Repeat fun, unplugged

A really fun and easy way to introduce your young coder to loops and repeating actions is to throw a dance party! Identify five dance moves: clap, stomp, wiggle, jump, and squat. Then write a series of moves on one line on the white board (or even a piece of paper):

Repeat 2 times:

Clap, clap, clap

Repeat 3 times:

Stomp, stomp, clap

Wiggle, jump, squat, jump

Clap

The entire dance would consist of 33 actions in total, but you only have to write 11! You can even have your young coders write their own dances using the same actions!

Random loop conditions, unplugged

One fun way to get your coder understanding how the conditions and actions of loops work is to pass out a sheet with a number line on it from 1 to 18 and give each person a die. In teams of two, have each person roll the die five times. The first number that they roll is the starting position on the number line, the next three numbers they roll are added together and is the ending position on the number line, and the fifth number that they roll is the increment value that they use as their action. Code.org offers a really great worksheet for this activity as part of its For Loop Fun puzzle found at https://studio.code.org/s/course4/stage/8/puzzle/1.

Figure 10-1 shows an example of two players playing one round of the game. Player 1 rolled a 3, then rolled three numbers that added to 12 (maybe 5, 6, and 1), then rolled a 4. So she started at 3, ended at 12, and added each 4^th number, totaling 21 points!

Infinite loops

Something that often trips up young coders is infinite loops. An infinite loop is any type of loop that never stops repeating. This can happen pretty easily if you set your conditions incorrectly. For example, if your young coder is 8 years old and you have a while loop that says “while you are older than 7 years old, do a jumping jack,” then your young coder has to do jumping jacks FOREVER!

Infinite loops often make your program either look like it’s not doing anything, or just crash. In Chapter 13 you can find out how to debug infinite loops.

You can also create a specific form on an infinite loop on purpose, for instance, coding a forever loop in Scratch to play background music that loops the entire time a game is being played. To break an infinite loop, you might need to use an if-then statement such as “if the player loses all lives in frogger, then stop all.” (See Chapter 2 for a description of these conditional statements.)

Actions repeated in loops

The last thing to know about all loop types is that you can have as many actions as you want in the loop. It could just be one action — “take a bite” — that you have to repeat, but it could also be eight actions — for example, if you're wrapping a lot of gifts, your actions to repeat might be “grab a gift from the unwrapped pile of gifts, take the price tag off the gift, measure the quantity of wrapping paper, cut the paper to the right measurements, fold the paper around the gift, tape the paper closed, write a message on the gift, place in the pile of wrapped gifts.”

Conditions of loops

As you might have noticed already, the conditions of loops are always of Boolean type. Chapter 7 introduces you to Boolean types if you need a refresher, but simply speaking a Boolean gives two choices: true or false. The condition of a loop always evaluates to either true or false, to know if the action(s) inside the loop repeats again, or the loop ends and the rest of the program continues.

Using pseudocode

Most programming languages have the same general structure for while loops, do-while loops, and for loops. Loops are written with indentation to make it clear which actions are a part of the repetition, and which actions simply come after the loop.

• While loops:

  while(condition)

  action1

  action2

  …

  END_WHILE

• Examples:

  while(sunIsUp)

  playOutside

  END_WHILE

  while(rightArrowKeyIsPressed)

  moveRightOneStep

  END_WHILE

• Do-while loops:

  do

  action1

  action2

  …

  while(condition)

• Examples:

  amount = 0

  do

  amount = amount + 5

  while(amount < 50)

  do

  takeBiteOfFood

  while(plateHasFood)

• For loops:

  for(starting_bound; condition; increment)

  action1

  action2

  …

  END_FOR

• Examples:

  balance = 0

  for(days = 1; days <= 365; days++)

  balance = balance * 1.01

  END_FOR

  for(walls = 1; walls <= 4; walls++)

  walkForwardLengthOfWall

  turnRight(90degrees)

  END_FOR

• For-each loop:

  for(item in list)

  action1

  action2

  …

  END_FOR_EACH

• Examples:

  for(classmate in class)

  highFive

  END_FOR_EACH

  totalScore = 0

  for(score in testScores)

  totalScore = totalScore + score

  END_FOR_EACH

• Explicit infinite loop:

  while(TRUE)

  action1

  action2

  if(condition)

  BREAK_LOOP

  …

  END_WHILE

• Examples:

  while(TRUE)

  exploreCave

  if(flashlightDies)

  BREAK_LOOP

  END_WHILE

  while(TRUE)

  number = askUserForNumber

  if(number is 5)

  BREAK_LOOP

  END_WHILE

An expression such as points++ means points = points + 1. Similarly, countdown-- means countdown = countdown - 1.

Using Scratch

Scratch offers three different type of loops, all shown in Figure 10-2, found under the Control category. The repeat loop is just like a for loop; it repeats exactly the number of times specified by the coder. The forever loop is one that is unique to Scratch; it repeats until the user presses the red stop sign, or uses a Stop block. It's basically a deliberate infinite loop! The repeat-until loop is just like a while loop; it repeats until the condition that the coder specifies is reached.

A typical while loop continues repeating the actions while the condition is true, and stops when the condition becomes false. In Scratch, the repeat-until loop continues repeating the actions while the condition is false, and stops when the condition becomes true. This can be really tricky for anyone, but especially young coders who’re switching between Scratch and other programming languages. The best way to mitigate this problem is to read the code in English. For example, the code for a typical while loop might read as “While my plate has food, take a bite”, whereas the code for a Scratch repeat-until loop might be read as “Repeat taking a bite until my plate is empty.” When in doubt, write it out!

Here are some examples of simple Scratch programs you might want to write using each of the types of loops that Scratch offers.

Repeat loop

The program shown in Figure 10-3 shows a repeat loop where you can have your sprite draw a shape on screen. You can change the number of times the loop repeats and the turn angle to create other shapes too. Give it a try!

Forever loop

The program shown in Figure 10-4 shows a forever loop where the character follows the mouse pointer! So anywhere you move your mouse, the character points towards it and moves 10 steps. Try changing the number of steps that the sprite takes each time to see what happens when you press the green flag!

Repeat-until loop

The program shown in Figure 10-5 shows a repeat-until loop where the character moves forward until it touches the color red. This can be used with random numbers and more than one character to create a race game!

Using Python

Python has two types of loops that it supports: while and for-each. A while loop can be written as a do-while loop however, and a for-each loop can be written to simulate a basic version of the for loop (where you start at one number and increment by one each iteration of the loop).

When you want to use a Python for-each loop like a for loop, use the range function. In this case, range includes the first number and excludes the second number. So if you want to print all the numbers from 1 to 10, your range parameters is range(1, 11).

In Python you must make sure that everything is indented properly. This means that any action that is repeated is indented once from the loop. If you have an action that you want to happen when the loop is done iterating, it should not be indented. For example:

while condition

actionToBeRepeated1

actionToBeRepeated2

actionToHappenAfterLoopCompletes

• While loops:

  while condition:

  action1

  action2

  …

• Examples:

  count = 1

  while count <= 10:

  print count

  count = count + 1

  number = 0

  while number != 5:

  number = input("Guess the number ")

  print "You guessed the right number"

• Do-while loops:

  action1

  while condition:

  action1

• Example:

  number = input("Guess the number ")

  while number != 5:

  number = input("Guess the number ")

  print "You guessed the right number!"

• For loops:

  for variableName in range(number1, number2):

  action1

  action2

  …

• Example:

  for count in range(1, 10):

  print count

• For-each loops:

  for item in list:

  action1

  action2

  …

• Example:

  classList = ["Sarah", "Camille", "Steve", "Rebecca"]

  for classmate in class:

  print "Hi " + classmate”

  name = "Sarah"

  for letter in name:

  print letter

• Explicit infinite loop:

  while True

  action1

  action2

  if condition:

  break

  …

• Example:

  while True:

  password = raw_input("what is the password? ")

  if password == "password":

      break

  print "Congratulations, you now have access"

Using pseudocode

Nesting loops can happen in any combination. Here are some examples of nesting loops, but this list is not exhaustive.

• For-each loop nested inside a for-each loop:

  for(item in list)

  action1

  for(item in list)

  action2

  action3

  …

  END_FOR_EACH

  action4

  END_FOR_EACH

• Example:

  jellyBeanCount = 0

  for(jar in jars)

  for(jellybean in jar)

  jellyBeanCount = jellyBeanCount + 1

  END_FOR_EACH

  END_FOR_EACH

• While loop nested inside a for-each loop:

  for(item in list)

  action1

  while(condition)

  action2

  action3

  …

  END_WHILE

  action4

  END_FOR_EACH

• Example:

  for(room in house)

  while(wallsWhite)

  paintWallsBlue

  END_WHILE

  END_FOR_EACH

Using Scratch

Nesting loops in Scratch can be a lot of fun when you introduce the pen and start drawing fun shapes. Here are some basic examples of nesting loops in Scratch.

Repeat loop nested inside a repeat loop

Figure 10-6 shows an example of a Scratch program that draws 10 concentric squares. The outer repeat loop is responsible for repeating 10 times for each square. The inner repeat loop is responsible for repeating 4 times, for each line and turn in each square.

Repeat loop nested inside a repeat-until loop

Figure 10-7 shows an example of a Scratch program that draws squares in random places around the screen until the user presses the space key.

Using Python

One of the most fun programs to write in Python with nested loops is ASCII art. In coding, each character and symbol that you can type on a keyboard has a number representation called its ASCII number. Although in today’s programming languages, you can type the character or symbol in your program, in the past you would have to use the ASCII number representation. ASCII art is basically when you create a picture using characters or symbols. There are some pretty amazing ASCII art examples, such as the one in Figure 10-8. As a kid, Camille used to have ASCII portraits of all the Star Trek: The Original Series cast members on her wall!

You can create a simple ASCII art program in Python using nested loops! For example, to create this pattern:

#

##

###

####

#####

######

#######

########

#########

The algorithm might be:

Print 1 # on Row 1

Print 2 # on Row 2

Print 3 # on Row 3

…

You could represent this using nested loops too!

for row in range(1, 11):

        rowText = ''

        for column in range(1, row):

                rowText = rowText + '#'

        print rowText