1. Counted (For) Loops in Alice
Stephen Cooper
Wanda Dann
Randy Pausch
Barb Ericson
July 2009
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2. Learning Goals Introduce the concept of a counted loop
A loop that keeps a counter or index
Show how a loop leads to easier to read and shorter code
Show an infinite for loop
Show a nested loop
Show a function call inside a loop
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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 then disappear only to appear elsewhere in the scene.
Of course, actions are made to happen again and again by running an animation instruction (or a method) more than once
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4. 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.
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5. bunny.hop
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6. One solution
Creating the same instruction again and again is somewhat tedious and the code gets longer and longer.
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7. Counted Loop
A counted loop is an alternate way to write repetitive code
Repeats instructions a counted number of times
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8. Demo BunnyHop Concepts illustrated in this example
The loop instruction executes a definite number of times, specified by a count or index
Using a loop instruction
saves time
is convenient
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9. Demo Carouselinfinity
Concept illustrated in this example
If “Infinity times” is selected for a loop, this means the loop will run until the program is shut down
This is an infinite loop
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10. More complicated loops
It is also possible to place a loop statement within another loop statement
This is called a nested loop
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11. 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.
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12. Demo FerrisWheel Concept illustrated in this example
The inner loop runs completely each time the outer loop runs once.
An outer loop that executes 2 times and an inner loop that executes 5 times will actually execute the inner loop 10 times.
In an animation, opposite actions that occur simultaneously effectively cancel each other out in terms of the rendered action. To demonstrate, set the outer loop to 1 time and the inner loop to 2 times. The inner wheel will look as though it rotates only once. As another example, you would see the same kind of result if you try to turn a person's head left and right ¼ revolution at the same time.
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13. Using a function
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.
Demo: LoopWithFunctionCall
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14. Challenges And have the bunny hop in a square using a nested loop
Copy BunnyHop
So that as the bunny nears the broccoli Papa rabbit (use Hare from the Animals folder) appears at the garden gateway and taps his foot
Have the bunny turn and hop out of the garden
Using a loop
Copy BunnyHop
And have the bunny hop in a square using a nested loop
Loop 4 times
Loop 3 times
Bunny hop
Bunny turn ¼ revolution
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15. Summary A for loop is similar to a while loop
But is useful when you need a counter or index in the loop
An infinite loop never stops
Loops can be nested inside of other loops
You can call a function inside the test for a loop
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