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Recursion Alice. Repetition In some situations, we dont know exactly how many times a block of instructions should be repeated. All we know is that repetition.

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Presentation on theme: "Recursion Alice. Repetition In some situations, we dont know exactly how many times a block of instructions should be repeated. All we know is that repetition."— Presentation transcript:

1 Recursion Alice

2 Repetition In some situations, we dont know exactly how many times a block of instructions should be repeated. All we know is that repetition is needed For example, in a board game like chess or checkers, we dont know exactly how many moves it will take for a player to win or lose the game – all we know is that several moves will be needed.

3 Indefinite Repetition In programs where a count of repetitions is not known (indefinite), we can use one of two repetition control mechanisms: Recursion While statement This session focuses on Recursion.

4 Recursion Many of the pieces we use to create a program are identified by using special words. For example, Do in order Do together If/Else Loop Recursion is not a program statement with a special word that identifies it as part of the programming language. Recursion means that a method (or a question) calls itself.

5 Example – horse race A carnival style horse race. In repeated moves, one horse is randomly selected to move forward. The selected horse runs straight ahead to the finish line. A horse is the winner if it gets to the finish line before any other horse. When one horse wins, the game ends.

6 Storyboard "do everything again" means that the entire method should be repeated – this is where recursion occurs. race If one of the horses has won the winner says, I won!!! Else randomly choose one horse and move it forward a small amount do everything again

7 Question How do we implement do everything again ? Create a call to the race method itself. Recursion means that a method calls itself. race If one of the horses has won the winner says, I won!!! Else randomly choose one horse and move it forward a small amount call the race method

8 Stepwise Refinement race If one of the horses has won the winner says, I won!!! Else randomly choose one horse and move it forward a small amount call the race method isGameOver? whichHorseWon?moveRandomHorseForward

9 isGameOver To determine whether the game is over, ask the question: Is the finish line < 0.5 meters (arbitrary distance) in front of a horse?

10 moveRandomHorseForward To randomly choose a horse and move it forward, use the built-in random selection question.

11 whichHorseWon To determine the winner, we ask the question: "Which horse is within 0.5 meters of the finish line?"

12 Putting together the pieces Recursive call. Note: The winning horse says "I won!!!" -- a lame ending, we know.

13 Demo!

14 Testing Testing a program that used random numbers requires extra caution. In this example, we ran the program 20 times and found that racehorse1 won 7 times racehorse2 won 3 times racehorse3 won 10 times Something is wrong! Each horse should win approximately 1/3 of the time.

15 Removing the bug The bug in this code is that we have nested If statements, and we used a 33% probability for each If statement What we didn't consider is that if racehorse1 was not selected, then we have a 50% probability of selecting either racehorse2 or racehorse3.

16 Revised moveRandomHorseForward The corrected code:

17 Assignment Read Chapter 8-1, Recursion Lab 8-1


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