INQ241A
Mobile Apps

Move To

• Draw a star

  function main()
    var size = 10
    for 0 <= i < 5 do
      ♻turtle → forward(size)
      ♻turtle → left turn(144)
    end for
  end function

• What if I want the star in a particular location?
• We can pick up the pen, move to the location, put the pen down, and draw
• But it can be a bit of pain to calculate the turn angle and move distance to get to a particular location
• To make it easier, we can show a grid, turtle → show grid
• Note, that the coordinates may be different than you are used to
• The point (0,0) is in the upper left, and y values increase downward

• To move to a location on the grid can use turtle → move to

  function main()
    turtle  → move to(200, 300)
    var size = 10
    for 0 <= i < 5 do
      ♻turtle → forward(size)
      ♻turtle → left turn(144)
    end for
  end function

• The direction that the turtle is facing after a move to may be unpredictable
• If you are using the forward and turn functions, they assume a certain heading.

• To ensure a certain heading, use the turtle → set heading function

  function main()
    turtle  → move to(200, 300)
    turtle  → set heading(0)
    var size = 10
    for 0 <= i < 5 do
      ♻turtle → forward(size)
      ♻turtle → left turn(144)
    end for
  end function

• If we want to draw a bunch of stars, we could use a loop, but then they would have to be in a regular pattern

• To put them at arbitrary locations without copying code, we need something new…

Functions

• A variables are named values, functions are named code
• Currently we have one function, main, it is what is executed when you choose run
• To make a new one, click on the script button above your code and choose ‘add new’
• From the menu, choose function
• Your code disappeared! Don’t worry we can get back to it
• What is created is a new function, it has the name ‘do stuff’ and does nothing, let’s fix that
• First change the name to draw circle, click on the name to change it to a text field

• Now we can put whatever code we want inside this function, let’s put the circle drawing code here

  function draw star()
    for 0 <= i < 5 do
      ♻turtle → forward(10)
      ♻turtle → left turn(144)
    end for
  end function

• Now we can run, or call, this function from the main function
• To get back to the main function, click the script button at the top and click on the main function

• Delete the code that draw the circle and replace it with the function call

  ▷ draw star

• Using functions makes creating large programs easier

• You have been doing this, every time you do turtle something, you are running code that someone else wrote

Parameters

• But now the draw circle makes the same size star every time
• We can modify the behavior of the function using paramters
• If you click on the function name line, without clicking on the function name, because that changes the function name
• There is a + button for ‘add input parameter’
• click that and inside the parentheses after the function name there is a text box where you can name the parameter
• Name it size

• A parameter is a variable that gets it’s value set when the function is called.

  function draw star(size: Number)
    for 0 <= i < 5 do
      ♻turtle → forward(size)
      ♻turtle → left turn(144)
    end for
  end function

• Use the new parameter in the main function
• Note, the main function has an error ‘not enough parameters supplied’

• Must specify the value, delete and re-add

  function main()
    ▷ draw star(10)
  end function

• Now we can call the function multiple times and get multiple stars

  function main()
    ▷ draw star(10)
    ♻turtle → pen up
    ♻turtle → move to(200, 300)
    ♻turtle → pen down
    ▷ draw star(20)
  end function

Multiple Parameters

• Can also have multiple parameters
• Add extra parameters for the x and y location to move to

• Use move to and set heading before drawing the star

  function draw star(size: Number, x: Number, y: Number)
    ♻turtle → pen up
    ♻turtle → move to(200, 300)
    ♻turtle → set heading(0)
    ♻turtle → pen down
    for 0 <= i < 5 do
      ♻turtle → forward(size)
      ♻turtle → left turn(144)
    end for
  end function

• Now, it only takes one line to draw a star at different locations

  function main()
    ▷ draw star(10, 200, 300)
    ▷ draw star(20, 300, 200)
  end function

• Notice that all of the parameters have : Number after their name
• This signifies their type, they are all numbers
• All of the variables we have seen so far have also been numbers
• One other type we can have is : Color
• To change the type of a parameter delete the current type and choose the desired type

• So to set the color of the star with a parameter

  function draw star(size: Number, x: Number, y: Number, color: Color)
    ♻turtle → pen up
    ♻turtle → move to(200, 300)
    ♻turtle → set heading(0)
    ♻turtle → pen down
    ♻turtle → set pen color(color)
    for 0 <= i < 5 do
      ♻turtle → forward(size)
      ♻turtle → left turn(144)
    end for
  end function
  
  function main()
    ▷ draw star(10, 200, 300, colors → red)
    ▷ draw star(20, 300, 200, coloras → blue)
  end function

Quiz

• socrative.com
• Room name ‘INQ241’

Activity

• Activity 5