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Exploring Loops

Using a Loop

A loop is a way to repeat the same action.

Instead of writing:

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# Draw a square:
jet.forward(distance)
jet.right(angle)
jet.forward(distance)
jet.right(angle)
jet.forward(distance)
jet.right(angle)
jet.forward(distance)
jet.right(angle)

We can use a loop: 

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# Draw a square:
for i in range(4):
    jet.forward(distance)
    jet.right(angle)

Let's examine the pieces of the for statement:

  1. Starts with the reserved word for
  2. Uses a variable that iterates
  3. Next is the reserved word in
  4. Finally, there is the data that is iterated over. In this example, we have a set (or range) of numbers. Note: a range is defined (by default) as starting at zero, so range(4) means that we are counting 0, 1, 2, 3.

See a loop in action: https://trinket.io/python/11386c7fa2

More on Loops

  • We can change the starting value of a range, for example: range(2, 6)
    • What numbers are in this sequence?
  • We can change the way a loop counts, for example: range(1, 100, 2)
    • What numbers are in this sequence?
  • We can nest loops: 
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    for x in range(2, 6):
    for y in range(1, 100, 2):
        z = x * y
  • Remember that Python variables can be words as well as numbers, so what happens if we do this? 
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    animal = 'squirrel'
for x in animal:
    print(x)

Experiments

  1. Can you make the turtle draw a larger square? Hint: change the distance to be 80. How big can you make the square before the turtle goes off the screen?
  2. A polygon's internal angle can be calculated as: angle = 360 / Number of sides. A square is a type of polygon. It has four sides, so the internal angle = 360 / 4 = 90 degrees!
  3. Can you make a hexagon? This is a figure with six sides. Hint: Use the equation above to calculate the internal angle.
  4. Can you make an octagon? An Octagon has eight sides.
  5. Can you make a stop sign? You will need to use a jet.color('red'). a jet.beginfill() and a jet.endfill(). You can add the text of the word "stop" by using jet.moveto(x,y) and jet.write('STOP', font=("Arial", 30, "normal")). You can also use the jet.hideturtle() so that the outline of the turtle is not displayed at the end.

Drawing the Stop Sign: https://trinket.io/python/eb27ed7fef