Day 1 review

1. Day 1 review • What is hardware. • What is software. • What is a vector. • What is a matrix. • Let's fire up Matlab.

2. Day 2 plan • What is a script file. • Flow control(1): for loops. • Logic (tomorrow) • Let's fire up Matlab.

3. What is a script file? Files of the type “.m” are simply a collection of orderly command- line instructions (scripting language) type: > edit moo.m and save it in your matlab directory.

4. script files Inside moo.m type the following sequence of commands: >str='this is my first program'; Save your file, close it and on the command line, execute it by typing: >moo

5. script files What happened? Why? Allow your program to show you its output and run it again.

6. script files Change your program to this: str='this is my first program'; disp(str); Run moo Differences?

7. clear all

8. script files Erase everything on moo and type: a=[… 1 2 3 6 4 5 6 7.811]; Save moo and execute it. CHECK WORKSPACE

9. script files Two types of m files: - programs (aka scripts) don't take inputs. don't produce outputs. but leave variables in workspace. - functions (Day 4). can take input (passing variables) can return outputs. variables are internal.

10. script files (1) Write a program that creates a variable named u that has 10 rows, and one column using the function zeros

11. script files (2) Add code that creates a variable v that has all the elements from 1 to 10, using the command: X = startN:Increment:endN

12. script files (2) Finally, create a variable w that has zeros in its first column and the numbers 1 through 10 in its second column using the “concatenate” command [ ] concatenatedAB= [A B]; or concatenatedAB= [A;B]; You will also need the function transpose: ' example: if A=[1 2 3] B=A' produces B=[ 1 2 3]

13. Flow control p.12 • REPEATING ORDERS: • 'FOR' loops • If you want to do something a fixed number of times: • you say “FOR the next ten times do this.”

14. script files FOR EXAMPLE: UofI.m • WRITE A SCRIPT TO CALCULATE THE FIRST 10 numbers in the sequence:u(n) = u(n-1) x (n+1)with u(1) = 1 • What we want is a vector U with 10 rows, U(i) = u(i)

15. script files This is a “list” the “FOR” command will run through… Any list will do! FOR syntax: • for n=first:increment:final . . . . . . end --> USE THE COMMAND “zeros” to get your variable U started. REMEMBER! Type n=1:10:100 on your command prompt What happened? And k=1212:-pi:0

16. script files A NOTE ABOUT HELP The first continuous lines of commented text atop of your script are read as the “help” file for your program. Etiquette dictates that you specify at least: What the program does and the “how to” Who wrote it When it was last updated Quirks & bugs GO AHEAD AND INCLUDE HELP FOR UofI

17. UofI.m % this is my first program, it does ... % created on august 4, 2005 by Alejo. u=zeros(10,1); u(1)=1; for i=2:10 u(i,1) = u(i-1,1) .*(i+1); %INDENT end; u .

18. UofI.m Issue of first and last CASE!! In your mind, testing these two cases is critical!!!!

19. UofI.m • It would be nice to add the index to U. So each row show index i and U(i). • Go ahead…

20. UofI.m % this is my first program % created on august 4, 2005 U=zeros(10,2); U(1,1)=1; U(1,2)=1; for n=2:10 U(n,1) = U(n-1,1) .*(n+1); U(n,2) = n; end; U

21. UofI.m It would be even nicer, if we asked the user the first value of the series. Use 'input'

22. UofI.m Syntax: VAR = input('your text here'); (EVALUATED INPUT) For entering strings of charactersVAR = input('Your text here','s'); (NOT EVALUATED INPUT)

23. UofI.m % this is my first program % created on august 4, 2005 U=zeros(10,2); U(1,1)=input('first value? '); U(1,2)=1; for n=2:10 U(n,1) = U(n-1,1) .*(n+1); U(n,2) = n; end; U

24. UofI.m RUN UofI and as input put: 1220-1219 That's why it is evaluated input.

25. Embedding for loops Multiplication table example. Write a 10 x 10 matrix such that each element is the product of the row and column number multable(i,j) = i*j;

26. Multiplication table For i=1:10 for j=1:10 Multable(i,j)=i*j;end end

27. A second look at for What is the variable: myname= 'ALEJO'; Answer: A list of 5 characters! Try switching the case of myname by adding 32 to each ascii code… use function “double”

28. Check the Help of double What does it mean??? Well, a “character” is not a double, so double transforms it into its “number equivalent”, which is its ASCII code. So, asciinumberofa=double('a')

29. How do we get the letter back? Try “edit char” so, now type: backtoa=char(asciinumberofa)

30. A second look at “for” What is the variable: myname= 'ALEJO'; Answer: A list of 5 characters! Try switching the case of myname by adding 32 to each ascii code…

31. A second look at for Try switching the case of myname by adding 32 to each ascii code… (edit moo2.m) myname='ALEJO'; counter =1; for l=myname lowname(counter)=double(l)+32; counter = counter +1; end; lowname=char(lowname) Did the size of my name mattered??? Try running the program with: myname='Alejo'

32. Without the “for” Try switching the case of myname by adding 32 to each ascii code… (edit moo2.m) myname='ALEJO'; asciiname=double(myname); newname=asciiname+32; lowname=char(newname); We've VECTORIZED the For Loop!An order of magnitude faster. Or EMBEDDING:lowname=char(double(myname)+32);

33. One more example: WRITE Moo3.m in which you add all the elements of a two dimensional matrix entered by the user. USING: “input” for loop “size” (help size)

34. Moo3.m matrix=input('enter two dimensional matrix '); s=size(matrix); tot=0; for i=1:s(1) for j=1:s(2) tot=matrix(i,j)+tot end end WORK ON A PIECE OF PAPER THE STEPS OF THIS FOR LOOP.

35. Moo3.m matrix=input('enter two dimensional matrix '); s=size(matrix); tot=0; what is sum(matrix)? %help sum so what is tot=sum(sum(matrix));

36. Multiplication table Can you do it with two embedded loops now?

37. Multiplication table For i=1:10 for j=1:10 Multable(i,j)=i*j;end end

38. LOGIC p.8 AND: & OR: | Complement: ~ Exclusive OR: xor

39. LOGIC p.8 What is TRUE in Matlab? NON ZERO ELEMENTS. TRY: a=[0 1 1 0 1]; b=[1 1 0 0 234009]; c = a&b d = a|b e = ~a f = xor(a,b)

40. LOGIC p.8 Question (without typing): Mylove=100009999; Yourlove=0; Ourlove = xor(Mylove,Yourlove); Whose love is true?

41. LOGIC p.8 BRUSH UP ON YOUR LOGIC!! --> YOU WILL MAKE LOCIGAL MISTAKES and you'll think your computer has gone crazy. … humbling experience -->TEST AND RETEST LOGICAL STATEMENTS. -->DO NOT MAKE CONVOLUTATED logical statements in one swoop:such as “if (this is true) or (that is wrong while x is also true) but not y”.

42. IF IF something is true do {list of commands} Else { list of commands2} end

43. IF syntax if (conditional statement) … elseif (conditional statement) … else … end Conditional statements have RELATIONAL Operators p. 8: Larger than, less than, equal to, not equal to…

44. IF syntax So… if (Mylove) Mylove=0; else Yourlove=0; end; Whose love is true?

45. IF syntax So… if (Mylove == true) % == is not = Mylove=0; else yourlove=0; end; Whose love is true? WHY????%help true

46. IF exercise Ask user for password. If password is less than 6 characters long, return "invalid password"

47. IF exercise Algorithm steps: 1. ask for user input (password) 2. check size of password 3. if size is too small, reject password

48. IF exercise > pwd = input('Please enter password: '); pwdsize = length(pwd); %check size of pwdsize if (pwdsize < 6) 'password invalid' else 'valid password' end

49. SWITCH: a more convenient if If you know in advance SEVERAL OUTCOMES ARE POSSIBLE, use SWITCH. For example: ask user to choose a number from 1 to 10, each number prints an associated joke.

50. SWITCH: syntax Something like: choice = input('number from 1 to 10? ') switch choice case 1 … case 2 … … otherwise … end. Choice is the name of the variable that can take the values listed after the “case” word.