CS320n Visual Programming

1
CS320n Visual Programming

• Definite / Counted Loops
• (Slides 7-1)

Thanks to Wanda Dann, Steve Cooper, and Susan
Rodger for slide ideas.
2
What We Will Do Today

• Learn how what definite loops are and how to use
  them in a program

3
Repetition

• In many kinds of animations, especially
  simulations and games, some actions happen again
  and again.
• Example Gallery games where targets appear
  randomly on screen and are caught or shot down,
  only to appear elsewhere in the scene
• Actions are made to happen again and again by
  running an animation instruction (or a method)
  more than once

4
Computers are Fast

• This is why we use computers.
• The number of instructions a CPU can carry out is
  mind boggling
• Intel Processor 3.20 gigahertz.
• CPU doing 3,200,000,000 instructions per second.
• Simple instructions like adding two numbers,
  looking up the value of a variable
• Lots of simple instructions can so quickly can
  accomplish a lot
• 3D animations, working with DNA, complex
  simulations.

5
Example

• A bunny sneaks into a garden and wants to eat the
  broccoli. The bunny will need to hop several
  times to get to the broccoli.

6
bunny.hop
7
bunny.hop

• Makes bunny hop up and down, making a sound and
  traveling forward 0.8 meters
• Code on next slide
• How do we get the bunny to hop enough times to
  get over to the broccoli?

8
One solution

• Does this work?
• Are there any problems with it?

9
Counted Loop

• A counted loop is an alternative way to
  repetitive code
• Repeats instructions a counted number of times
• One of the commands available in Alice

10
Code for Loops

• The loop instruction executes a definite number
  of times, specified by a count. (Set when insert
  loop)
• Using a loop instruction
• saves time when coding
• is convenient, easy to change count
• can use a function that returns a number in place
  of the count

11
Modify the Bunny Animation

• Make the bunny hop over to a broccoli and eat it
• Then hop to next broccoli and eat it
• move broccoli so not all together
• only do with 3 broccoli
• fairly easy to expand to more broccoli

12
Infinity times

• If Infinity times is selected for a loop, this
  means the loop will run until the program is shut
  down

13
Example 1 Add a bunny

• Add a bunny to the broccoli scene
• this bunny should jump up and down while the
  other bunny eats all the broccoli

14
Does this Work?
15
How Do We fix This?

• Get both bunnies to move
• one definitely (the one that eats the broccoli)
• one indefinitely (the one that hops up and down
  until the movie stops?)

16
Example 2 Carousel

• To make the carousel go round continuously in an
  amusement park world

17
More complicated loops

• What can be placed in a loop?
• any number of other commands
• including other loops
• a loop within a loop
• this is a called a nested loop
• Example in book a double ferris wheel

18
An example of nested loops
The whole Ferris wheel will rotate clockwise,
while the two inner wheels will rotate
counterclockwise. The inner wheels should perform
2 revolutions for each outer loop revolution.
19
Rotating each of the wheels
Rotating the outer wheel 10 times
Rotating the inner wheels 2 times
20
The Complete Program
The two loops structures, inner wheel loop nested
within the outer wheel loop.
How many times does the inner loop execute?
21
Using a Function for Count

• A loop count can be computed by calling a
  function that returns a number value.
• The loop instruction automatically rounds the
  returned value to the nearest whole number.