How to Teach Kids to Code Nesting Loops

Nested loops can seem complicated to young coders, but they are basically a loop within a loop. This can be useful for making drawings like fractals or fun shapes that repeat but slightly change each time, creating ASCII art, or even having complex repetitions. The examples found here show how nesting loops can help create some fun programs. You can nest any kind of loop inside any other kind of loop. For example, you can nest a for loop inside a while loop!

A fractal is a geometric object that has self-similarity — it shows the same pattern whether you’re looking at it far away or close up. Fractals can provide mathematically accurate models of coastlines, mountains, and other real objects from nature. Loops (specifically, recursive loops) are used to code fractals. One of the earliest uses of computer graphics in big-screen movies was the creation of fractal-based scenery for the transformation of the Genesis planet in Star Trek II: The Wrath of Khan. Khhannnnnn!

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

The image below 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

Take a look at the image below to see 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. 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</pre