

Strings are equal if they have the same sequence of runes (letters, punctuation symbols etc.) and have the same number of runes. Otherwise the strings are not equal.

The not equals, !=, operator also works for strings. If the strings are not the same length or any letter in the strings is different the != operator returns true. Otherwise the result is false

Strings are ordered alphabetically i.e. dictionary order. Or more correctly order lexicographically. Numbers in the string come before letters and upper case letters come before lower case letters.

This is the ordering of the letters etc. in the Unicode Table. Given that all characters are represented by a number in the Unicode table this means that string comparison involves comparing one list of numbers against a second list of numbers.

Challenge

Notes for Slide 7

The challenge has two parts. Firstly, the pupils are asked to compare two strings using the comparison operators. Secondly, the pupils have to think logically and create two if statements. The pupils will have to come up with their own names for variables to represent the height of The Shard and Nelson’s Column.

Notes

You may have to explain that The Shard is a tall building in London (actually the tallest in Europe) and Nelson’s Column is the monument in the centre of Trafalgar Square in London.

The answer to the first challenge is

    1 if "robin" < "blackbird" {
    2     fmt.Println("Blackbird comes before robin in the dictionary")
    3 }

the condition in the if statement is false so nothing is printed. So the string “robin” is not less than the string “blackbird” i.e. it is considered bigger. This is correct based on the dictionary order.

Notes

This can be proved if your pupils look up both “robin” and “blackbird” in the dictionary.

The second if statement

    1 if "robin" > "blackbird" {
    2     fmt.Println("Blackbird comes before robin in the dictionary")
    3 }

Is true, so the output would be

    1 "Blackbird comes before robin in the dictionary"

The if tests required for the second part could be written as:

    1 if heightOfShard > heightOfNelsonsColumn {
    2     fmt.Println("The Shard is the tallest")
    3 }
    4 if heightOfNelsonsColumn > heightOfShard {
    5     fmt.Println("Nelsons Columns is tallest")
    6 }

The pupils can chose any variable names, but the variable names must be consistent. The same variable names must be used in both if statements.

The pupils will need two distinct if statements to complete the challenge.

If the pupils also declare the variables, both should be declared as int types to approximate the height to the nearest metre.

Clearly in reality The Shard is much taller.

Let’s Look at the Challenge Again

Notes for Slide 8

The challenge is also about two logically opposite statements. If the height of The Shard is larger than the height of Nelson’s Columns then the height of Nelson’s Columns cannot be higher then the height of The Shard.

Logically only one of the two statements can be true at any one time. If the first statement is true the second statement must be false. The second statement is true then the first statement must be false.

This situation can be solved using an if else statement.

if and his friend else

Notes for Slide 9

In a situation where you have two statements but only one of them can be true at any one time two if statements are not required. A single if else statement will do.

The if else pattern is

if condition {
    true-statment-block
} else {
    false-statement-block
} // this is the last brace

This is a simple extension of the if pattern already introduced.

If the condition is true then the if else behaves like an if statement. The true-statement-block is executed. Once executed the next line to be executed is the line after the last }. The false-statement-block is skipped over and not executed.

If the condition is false then the false-statment-block is executed. Once executed the execution continues on the line after the last }. The true-statement-block is skipped over and not executed.

Only one of the two statement blocks is executed. Which is executed depends on the result of the condition.

else is another of Go’s keywords.

One if else Statement Solution

Notes for Slide 10

The challenge on Slide 7 can now be solved with a single if else statement like this

    1 if height_of_shard > height_of_nelsons_column {
    2     fmt.Println("The Shard is tallest")
    3 } else {
    4     fmt.Println("Nelsons Column is tallest")
    5 }

Only one of the Println statements will be executed. Never both.

if else and if again

Notes for Slide 11

There is one slight tweak to the standard if else pattern

If the first statement in the false-statement-block is an if statement you are allowed to write this

if first-condition {
    true-statement-block-for-first-condition
} else if second-condition {
    true-statement-block-for-second-condition
} else {
    statement-block-if-both-conditions-are-false
} // this is the last brace

So you are allowed to squeeze the extra if statement in after the else. This is the usual practice so there is an example of this in example program.

The programmer can have as many else if lines as they require to solve the problem. There is no limit.

The timesquestion Program

Notes for Slide 12

The timesquestion program asks the pupils multiplications from the 1-12 times-tables. If the pupils answer correctly the program displays a congratulations message. If the pupils are wrong the program tells them if their guess was too large or too small and then prints out the correct answer.

At the heart of the program is an if else statement.

Lets look at the timesquestion program.

    1 package main
    2 
    3 import (
    4    "fmt"
    5 
    6    "github.com/gophercoders/random"
    7    "github.com/gophercoders/simpleio"
    8 )
    9 
   10 func main() {
   11 
   12    var a int
   13    var b int
   14    var answer int
   15 
   16    fmt.Println("The timesquestion shows you how to use if and else.")
   17    fmt.Println("Can you remember your times tables?")
   18    fmt.Println("")
   19 
   20    a = random.GetRandomNumberInRange(1, 12)
   21    b = random.GetRandomNumberInRange(1, 12)
   22 
   23    fmt.Print("What is ")
   24    fmt.Print(a)
   25    fmt.Print(" * ")
   26    fmt.Print(b)
   27    fmt.Println("?")
   28 
   29    answer = simpleio.ReadNumberFromKeyboard()
   30 
   31    if answer == a*b {
   32        fmt.Println("Congratulations! You are correct! ")
   33    } else if answer > a*b {
   34        fmt.Println("Sorry, your guess was to big.")
   35        fmt.Print("The correct answer is ")
   36        fmt.Print(a)
   37        fmt.Print(" * ")
   38        fmt.Print(b)
   39        fmt.Print(" = ")
   40        fmt.Println(a * b)
   41    } else {
   42        fmt.Println("Sorry your guess was to small.")
   43        fmt.Print("The correct answer is ")
   44        fmt.Print(a)
   45        fmt.Print(" * ")
   46        fmt.Print(b)
   47        fmt.Print(" = ")
   48        fmt.Println(a * b)
   49    }
   50    fmt.Println("Run the program again to try another question.")
   51 }
Fig-1. The timesquestion code

If the pupils answer correctly the output is

The timesquestion shows you how to use if and else.
Can you remember your times tables?

What is 4 * 12?
48
Congratulations! You are correct!
Run the program again to try another question.

If the pupils guess is to small the output is

The timesquestion shows you how to use if and else.
Can you remember your times tables?

What is 12 * 10?
5
Sorry your guess was to small.
The correct answer is 12 * 10 = 120
Run the program again to try another question.

If the pupils guess is to large the output is

The timesquestion shows you how to use if and else.
Can you remember your times tables?

What is 3 * 8?
44
Sorry, your guess was to big.
The correct answer is 3 * 8 = 24
Run the program again to try another question.

Important

Both numbers involved in the multiplication are chosen randomly. The numbers the pupils see will be different each time the program is run. Therefore the numbers will be unlikely to match those shown in the sample output.

Lets look at the key points of the program. The first key line is line 29.

answer = simpleio.ReadNumberFromKeyboard()

This is a straight forward variable assignment statement. The value of answer is assigned the result of the function ReadNumberFromKeyboard in the simpleio package. So, the number that the user types is assigned to the answer variable.

The next key lines are lines 31 to 33.

if answer == a*b {
    fmt.Println("Congratulations! You are correct! ")
} else if answer > a*b {

This is the if else pattern. Lets break this down. a and b are the numbers picked by the program. The pupil is asked to solve a * b. Their answer is held in answer.

If the pupil’s answer is equal to a * b then the condition of the if statement is true. If the condition is true then line 32 is executed and

Congratulations! You are correct!

appears in the output. The else block that extends between lines 33 and 49 is skipped. The next line that is executed is therefore line 50, resulting in

Congratulations! You are correct!

appearing in the output.

But what happens if the answer is wrong? What happen when the condition is false?

Lets look at the whole block from lines 31 to 49.

if answer == a*b {
    fmt.Println("Congratulations! You are correct! ")
} else if answer > a*b {
    fmt.Println("Sorry, your guess was to big.")
    fmt.Print("The correct answer is ")
    fmt.Print(a)
    fmt.Print(" * ")
    fmt.Print(b)
    fmt.Print(" = ")
    fmt.Println(a * b)
} else {
    fmt.Println("Sorry your guess was to small.")
    fmt.Print("The correct answer is ")
    fmt.Print(a)
    fmt.Print(" * ")
    fmt.Print(b)
    fmt.Print(" = ")
    fmt.Println(a * b)
}

If the user types the wrong answer then the first condition

if answer == a*b {

is false. So the else on line 33 is executed.

} else if answer > a*b {

But the else block itself starts with another if statement.

If this second if test

if answer > a*b {

is true, then the user’s guess is too large. In this case the next block to be executed is from line 34 to 40

fmt.Println("Sorry, your guess was to big.")
fmt.Print("The correct answer is ")
fmt.Print(a)
fmt.Print(" * ")
fmt.Print(b)
fmt.Print(" = ")
fmt.Println(a * b)

which results in

Sorry, your guess was to big.
The correct answer is 3 * 8 = 24

appearing in the output. So even though the if statement follows an else it behaves in the same was as any other if statement. If this if test is true the else block in lines 42 to 49 are skipped over.

If the if test on line 33 is false

} else if answer > a*b {

then answer is less than a * b then the else block on lines 42 to 49

fmt.Println("Sorry your guess was to small.")
fmt.Print("The correct answer is ")
fmt.Print(a)
fmt.Print(" * ")
fmt.Print(b)
fmt.Print(" = ")
fmt.Println(a * b)

which results in

Sorry your guess was to small.
The correct answer is 12 * 10 = 120

appearing in the output.

Important

Remember that lines 42 to 49 are only executed if both if tests are false.

In either case line 50 is always executed.

If you look closely at the program there are two if else blocks. The fist one extends between lines 31 and 49. The second one is enclosed within the first one, and extends between lines 33 and 49.

The else on line 33 matches the if on line 31. The else on line 41 matches the if on line 33.

The only other significant lines are lines 20 and 21.

a = random.GetRandomNumberInRange(1, 12)
b = random.GetRandomNumberInRange(1, 12)

Both of these use the new random package we have developed. The function GetRandomNumberInRange returns a random number in the range 1 to 12. 1 being the bottom of the range and 12 being the top of the range.

The random package is imported on line 6

"github.com/gophercoders/random"

The remainder of the program is a mixture of variable declarations, lines 12, 13, 14 and Print and Println statements.

Plenary

Notes for Slide 13

The plenary is about repetition, the subject of the next lessons.

At the moment the pupils have to restart the program completely to re-run the program. They don’t yet know how to repeat just part of the program without restarting it. But they might be able to guess this. You are looking for the pupils such as repeat, or loop or phrases like “do that bit again”, “keep doing this bit”, “over and over (again)”.

Can the pupils work out that they need to repeat the part that asks the question and then checks the answer?

If the program is run again numbers will be different. The GetRandomNumberInRange guarantees this.

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