QUIZ: What is the output of this MATLAB code? >> A = [2,4,10,13;16,3,7,18; 8,4,9,25;3,12,15,17]; >> length(a) >> size(a) >> B = A(2:3, 1:3) >> B(5)

Transcription

1 QUIZ: What is the output of this MATLAB code? >> A = [2,4,10,13;16,3,7,18; 8,4,9,25;3,12,15,17]; >> length(a) >> size(a) >> B = A(2:3, 1:3) >> B(5)

2 QUIZ

3 Ch.3 Introduction to MATLAB programming

4 3.1 Algorithms

5 Algorithm for buying a car Car 1 Cost: $35,000 MPG: 45 Car 2 Cost: $10,000 MPG: 10 Which car is best? a. Car 1 d. Not enough information b. Car 2 c. Same Inspired by : Udacity Introduction to Programming

6 Algorithm for buying a car Car 1 Cost: $35,000 MPG: 45 Car 2 Cost: $10,000 MPG: 10 What other information do we need?

7 Algorithm for buying a car Car 1 Cost: $35,000 MPG: 45 Car 2 Cost: $10,000 MPG: 10 What other information do we need? Total miles to drive: 60,000 Average cost of gasoline: $3.50

8 Algorithm for buying a car Car 1 Cost: $35,000 MPG: 45 miles: 60,000 cost: Car 2 Cost: $10,000 MPG: 10 $4/gal What is wrong with this algorithm? 1. Calculate costs for each car. 2. Choose the one with lower cost.

9 Algorithm for buying a car Car 1 Cost: $35,000 MPG: 45 miles: 60,000 cost: Car 2 Cost: $10,000 MPG: 10 $3.50/gal What is wrong with this algorithm? 1. Ask a friend for formula. 2. If friend doesn t know, GO TO step Plug in numbers for each car. 4. Choose car with lower cost.

10 Algorithm for buying a car Car 1 Cost: $35,000 MPG: 45 miles: 60,000 cost: Car 2 Cost: $10,000 MPG: 10 $4/gal Conclusion An algorithm must be: 1. Unambiguous, i.e. precise enough to be implemented in a program. 2. Guaranteed to terminate.

11 Algorithm for buying a car How would you change this algorithm so it s guaranteed to terminate? 1. Ask a friend for formula. 2. If friend doesn t know, GO TO step Plug in numbers for each car. 4. Choose car with lower cost. Hint: Ask at most 5 friends!

12 Algorithm for buying a car How would you change this algorithm so it s guaranteed to teminate? 1. Ask a friend for formula. 2. If friend doesn t know and I asked less than 5 friends 1. GO TO step If # of friends asked is less or equal to 5 1. Plug in numbers for each car. 2. Choose car with lower cost. 4. Else 1. Give up

13 Algorithm for buying a car Is this unambiguous/precise enough? 1. Ask a friend for formula. 2. If friend doesn t know and I asked less than 5 friends 1. GO TO step If # of friends asked is less or equal to 5 1. Plug in numbers for each car. 2. Choose car with lower cost. 4. Else 1. Give up

14 Algorithm for buying a car Add steps to keep track of the number of friends we have asked Hint: Add a variable nr_friends 1. Ask a friend for formula. 2. If friend doesn t know and I asked less than 5 friends 1. GO TO step If # of friends asked is less or equal to 5 1. Plug in numbers for each car. 2. Choose car with lower cost. 4. Else 1. Give up

15 Algorithm for buying a car 1. nr_friends = 0 2. Ask a friend for formula. 3. nr_friends = nr_friends If friend doesn t know and nr_friends < 5 1. GO TO step If nr_friends 5 1. Plug in numbers for each car. 2. Choose car with lower cost. 4. Else 1. Give up

16 A better Algorithm for buying a car 1. For each car: 1. gas_cost = miles/mpg*gallon_price 2. total_cost = gas_cost + purchase_cost 2. If total_cost1 < total_cost2 1. Buy car 1 3. Else 1. Buy car 2

17 A better Algorithm for buying a car 1. For each car: 1. gas_cost = miles/mpg*gallon_price 2. total_cost = gas_cost + purchase_cost 2. If total_cost1 < total_cost2 1. Buy car 1 3. Else 1. Buy car 2 Is this unambiguous/precise enough?

18 A better Algorithm for buying a car 1. For each car: 1. gas_cost = miles/mpg*gallon_price 2. total_cost = gas_cost + purchase_cost 2. If total_cost1 < total_cost2 1. Buy car 1 3. Else 1. Buy car 2 Add all the initialization (input) and output steps needed! Car 1 Cost: $35,000 Car 2 Cost: $10,000 MPG: 45 MPG: 10 miles: 60,000 cost: $3.10/gal

19 1. gallon_price = 4 2. mpg1 = 45, mpg2 = miles = purchase_cost1 = 35000, purchase_cost2 = For each car: 1. gas_cost = miles/mpg*gallon_price 2. total_cost = gas_cost + purchase_cost 6. If total_cost1 < total_cost2 1. Output Buy car 1 7. Else 1. Output Buy car 2 Now follow the algorithm and decide!

20 Conclusion: building blocks for algorithms Actions (input, output, computations) Decisions Loops (repetition)

21 1. For each car: 1. gas_cost = miles/mpg 2. total_cost = gas_cost + purchase_cost 2. If total_cost1 < total_cost2 1. Buy car 1 3. Else 1. Buy car 2 Is there a loop in this algorithm?

22 1. For each car: 1. gas_cost = miles/mpg*gallon_price 2. total_cost = gas_cost + purchase_cost 2. If total_cost1 < total_cost2 1. Buy car 1 3. Else 1. Buy car 2 Is the decision at the beginning or at the end of the loop?

23 Simple algorithm 1. Get input(s) 2. Calculate 3. Display output(s)

24 3.2 Scripts Computer languages: Machine Assembly High-level Compiling vs. interpreting Programs are compiled Scripts are interpreted MATLAB scripts are stored in m-files

25

26 Script that calculates area of circle Scripts can be run from the command window or directly from the editor window

27 Write a script that calculates the circumference of a circle

28 Write a script that calculates the volume of a sphere

29 Write a script that calculates the area of a rectangle

30 Documenting scripts with comments The first comment (or block of comments see next slide) in a script is returned by the help function

31 Comment blocks

32 3.3 Input and output The same function input() can be used to input numbers, characters and strings

33

34 Write a script that calculates the area of a rectangle, with the two edges being input by the user at runtime

35 Inputting strings: leading and trailing spaces Case 1: only spaces String is empty Case 2: leading spaces String includes them!

36 Inputting strings: leading and trailing spaces Case 3: trailing spaces String includes them!

37 Output statements disp() no formatting (simple!) fprintf() with formatting (complex!) >> disp('hello!') Hello! >> disp('6*7') 42 The output is not assigned to the default variable ans

38 We covered pp of our text

39 Output statements - fprintf placeholder Control character: newline >> fprintf('value is %d, surely\n!',6*7) Value is 42, surely! >> fprintf('int is %d, char is %c\n', , 'a' ) Int is 42, char is a

40 QUIZ: Output statements The numbers 3 and 10 are stored in the variables a and b. Print this to the screen: The sum of 3 and 10 is 13.

41 QUIZ: fprintf >> myname = 'Riddick' Print this to the screen: How can we print a quote inside a string? The length of the name 'Riddick' is 7. Don t hardcode 7 it has to work with any name!

42 Inside the placeholder, we can specify the width of the field >> fprintf('value is %5d!\n',6*7) Value is 42! Use the width option to print the following 3 numbers, aligned in a column: (do not cheat by putting spaces by hand!)

43 If we want left alignment, we use a dash >> fprintf('value is %-5d!\n',6*7) Value is 42! Print the following 3 numbers in a column, aligned to the left: (do not cheat by putting spaces by hand!)

44 Printing vectors and matrices In a script, we can fix this problem!

45 Printing vectors and matrices

46 Write a complete script that creates a row vector of all the prime numbers smaller than 10, and then prints them nicely on one line. On the line above the numbers, there should be printed the word Primes.

47 Printing vectors and matrices Remember: MATLAB stores matrices by columns!

48 Printing vectors and matrices Finally!

49 Extra-credit

50 Straight from the horse s mouth Source:

51 Printing vectors and matrices the painless way!

52 3.4 Scripts with I and O

53

54 Section 3.5 Simple Plots will be covered in the lab

55 3.6 File I/O Three operation modes for files: Read use load Write use save Append use save

56 3.6 File I/O Caution: If the file already exists, save will overwrite it! The ascii option indicates a text file

57 Use the append option in order to keep the old data in the file

58

59 Reading from a file

60 create a third vector, having as elements the differences between the second and the first divisions sales

61 We covered pp and of our text

62 QUIZ: Exercise 5

63 What will this script output? m = [1 2 3;4 5 6]; save mat.txt m ascii m = [1 2 3;4 5 6;7 8 9]; save mat.txt m ascii load mat.txt fprintf('%-3d%-3d\n', mat);

64 3.7 User-defined functions

65 3.7 User-defined functions Write a function that returns the volume of a sphere

66 3.7 User-defined functions Write a function that returns the volume of rectangular box

67 User-defined functions are called and used just like MATLAB built-in functions

68 Documenting user-defined functions

69 Calling user-defined functions with array arguments

70 QUIZ Write the function calcrectarea() that is called as shown

71 3.7.6 Local vs. global variables

72 Workspaces All scripts and the command window have the same workspace. Each function has its individual workspace.

73 Homework for Ch.3 End-of-chapter: 8,9,11,23,25,33, 35 Hint: use repmat() (p.47 of text) 39 Due: Wednesday, Oct.1