Lecture 11: Logical Functions & Selection Structures CMPSC 200 Programming for Engineers with MATLAB

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1 Lecture 11: Logical Functions & Selection Structures CMPSC 2 Programming for Engineers with MATLAB Brad Sottile Fall 214

2 Slide 2 of 3 Midterm 1 Details You must bring your PSU student ID card Scantron must be in #2 pencil 5 Questions/1 points 3 m/c questions (1 point each) 1 true/false questions (1 point each) 5 short answer questions (2 points each) 5 programming questions (1 points each) You ll have 2 hours (hopefully won t need all of it) You may bring one (1) 8.5 x 11 sheet of notes, equations, syntax, etc. You may write on both the front and back. We won t collect it. Will send an about room assignments once I ve confirmed the number of conflict exams. Only material through last week (i.e. up through and including Chapter 7) will be on the exam. Knowledge learned from labs, project 1 and in-class practice/review exercises is all fair game.

3 Slide 3 of 3 Advice of the Day Being smart isn t good enough anymore Look around you everyone in this room was a good or excellent student in HS or they wouldn t be here What sets good scientists and engineers apart from the rest then? Teamwork Communication skills Knowing when to lead, and when to follow Specialization (becoming an expert at topic X ) Knowing when and how to ask for help Others?

4 Slide 4 of 3 Categorizing Code Sections of computer code can be classified into three categories Sequences lines of code are executed one after another (i.e. what we ve been doing) Selection Structures executes some piece of code if some known condition is true, otherwise executes some sort of alternative code (can be many branches). Repetition Structures (loops) causes a group of statements to be executed multiple times (either a fixed number, or until some stated condition is met). [We ll talk about this one next week.]

5 Slide 5 of 3 Graphical View of Classification Sequence Straight through Selection Repetition As many branches as Fixed or Variable needed; may Number of loops re-converge to sequence

6 Slide 6 of 3 Relational Operators One can perform comparisons between the relationships between scalars or vectors) using the relational operators show in the table below. Beware: means equal to/assignment means comparison for equality Remember: is false, 1 is true (binary logic!) Relational Operator Interpretation < less than < Less than or equal to > greater than > greater than or equal to equal to ~ not equal to Source: Table 8.1 in Moore, H., MATLAB for Engineers, 4th ed., p. 272

7 Slide 7 of 3 Examples of Relational Operators (1 of 2) Input: Output: x 1; y 2; a x < y b x > y c x y d x ~ y a 1 b c d 1 Remember: is false 1 is true

8 Slide 8 of 3 Examples of Relational Operators (2 of 2) Input: x y a b c d Output: [1 2 3]; [2 1 3]; x < y x > y x y x ~ y a b c d Remember: is false 1 is true 1

9 Slide 9 of 3 Logical Operators One can augment performing comparisons using and, not, and or Beware: is the pipe (underneath backspace) Remember: is false, 1 is true (binary logic!) Logical Operator Interpretation & and ~ not or xor exclusive or && short cut and (used with scalars) short cut or (used with scalars) Source: Table 8.2 in Moore, H., MATLAB for Engineers, 4 th ed., p. 273

10 Slide 1 of 3 Examples of Logical Operators (1 of 2) Input: x 1; y 2; z 1; a b c d z > x & z < y z > x z < y ~x xor(x,y) Output: a b c d 1 Remember: is false 1 is true

11 Slide 11 of 3 Examples of Logical Operators (2 of 2) Input: Output: x [1 2 3]; y [2 1 3]; z [1 2 ]; a b c d a b c d z > x & z < y z > x z < y ~z xor(x,y) 1 Remember: is false 1 is true 1 1

12 Slide 12 of 3 Think-Pair-Share What will be the result of these lines of code? Think about these, then find a partner/small group and discuss. We ll come together and discuss. x [1 4 5]; y [1 1 3]; z [ 2 1]; a z < x & z > y b x y c ~z d y z

13 Slide 13 of 3 Pseudocode Verbal description of code Often is language independent Intermediate step between everyday language and a programming language Example: Pseudocode: Request character string input from user Print the string to the screen MATLAB code: str input('enter string: ', 's'); disp(str)

14 Slide 14 of 3 Flowcharts A flowchart is exactly what it sounds like a graphical representation of how code flows or progresses. Symbol Description Indicates the beginning or end of a section of code Indicates input or output processes Indicates a decision point Indicates calculations Source: Table 8.3 in Moore, H., MATLAB for Engineers, 4th Ed., p. 275.

15 Slide 15 of 3 Flowchart Example Start This is the same problem that we created psuedocode for Request character string from user Output character string to the user End

16 Slide 16 of 3 find The command find searches a matrix, finding what elements meet the search criteria The command returns the index/indices of the valid results Compare this technique to if statements and switches later in the lecture Versions of the command: index index index [row, [row, find(x) find(x,k) (first k elements) find(x,k,'last') (last k elements) col] find(x, ) col, vector] find(x, )

17 Slide 17 of 3 find examples (1 of 2) Code: A [ ]; B find(a>3) C find(a4) D find(a~2) Output: B C 4 6 D

18 Slide 18 of 3 find examples (2 of 2) Code: E [1 5; 2 6]; [F_row, F_col] find(e>1) [G_row, G_col, G_vec] find(e<6) Output: (NOTE: automatically prints as columns printed as rows only to fit output onto the slide) F_row F_col G_row G_col G_vec 1 1 1

19 Slide 19 of 3 Selection Structures: if The simplest classical selection structure is the if statement Takes the following form: if comparison do something end If the comparison evaluates to be true, then the do something statements are executed. If the comparison evaluates to be false, then the do something statements are ignored.

20 Slide 2 of 3 Selection Structures: if/elseif/else Can also use elseif and else as choices Takes the following form: if comparison do something elseif comparison do something else else do something different end You can have an else without an elseif, and an elseif without an else. However, both else and elseif are dependent on having an if If the first statement (the if statement) doesn t evaluate to true, it checks the elseif statement(s) If nothing is true by the time one gets to else, the else commands are executed Can mix and match these as much as you need, and not everyone needs to be present

21 Slide 21 of 3 if example Code: x 4; if x 4 disp('x 4') end if/else example Code: Checks WHOLE matrix beware! y [1 2]; if y 1 disp('works.') else disp('ut oh.') end Output: Output: x 4 Ut oh.

22 Slide 22 of 3 if/elseif/else Example Code: x 2; if x 1 disp('then elseif x > 1 disp('then elseif x < 1 disp('then else disp('then end Output: Then x > 1 x 1') x > 1') x < 1') I''m confused')

23 Slide 23 of 3 Another if/elseif/else Example Code: y []; if y 1 disp('then y 1') elseif y > 1 disp('then y > 1') elseif y < 1 disp('then y < 1') else disp('then I''m confused') end Output: Then I m confused

24 Slide 24 of 3 Switches Switches have a similar purpose to if statements Anything you can do with a switch can be done using if/elseif/else Why bother? Often personal preference, though you ll often see switches when checking strings Takes the form: Important: In MATLAB, once a true case has been found MATLAB will NOT check the other cases make sure you plan according. switch var case option executes if var is equal to option otherwise Not required to be present executes if var doesn t match a case end

25 switch and otherwise example Code Slide 25 of 3 location 'lion shrine'; switch location case 'lion shrine' disp('i''m at the lion shrine') otherwise disp('i''m lost') end Output I'm at the lion shrine

26 Slide 26 of 3 Think-Pair-Share What will be the result of these lines of code? Think about these, then find a partner/small group and discuss. We ll come together and discuss. x 1; y 3; switch x case if y > 2 disp('x else disp('x end case 1 if y > 2 disp('x else disp('x end otherwise disp('beats end, y > 2'), y < 2') 1, y > 2') 1, y < 2') me')

27 Slide 27 of 3 Creating menus in MATLAB Instead of requesting input from the Command Window, you can have MATLAB collect input from a menu box. Syntax: var menu('title','option 1', 'option 2', '...')

28 Slide 28 of 3 Menu Example Code: city menu('what campus?', 'UP', 'BD', 'AA'); switch city case 1 disp('university Park') case 2 disp('erie') case 3 disp('altoona') otherwise disp('not one of the 3 biggest campuses') end

29 Slide 29 of 3 Menu Example Cont d Popup: Output: (assuming that you clicked BD) Erie

30 Slide 3 of 3 Questions? No quiz this week Wednesday s Class Interactive Lecture in Computer Labs Midterm No 1 Thursday night class on Friday Exam Release

BIL101E: Introduction to Computers and Information systems Lecture 8

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