CS320n –Visual Programming

CS320n –Visual Programming

Definite / Counted Loops(Slides 7-1)

Thanks to Wanda Dann, Steve Cooper, and Susan Rodger for slide ideas.

Visual Programming Definite / Counted Loops 2

What We Will Do Today• Learn how what definite loops are and

how to use them in a program


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


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.


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.


bunny.hop


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?


One solution

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


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


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


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


Infinity times…• If “Infinity times” is selected for a loop, this

means the loop will run until the program is shut down


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


Does this Work?


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?)


Example 2: Carousel

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


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


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.


Rotating each of the wheelsRotating the outer wheel 10 times

Rotating the inner wheels 2 times


The Complete ProgramThe two loops structures, inner wheel loop nested within the outer wheel loop.

How many times does the inner loop execute?


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.

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CS320n –Visual Programming