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Section 17.6 Multiple Parameters

Below we have a program that draws a square of size 100. What if we want to draw a different size square, like one with a side length of 50? We could do that by changing each of the 100s into a 50.
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But, this means we have to change each of the four forward statements and we could make a mistake and not set all of them to the same number. Is there a better way? What if we create a variable size and set its value to the amount to move forward?
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Now the program is easier to change since we only have one line to change - size = 50 - to draw another size square. Once we change the size variable, all of the forward commands will use that new value.
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But, the procedure can still only draw a square of one particular size. We can’t use it to draw two different size squares. If we want grace to draw a small square and then a large square, we would need two different procedures:
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def square(turtle):
    size = 50
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)

def largesSquare(turtle):
    size = 100
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
Yuck. How repetitive. Now imagine we want to make squares of size 75 and 25 as well. We would need 4 different procedures that all look basically the same! Remember that if you find yourself writing the same code multiple times, there is probably a better way to do things.
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In this case, we can add an additional parameter to the procedure that specifies the size of the square. Recall that a procedure can have as many parameters (inputs) as we like - just separate the names for the parameters with a comma like: (turtle, size).
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Now that square has two parameters, any call to square must provide two arguments. We need to specify the name of the turtle to draw with and the size of the square to draw. Something like square(grace, 100) or square(grace, 50).
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Checkpoint 17.6.1.

What shape would the following code draw?
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def mystery(turtle,size):
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size / 2)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size / 2)
    turtle.right(90)

from turtle import *    # use the turtle library
grace = Turtle()        # create a turtle named grace
mystery(grace, 100)     # draw something with size = 100
  • square
  • Check the 2nd and 4th forwards. How much do they move forward by?
  • rectangle
  • This will draw a rectangle with two sides with the specified size and two sides half that size. Copy this code into the area above and run it.
  • triangle
  • A triangle has 3 sides.
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Checkpoint 17.6.2.

In the following code what are the arguments?
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def square(turtle,size):
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)
    turtle.forward(size)
    turtle.right(90)

from turtle import *    # use the turtle library
imani = Turtle()        # create a turtle named imani
square(imani, 25)      # draw a square with size 25
  • turtle and size
  • These are the names of the parameters (formal parameters).
  • grace and 25
  • Look again at the code above. Is that the name of this turtle?
  • imani and 25
  • The turtle is named imani and the size is 25 in the code: square(imani, 25).
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Checkpoint 17.6.3.

The following code assumes that a procedure square has been defined that takes a size. The code should create a turtle and then use it to draw a square, move forward, and draw a second square as shown below, but the lines are mixed up.
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