Algorithms (continued)

Transcription

1 Algorithms (continued)

2 QUIZ What are the 3 (or 4) fundamental control structures that we use as building blocks for algorithms?

3 Building blocks for algorithms Sequential Actions (input, output, computations) Decisions Loops (repetition)

4 COSC 1100 Freshman Seminar Fall 2016 Source: AP CS Principles Course and Exam Descriptions

5 QUIZ What is pseudocode?

6 QUIZ What are the symbols that we use in flowcharts?

7 Flowchart Symbols Input/Output Process step or instruction Conditional Test (decision) Start/Stop Flowchart Rules Always have a Start and a Stop Blocks are connected with lines and arrows indicating the direction of process flow Flow should mainly be top to bottom or left to right Connector

8 What is printed by this pseudocode? Set of purchase prices: {42, 100, 10} Total_cost = 0 For each purchase: tax = purchase_price * 0.08 Total_cost = purchase_price + tax Print Total_cost

9 What is printed by this pseudocode? Set of purchase prices: {42, 100, 10} Total_cost = 0 For each purchase: tax = purchase_price * 0.08 Total_cost = purchase_price + tax Print Total_cost Watch the indentation carefully!

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11 Modify the pseudocode to get the correct algorithm Hint: Use else

12 What does this algorithm print?

13 What does this algorithm print?

14 What does this algorithm print? N = 20 While N > 0 Print N if N is even N = N/2 else N = 3*N + 1

15 The algorithm does not terminate! N = 20 (infinite loop) While N > 0 Print N if N is even else N = N/2 N = 3*N + 1 Make a small modification so it terminates!

16 What does this algorithm print? N = 20 While N > 1 Print N if N is even N = N/2 else N = 3*N + 1 EOL 1

17 COSC 1100 Freshman Seminar Fall 2016 Source: AP CS Principles Course and Exam Descriptions

18 COSC 1100 Freshman Seminar Fall 2016 Source: AP CS Principles Course and Exam Descriptions Is the test performed at the beginning or at the end of the loop?

19 COSC 1100 Freshman Seminar Fall 2016 Source: AP CS Principles Course and Exam Descriptions Is the test performed at the beginning or at the end of the loop?

20 What is displayed? Source: AP CS Principles Course and Exam Descriptions COSC 1100 Freshman Seminar Fall 2016

21 What is displayed? A: Nothing it is an infinite loop, b/c i never reaches 4! Source: AP CS Principles Course and Exam Descriptions COSC 1100 Freshman Seminar Fall 2016

22 How can we fix the algorithm to avoid the infinite loop? What is displayed? A: Nothing it is an infinite loop, b/c i never reaches 4! Source: AP CS Principles Course and Exam Descriptions COSC 1100 Freshman Seminar Fall 2016

23 Counting marbles There are 20 marbles in a bag, some green, some blue. The only action available to us is to draw a marble (at random) and examine its color. Develop an algorithm to determine and print the numbers of green and blue marbles.

24 There are 20 marbles in a bag, some green, some blue. The only action available to us is to draw a marble (at random) and examine its color. Develop an algorithm to determine and print the numbers of green and blue marbles. Imagine yourself doing the counting. What would you write down on your scratchpad? What variables would a computer program need to keep track of the same information?

25 There are 20 marbles in a bag, some green, some blue. The only action available to us is to draw a marble (at random) and examine its color. Develop an algorithm to determine and print the numbers of green and blue marbles. What control structures would the computer need to implement the algorithm (initializations, loops, decisions)?

26 There are 20 marbles in a bag, some green, some blue. The only action available to us is to draw a marble (at random) and examine its color. Develop an algorithm to determine and print the numbers of green and blue marbles. Write the pseudocode!

27 Pseudocode Solution 1. Initialize color counters: green = 1 and blue = 1 2. Initialize marble count: count = 1 3. Draw a marble 4. If the marble is green, increment the green count green = green If the marble is blue, increment the blue count blue = blue Are there any marbles left? (is count 20?) 1. If yes, increment marble count count = count + 1 and go to step 3 2. If no, then output green and blue. 7. The end

28 Draw the flowchart!

29 START Partial solution green=1 blue = 1 count = 1 Select a marble Y Is green? N

30 START Partial solution green=1 blue = 1 count = 1 Select a marble Y green = green + 1 Is green? N blue = blue + 1

31 START Partial solution green=1 blue = 1 count = 1 Select a marble Y green = green + 1 Is green? N blue = blue + 1 count<20? Y N

32 START green=1 blue = 1 count = 1 Y green = green + 1 Select a marble Is green? N blue = blue + 1 count = count + 1 count<20? N Y OUTPUT blue, green STOP

33 Your turn: Can you find the mistakes? Hint: What would be printed if there were 20 green marbles?

34 Solution What would be printed if there were 20 green marbles? Conclusion: off by one 21 1

35 Corrected pseudocode 1. Initialize color counters: green = 0 and blue = 0 2. Initialize marble count: count = 1 3. Draw a marble 4. If the marble is green, increment the green count green = green If the marble is blue, increment the blue count blue = blue Are there any marbles left? (is count < 20?) 1. If yes, increment marble count count = count + 1 and go to step 3 2. If no, then output green and blue. 7. The end

36 Assignment #4 (final): There are 42 marbles in a bag, some green, some blue, and some red. The only action available to us is to draw a marble (at random) and examine its color. Develop an algorithm to determine the number of R, G, and B marbles: Write a pseudocode solution. Write a flowchart solution. Hint: We need two decision in the increment part of the algorithm Due Wed., Oct.11

37 COSC 1100 Freshman Seminar Fall 2016 Source: AP CS Principles Course and Exam Descriptions