ForLoopsInAlice1 Stephen Cooper Wanda Dann Randy Pausch Barb Ericson Oct 2009 Counted (For) Loops in Alice

ForLoopsInAlice2 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

ForLoopsInAlice3 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

ForLoopsInAlice4 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.

ForLoopsInAlice5 bunny.hop

ForLoopsInAlice6 One solution Creating the same instruction again and again is somewhat tedious and the code gets longer and longer.

ForLoopsInAlice7 Counted Loop A counted loop is an alternate to repeated code –Repeats instructions a counted number of times

ForLoopsInAlice8 Demo 07BunnyHop 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 Easy to modify

ForLoopsInAlice9 Demo 07Carousel 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

ForLoopsInAlice10 More complicated loops It is also possible to place a loop statement within another loop statement This is called a nested loop

ForLoopsInAlice11 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.

ForLoopsInAlice12 Demo 07FerrisWheel 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.

ForLoopsInAlice13 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: 07LoopWithFunctionCall

ForLoopsInAlice14 Challenges Copy 07BunnyHop to 07BunnyOut –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 07BunnyHop to 07BunnySquare –And have the bunny hop in a square using a nested loop Loop 4 times –Loop 3 times »Bunny hop –Bunny turn ¼ revolution

ForLoopsInAlice15 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