INQ241B - Day 3 Notes

Review Polygon

Want to draw a hexagon withh circuference 400:

function main()
  for 0  <= i < 6 do
    ♻turtle → forward(66.67)
    ♻turtle → left turn(60)
  end for
end function

Arithmetic

Now, what if we wnat a pentagon instead
Need to change the iterations, the forward distance, and the turn angle turning the same amount 5 times

But there is a better way, make the program do the math

function main()
  for 0  <= i < 6 do
    ♻turtle → forward(400 / 6)
    ♻turtle → left turn(360 / 6)
  end for
end function

Now we can change the shape just by changing all of the sixes to fives
Anwhere you can put a number, you can put a mathmatical equations as well

Variables

But changing all three of those variables is a bit of pain, and prone to making mistakes in a large program
Again, there is a better way, named values, also called variables
We can name the 5, according to what it represents, in this case the number of sides
var number of side := 5

Replace all of the fives with the variable name

function main()
  for 0  <= i < number of sides do
    ♻turtle → forward(400 / number of sides)
    ♻turtle → left turn(360 / number of sides)
  end for
end function

And changing the shape is as simple as changing one number in the program
Added bonus, it’s easier to read
We can make it even more easy to read by using another variable
Replace 400 with the name circumference
Note, the order matters; must declare the variable’s value before using the variable name

Reusing Variables

It is also possible to change the value of a variable after it has been declared

Let’s say we want to draw an N

function main()
  ♻turtle → forward(100)
  ♻turtle → right turn(135)
  ♻turtle → forward(141.42)
  ♻turtle → left turn(135)
  ♻turtle → forward(100)
end function

can use a varible for the distance it moves

but that distance changes each time, so update the variable

function main()
  var distance = 100
  ♻turtle → forward(distance)
  ♻turtle → right turn(135)
  distance := 141.42
  ♻turtle → forward(141.42)
  ♻turtle → left turn(135)
  distance := 100
  ♻turtle → forward(100)
end function

Note, when changing the value, don’t use var
Also note, order still matters

Updating Variables

But what if we want to change the size of the N?
Need to change two variables, one dependent on the other
We can do better

Can update a variable using its own value

function main()
  var distance = 100
  ♻turtle → forward(distance)
  ♻turtle → right turn(135)
  distance := distance * math → sqrt(2)
  ♻turtle → forward(141.42)
  ♻turtle → left turn(135)
  distance := distance / math → sqrt(2)
  ♻turtle → forward(100)
end function

So now can change the distance variable and it will change the size

Accumulating Variables

Here is a neat drawing that uses a loop

function main()
  var distance = 100
  var angle = 5
  for 0  <= i < 360 / angle do
    ♻turtle → forward(distance)
    ♻turtle → back(distance)
    ♻turtle → left turn(angle)
  end for
end function

Note the use of variables that make it easier to change the drawing
By updating variables inside a loop, we can change what it does each iteration of the loop.

For example, changing the distance each iteration

function main()
  var distance = 5
  var angle = 5
  for 0  <= i < 360 / angle do
    ♻turtle → forward(distance)
    ♻turtle → back(distance)
    ♻turtle → left turn(angle)
    distance := distance + 5
  end for
end function

Quiz

m.socrative.com
Room name ‘INQ241’

Activity

Activity 3