What does C store?

1. What does C store? >>A = [1 2 3] >>B = [11] >>[C,D]=meshgrid(A,B) b) 1 2 3 1 2 3 a) 1 2 3 1 1 1 1 2 3 1 1 1 1 1 1 1 1 1

2. What does D store? >>A = [1 2 3] >>B = [11] >>[C,D]=meshgrid(A,B) b) 1 2 3 1 2 3 a) 1 2 3 1 1 1 1 2 3 1 1 1 1 1 1 1 1 1

3. What does C store? >>A = [1 2 3] >>B = [11] >>[C,D]=meshgrid(B,A) b) 1 1 1 1 1 1 a) 1 1 1 1 1 1 1 1 1 1 1 2 2 3 3 1 2 3 12 3

4. What does D store? >>A = [1 2 3] >>B = [11] >>[C,D]=meshgrid(B,A) b) 1 1 1 1 1 1 a) 1 1 1 1 1 1 1 1 1 1 1 2 2 3 3 1 2 3 12 3

5. LEARNING ABOUT FUNCTIONS IN MATLAB

6. A simple function (poly2) Save the file as poly2.m (same as the name of the function)

7. The function is available from the command window or from other M-file programs

8. Comments • You should comment functions, just as you would any computer code • The comment lines immediately after the first line are returned when you query the help function

9. Functions with Multiple Inputs A user defined function with multiple inputs

10. Functions with Multiple Outputs This function returns 3 output values If you don’t ask for all three results, the program just returns the first value

11. User Defined Input • To this point we have “hard coded” the values of variables into our M-file programs • The input function allows us to prompt the user to enter a value

12. The input function is used in an M-file program to prompt the user to enter a value The prompt is displayed in the command window

14. Input also can input in matrices and characters

15. Output Options • Use the disp function • Use the fprintffunction – same as C except single quotes

16. disp The display (disp) function can be used to display the contents of a matrix without printing the matrix name

17. The disp function can also be used to display a string

18. You can combine disp functions to create meaningful output from an M-file program, but the result of each disp function is on a separate line.

19. Using find and logical operators in Matlab

20. Logical Operators & and ~ not | or

21. find • The find command searches a matrix and identifies which elements in that matrix meet a given criteria.

22. index numbers element values

23. find used with a 2D matrix x =[1 2 3; 10 5 1; 12 3 2; 8 3 1] element = find(x > 9) [row, column] = find(x > 9) Returns a single element number Returns the row and column designation of an element

24. The rest of Chapter 5 shows how to use if/else if and for loops in Matlab – very similar to C

25. Homework – Chapter 5, problems 1 and 2 • These are a little tougher – so good luck!

26. Simple if if comparison statements end For example…. if G<50 count = count +1; disp(G); end

27. If statements • Easy to interpret for scalars • What does an if statement mean if the comparison includes a matrix? • The comparison is only true if it is true for every member of the array G=[30,55,10] if G<50 count = count +1; disp(G); end The code inside the if statement is not executed, because the comparison is not true!!

28. The if/else structure • The simple if triggers the execution of a block of code if a condition is true • If it is false that block of code is skipped, and the program continues without doing anything • What if instead you want to execute an alternate set of code if the condition is false?

29. Use an if structure to calculate a natural log • Check to see if the input is positive • If it is, calculate the natural log • If it isn’t, send an error message to the screen

30. The if/else/elseif structure • Use the elseif for multiple selection criteria • For example • Write a program to determine if an applicant is eligible to drive

31. Repetition Structures - Loops • Loops are used when you need to repeat a set of instructions multiple times • MATLAB supports two types of loops • for • while

32. When to use loops • In general loops are best used with scalars • Many of the problems you may want to attempt with loops can be better solved by vectorizing your code or with MATLAB’s logical functions such as • find

33. For Loops for index = [matrix] commands to be executed end • The loop starts with a for statement, and ends with the word end. • The first line in the loop defines the number of times the loops will repeat, using an index number. The loop is executed once for each element of the index matrix identified in the first line • The index of a for loop must be a variable.

34. Here’s a simple example In this case k is the index – the loop is repeated once for each value of k the index can be defined using any of the techniques we’ve learned

35. Here’s a simple example In this case k is the index – the loop is repeated once for each value of k the index can be defined using any of the techniques we’ve learned

36. While Loops while criterion commands to be executed end • While loops are very similar to for loops. • The big difference is the way MATLAB decides how many times to repeat the loop. • While loops continue until some criterion is met.

37. This loop creates the matrix a, one element at a time

38. Improving the Efficiency of Loops • In general, using a for loop (or a while loop) is less efficient in MATLAB than using array operations.

39. Here’s an example. This code creates a 40,000 element matrix of ones, then multiplies each element in the matrix by pi These two lines of code start a timer to measure the elapsed time required to run the lines of MATLAB code between them The amount of time it takes to run this code will depend on your computer

40. This code accomplishes the same thing with a for loop