csChapter 6_Conditional statements

6.1

Chapter 6: Conditional Statements

6.0.1 Objectives

Students typically find conditional statements (also known as selection, or decision statements) easy to

understand when compared to other constructs. The main area of confusion involved with conditionals

occurs when students begin using Boolean combinations: for example, the word “and” as it Is used

casually in English can have a different for the formal word “and” in Boolean logic. At the end of this unit,

students should be able to apply conditionals, creating Boolean expressions with comparisons, and they

should be able to reason correctly about the use of the Boolean operators “and” “or” and “not.”

6.0.2 Topic Outline

6.0 Chapter Introduction

6.0.1 Objectives

6.0.2 Topic Outlines

6.0.3 Key Terms

6.0.4 Key Concepts

6.1 Lesson Plans

6.1.1 Suggested Timeline

6.1.2 CSTA Standards

6.1.3 Lesson Plan I on using the control block- If, If… Else.

6.1.4 Lesson Plan II on re-visiting the spiral assignment on the Question Bot.

6.1.5 Lesson Plan III on using Boolean expressions.

6.1.6 Lesson Plan IV to designing a hi-lo game and the race car game.

6.2 Resources

6.2.1 Additional exercises.

6.0.3 Key Terms

Boolean Values: true / false

If then else, If then else if else

Simple & Complex expressions

match functionality

AND / OR – Operators

is (comparison), isnt, < ,>

Numerical comparisons

String comparisons

6.0.4 Key Concepts

Controlling Code Using Conditions

The word if can be used to put a block of code under the control of a condition, so it only runs when the

condition is true.

Below is an example that uses if to control turtle motion by testing keyboard presses.

The indented code fd 2 only runs when the condition pressed('W') is true, that is, when the user is

pressing the W key. Similarly, the two lines of code rt 2; dot blue, 5 only run when the user is

pressing the D key.

If neither key is pressed, neither block of indented code is run. If both keys are pressed, both blocks are

run.

6.2

forever ->

if pressed('W')

fd 2

if pressed('D')

rt 2

dot blue, 5

Testing key presses using if.

Using “else” For the Other Alternative

The “else” keyword allows you to program a second action to take when the “if” does not happen. The

second block of code will run when the condition is false.

forever ->

if pressed('W')

fd 2

else

rt 2

Providing two alternatives using if/else.

This program moves the turtle forward when W is pressed. When W is not pressed, it spins the turtle.

Chaining “else if” for Multiple Alternatives

When there are three or more actions, if and else can be chained.

forever ->

if pressed('W')

fd 2

else if pressed('S')

bk 2

else

rt 2

Chaining if, else if, and else for three alternatives.

This code moves forward if W is pressed and backward if S is pressed. It spins right if nothing is pressed.

A chained if/else only chooses the first condition that is true, so if both W and S are pressed at the same

time, this program will just do “fd 2” and not move backward.

Using “and”, “or” and “not” to Combine Boolean Expressions

The words and, or, and not are Boolean operators that can be used to combine conditions. For

example, the following program uses “and” and “not”. It draws a blue ring and then only moves the turtle

forward if W is pressed and the turtle is not already touching the blue ring.

6.3

dot blue, 500

dot white, 400

forever ->

if pressed('W') and not touches('blue')

fd 2

else

rt 2

Combining tests using and and not.

Although Boolean operators usually work in the same way as when reading them as English words, it is

important to understand exactly how they work as mathematical operators because it is easy to get

unexpected effects.

Confusing “and” With “or”

Consider a program that where “up” and “W” keys need to work equivalently, both working in the same

way to move the turtle forward. We might be tempted to use the “and” combiner to capture both cases

with a single “if” like this:

WRONG:

forever ->

if pressed('up') and pressed('W')

fd 2

Incorrectly using “and” to combine two alternatives.

This code, however, will not generate the desired effect! To understand why, we need to understand how

and and or operate on truth values.

Boolean Values and Boolean Tables

The words and, or and not are Boolean operations that combine “true” and “false” values (similar to the

arithmetic rules you get when using “+”, “*” to combine regular numbers). Just as we can learn about

addition and multiplication by creating addition and multiplication tables, we can understand and and or

by writing truth tables. Here are two truth tables related to the program above:

pressed('up') and pressed('W')

pressed 'up' = false

pressed 'up' = true

pressed 'W' = false

false

pressed 'W' = true

false

true

pressed('up') or pressed('W')

pressed 'up' = false

pressed 'up' = true

pressed 'W' = false

false

true

pressed 'W' = true

true

6.4

The conjunction and combines two Boolean values and creates “true” only when both of the values are

true. For example, pressed('up') and pressed('W') is true only when both the up and W keys are

pressed at the same time.

The disjunction or combines two Boolean values and creates “true” when either or both of the values are

true. For example, pressed('up') or pressed('W') is true when just the W key is pressed, or just the

up key is pressed, or both. This is what we want for our program.

To fix the program, the and should be switched to or.

Testing Numbers Using Comparison Operators

Boolean expressions can be used to test the properties of numbers. Most of the comparison operators

you would see in math class work in a programming language, but they may be written with slightly

different punctuation. For example, “is less than or equal to” is written <=. Here is a summary of some

common Boolean tests for numbers:

Expression

Description

What if x =

x is 3

x is equal to 3

false

true

false

x isnt 3

x is not equal to 3

true

false

true

x < 3

x is less than 3.

true

false

x <= 3

x is less than or equal to 3

true

false

x > 3

x is greater than 3

false

true

x >= 3

x is greater than or equal to 3

false

true

0 < x <= 6

x is greater than 0 and less than or equal to 6

false

true

x % 2 is 1

x is odd (because it has remainder 1 when

divided by 2)

false

true

false

x % 3 is 0

x is divisible by 3

false

true

Confusing “or” with Comparisons

Numerical comparisons can be combined with Boolean operators. For example, (x > 6 and x isnt 9)

means that x is a number greater than 6 other than 9, and (x is 5 or x is 11) means that x is either

5 or 11. It is important, though, to remember that the word “or” operates on truth values and not on

numbers, so the version of the program on the left does not do produce the desired result.

WRONG:

await readnum 'How many items?', defer n

if n is 1 or 2

write 'Come to the speedy checkout.'

RIGHT:

await readnum 'How many items?', defer n

if n is 1 or n is 2

write 'Come to the speedy checkout.'

The program on the left incorrectly results in the speedy checkout line no matter what number you enter.

6.5

To understand why, remember that or operates on truth values, so when you say “or 2”, it begins with

the question “is 2 true or false?” By convention, any number that is not zero is treated as “true”, so “or 2”

makes the expression always true regardless of the value of num. On the other hand, the program on the

right produced the desired result: “or n is 2” only makes the expression true when the number is 2.

Another way to think about the difference is with precedence of operators. The word “or” has lower

precedence than the word “is”, so the expression on the left reads like this: ((n is 1) or 2) and the

expression on the right reads like this: ((n is 1) or (n is 2)).

Testing Strings Using Pattern Matching

Text strings can also be tested to create Boolean values. It is common to test strings by comparing them

exactly (looking at their length) or by testing if the string matches a pattern using the “match” method.

Pattern matching can be used to determine if a string contains a particular pattern of letters within it.

The following table shows several examples.

Expression

Description

"appear"

"pear"

"peachy"

x is "pear"

x is exactly equal to the string "pear"

false

true

false

x.length is 6

x has exactly 6 characters

true

false

true

x.match(/pp/)

x contains the substring “pp”.

true

false

x.match(/pea/)

x contains the substring “pea”

true

x.match(/PEA/)

x contains the substring “PEA”

false

x.match(/Pea/i)

x contains the substring “Pea”, ignoring case

true

x.match(/^pea/)

x contains “pea” at the start of the string

false

true

x.match(/ear$/)

x contains “ear” at the end of the string

true

false

x.match(/a(p|ch)/)

x contains “a” followed by either “p” or “ch”

true

false

true

x.match(/ap*e/)

x contains “a”, then zero or more “p”, then “e”

true

false

The patterns used between the “/” symbols are called regular expressions.

A regular expression can be used to test whether a string contains a fixed pattern, for example whether it

contains the letters “pp”. Normally regular expressions are case-sensitive, so “PEA” does not match

“pea”, but putting an “i” after the regular expression makes it case-insensitive.

Regular expression patterns have several powerful features. For example, in a regular expression, “^”

matches the beginning of the string, “$” matches the end of the string, “(one|other)” is used to match

alternatives, and “*” allows a sub-pattern to be repeated zero or more times.

Although the types of patterns shown above are enough for most situations, regular expressions have

several more features. There are many excellent resources about regular expressions on the Internet if

you search for “regular expression lessons”. When exploring, it is important to know that the symbols

used in regular expression patterns are standardized, and the same pattern language is used in

JavaScript, CoffeeScript, Python, Perl, Java, C# and other languages.

6.6

6.1.1 Suggested Timeline: 1 55-minute class period

Instructional Day

Topic

2 Days

Lesson Plan I: If, If Then Else statements using fun visual elements

1 Day

Lesson Plan II: Question Bot & Lesson Plan III Complex If Statements

1 Day

Lesson Plan IV: Pair programming for the HiLo Game

1 Day

Lesson Plan V: Race Car Track game

6.1.2 Standards

CSTA Standards

CSTA Strand

CSTA Learning Objectives Covered

Level 3 A

(Grades 9 – 12)

Computational

Thinking (CT)

Explain how sequencing, selection, iteration and recursion

are building blocks of algorithms.

Level 3 A

(Grades 9 – 12)

Explain the program execution process.

Level 3 A

(Grades 9 – 12)

Describe how mathematical and statistical functions, sets,

and logic are used in computation.

6.1.3 Lesson Plan I

This lesson focuses on the Control Block, If statements and If Else statements. The lesson should take

about 30 to 40 minutes of a class period, providing time for students to code programs similar to the ones

modeled.

Content details

Teaching Suggestions

Time

Code

x = random [1..3]

write x

if x is 1 or x is 2

write 'Today is your lucky

day!'

else

if `` is ``

write 'Stay low. Let

everything happen tomorrow'

else

write 'I cannot see your

future.'

# Demonstrate complex IF

Statements

Explain the key concept of the evaluation of

a Boolean expression.

Show the Control Block and the ‘IF’

statement.

Type the code shown on the left column.

(Note: You will need speakers for this

program. You can use the write block

instead of say block.)

Teaching Tips: You can extend this lesson

further by adding a loop around the entire

code for the tune to be played for a fixed

period (e.g. 5 times). Explain that the

program gives an output based on the value

the variable to which “Day” is set.

Demonstration:

20 minutes

Code

Give another example using patterns.

Demonstrate how the conditional can impact

Demonstration:

20 minutes

6.7

Content details

Teaching Suggestions

Time

speed 50

pen red, 10

for x in[1..20]

fd 90

rt 100

if x is 8

pen purple, 10

else

if x is 12

pen lightsalmon, 10

the color in which the pattern is drawn.

Teaching Tips: Change the values within the

conditional to show how the pattern

changes.

Add another if statement to show another

color. (Point out the use of a nested if

statement.)

Add an ‘if’ statement to change the speed.

Here is the copy of the program:

http://teachersguide.pencilcode.net./edit/cha

pter6/pattern

Now students can start writing their own

programs. Students are expected to write

both the programs that were demonstrated.

Students should complete both programs by

the end of the class period (15 minutes of

class time).

Student Practice:

15 minutes

Code

speed 10

answer = 'yes'

hide()

while answer is 'yes'

diceRoll = (random 6)

label

String.fromCharCode(9856 +

diceRoll), 100

say 'Rolling dice now!'

if diceRoll is 6

say 'You made a 6!! Roll

again!'

answer = no

else

write diceRoll

say 'Tool Bad! Want to

Add a few fun elements to the program to

show that how using a loop and a

conditional increases a program’s power.

Here is the code to simulate the roll of a die.

(This is a starter program for a Yahtzee

game. The Additional Exercises section

provides the specifications for Yahtzee.)

Demonstrate and walk students through the

code. Point out the use of random numbers,

how the assignment of an exit condition lets

the loop exit eventually, and the If… Else

block.

Demonstration:

55 minutes

6.8

Content details

Teaching Suggestions

Time

try again?'

await read 'Roll

again?', defer answer

say 'Good bye!'

6.1.4 Lesson Plan II

This lesson revisits the Spiral assignment Question Bot introduced in the Chapters 3, 4, and 5. Students

will extend this program using all of the previously-learned concepts and add conditional statements.

Content details

Teaching Suggestions

Time

Code:

# chatbot

# CS1004 example chatbot using

loop and variable prompts

write 'Bob: Hi! My name is Bob.'

await read 'Bob: What\'s your

name?', defer name

write 'Bob: Hello ' + name

done = false

while not done

prompt = name + ' can you

guess who I am?' + ':'

await read prompt, defer q

if (q.match /quit|give up/)

write 'Bob: OK, nice talking

to you!'

done = true

else if (q.match /bot/)

write 'Bob: Close... But I

am a human, of course.'

else

write 'Bob: Good guesswork!'

Pull up the Question Bot (Loops version)

program. Ask students how its

functionality can be expanded.

Even though the program asks four

people’s names, it does not “remember”

them?

Making the lesson more interesting by

asking students for a good question.

Walk through the code and emphasize

the power of conditionals.

This example also introduces the

operator x.match () functionality.

In block-mode it looks like this:

Demonstration:

20 minutes

6.9

6.1.5 Lesson Plan III

This lesson plan demonstrates the use of complex if statements, specifically, combining multiple Boolean

expressions.

Content details

Teaching Suggestions

Time

Code

x = random [1..3]

write x

if x is 1 or x is 2

write 'Today is your lucky day!'

else

write 'I cannot see your

future.'

# Demonstrate complex IF

Statements

Explain the key concept of the

evaluation of multiple Boolean

expressions. Explain how an AND /

OR operator combines two

expressions and evaluates them.

The example provided here is a

simple program that acts like a

“Genie” and predicts the day based

on the random number generated.

Teaching Tip: Explain how an OR

operator works. Explain using the

example that the ‘if’ statement will

get executed even if one of the

expressions is true.

Ask the students to copy the code

provided and test the program.

Demonstration:

20 minutes

Code:

x = random [1..3]

write x

if x is 1 or x is 2

write 'Today is your lucky

day!'

else

if `` is ``

write 'Stay low. Let

everything happen tomorrow'

else

write 'I cannot see your

future.'

# Demonstrate complex IF

Statements

Now explain in-depth using the

modified program as show in the left

column.

Teaching Tip: Explain how an AND

operator works. Explain using the

example that the ‘if’ statement will

get executed IF AND ONLY IF both

expressions are true.

Ask students to copy the code

provided and test the program.

Give students 10 minutes to

experiment with if statements and

modifications of the code. Let them

test other programmers’ code and

see what kind of day the genie

predicts for them!

Demonstration:

20 minutes.

Student

Practice:

10 minutes

6.10

6.1.6 Lesson Plan IV

Students will now work with a partner to design a Hi-Lo Guessing game. The Additional Exercises section

provides the problem description. Students will watch a short video on pair programming before beginning

the exercise. The lesson plan below focuses on how to design a project with a partner. Students will take

about half a class period to design the program. They will have the rest of the class period and one more

class period or homework to complete the assignment. (Note: Homework may not result in true

collaborative work because it increases the temptation to find answers on the Internet and from people

outside of the team.)

Content details

Teaching Suggestions

Time

https://www.youtube

.com/watch?v=vgka

hOzFH2Q

Have students watch the video on pair programming.

Student Practice:

5 minutes

Split students into groups. The References section provides resources on forming successful

collaborative groups.

Using the “Rally Robin” co-operative learning® structure, have the students write the pseudocode for the

HiLo game. Their pseudocode should indicate:

i. Variables used

ii. Control structures needed

iii. Input / Output statements

Student Practice: 20 minutes

Rally Robin instructions:

On a piece of paper, take turns writing instructions on how to design the guessing game.

Each instruction should be a numerical bullet (e.g. 1,2,…)

Student 1 writes the first instruction.

Student 2 writes the second instruction.

Keep repeating this process until all of the instructions have been recorded.

Next, go back and take turns revising the instructions until both partners are satisfied.

Submit your work your teacher for grading.

Here is a sample Hi-Lo program.

Student Practice: 20 minutes

On your computer, type the program with your partner using the pair programming methods presented in

the pair programming video.

Student Practice: 40 minutes

6.11

6.1.7 Lesson Plan V

This interactive lesson involves having the students design a racecar game. We recommend that

students type this in text using CoffeeScript. Encourage the students to toggle between text-mode and

block-mode to venture out of their comfort zone and increase their confidence. Follow the activity Turtle

Race track as provided in this activity sheet

Content details

Teaching Suggestions

Time

http://activity.pencilcode.net/home/workshe

et/race.html

Teaching Tip:

Have all the students in the class start

on the worksheet at the same time.

Print out the worksheet and give it to

the students. This will eliminate the

temptation to copy-paste the code.

Follow the link to complete steps 1

and 2.

Instruct students to type the text and

stay in text-mode.

Student

Practice:

10 mins

http://activity.pencilcode.net/home/workshe

et/race.html

Ask students to answer Questions 1

and 2 verbally in the classroom.

Student

Practice:

15 mins

http://activity.pencilcode.net/home/workshe

et/race.html

Now let the students venture out at

their own pace as they respond to

Challenges 1 through 4. Students can

stay in text-mode or switch back to

block-mode. If they do switch to

block-mode, encourage them to

toggle to text and see how their code

looks.

The solution code is given in

CoffeeScript.

Teaching Tip: If the students type the

text, encourage them to indent their

code to convey intent. All good

programming practices (Refer

Appendix A) should be followed.

Student

Practice:

20 mins

The race car game activity can be completed by students.

1. http://activity.pencilcode.net/home/worksheet/race.html - the basic game

2. http://activity.pencilcode.net/home/worksheet/race-car.html - making the car look like a car,

and this emphasizes the idea of an “object” whose behavior your control

3. http://activity.pencilcode.net/home/worksheet/race-two.html - here you introduce a “second

object” - two instances - and now you can controls them separately

4. http://activity.pencilcode.net/home/worksheet/race-track.html - this is a review of drawing, but

used for a very different purpose - to create a track shape

5. http://activity.pencilcode.net/home/worksheet/race-speed.html - this is an introduction to

variables, used to keep track of how fast each car goes.

6. http://activity.pencilcode.net/home/worksheet/race-time.html - this is another use of variables,

this time to keep track of how much time has passed

7. http://activity.pencilcode.net/home/worksheet/race-menu.html - this is an example of use of

functions to divide your program into subprograms.

6.12

6.2 Resources

Additional Exercises:

Yahtzee

Design a modified version of Yahtzee where you roll three dice and the score is calculated based on

which of the following categories are satisfied. The game ends after each user has had three turns. The

user who has the largest point value at the end of three turns is the winner. The three categories are:

i. Three of a kind – 5 points

ii. Two of a kind – 10 points

iii. Yahtzee – You Win! Game over!

Hi-Lo- Guessing Game

Design a simple game where the computer generates a random number and the user must guess the

number. The computer helps the user by giving responses such as “too high or too low”. The user has a

fixed number of tries to guess the right answer. Once the allowed number of tries are completed, the

game displays the computer-generated number and says “Game Over”. You can get up to extra five

creativity points if you have added a new feature to the game.