Programming in MATLAB

Transcription

1 Programming in MATLAB Scripts, functions, and control structures Some code examples from: Introduction to Numerical Methods and MATLAB Programming for Engineers by Young & Mohlenkamp

2 Script Collection of commands without external data Can be written by simply harvesting a set of commands from the command history window and pasting them into an editor window Example:

3 Script can be annotated with comments, as shown below:

4 Functions Sets of commands, that, like scripts, can be named and saved for later execution Unlike scripts, functions can have parameters and return values Can be written as inline functions or as separate programs

5 Inline functions An inline function in MATLAB is defined in much the same fashion as a variable The syntax is: Name = inline('expression', 'argument(s)') Examples: f = inline('2*x.^2 3*x + 1', 'x') g = inline('sin(alpha*x)','x','alpha')

6 Function programs Like scripts, functions can be written as standalone.m files that, once written, can be invoked whenever needed The syntax for a function program dictates something similar to a function heading or prototype in C; the first line in a MATLAB functin looks like: function rname = fname(argument(s)) rname is a name for a return value (can be a vector) fname is the name of the function

7 Examples from the MATLAB Help pages Function with one argument, one return value: function y = average(x) y = sum(x) / length (x); end Function with one argument, multiple return values: function [m,s] = stat(x) n = length(x); m = sum(x)/n; s = sqrt(sum((x-m).^2/n)); end Note: in both cases, x is a vector; otherwise functions won't work

8 Calling functions As with C, the names of parameters (and return values) need not correspond to the names of arguments passed (or variables that get assigned return values) Examples of calls to functions on previous slide: nums = randi(100,100,1); avg = average(nums); [avg,sd] = stat(nums);

9 Keyboard Input in MATLAB The built-in functions input() performs basic input functions There are two forms of the function: Calling input with a single string argument returns a double value Calling input with 2 string arguments, where the second argument is 's' returns a string argument

10 Number input The input function takes a string argument that is, a set of characters enclosed in single quotes and returns the numeric value input by the user Example: >> x = input('give me a number: '); give me a number: 24 >> x x = 24

11 String input Calling the input function with 2 arguments, a string prompt for the 1 st and the value 's' for the second, returns a string value Example: >> y = input('enter a word: ', 's'); enter a word: word! >> y y = word!

12 String input The string version of input can take multiple words as input; example: >> z = input('enter several words: ', 's'); enter several words: words words words >> z z = words words words

13 Example program: displays the area & perimeter of a triangle

14 Logical & relational operators MATLAB uses the same relational operators as C, except for one notable exception: not equals is ~= instead of!= Similarly, ~ is the NOT operator rather than! AND is && and OR is for scalar operands For arrays, there are operators & and which operate as described on the next couple of slides

15 Logical array operations Like C, MATLAB considers 0 to mean false and nonzero values to mean true Applying the ~ operation to an array returns an array of 1s and 0s: 1 for every non-zero value 0 for every zero value Example: >> x = randi([0,3],2) x = >> ~x ans =

16 Logical array operations Applying the & and operations to arrays: Both arrays must have same dimension(s) The & operator returns an array with 1s where both arrays have non-zero values, 0s where either has a 0 value The operator returns an array with 1s where either array has a non-zero value, 0 where both have a 0 value

17 Control structures in MATLAB MATLAB has the same control structures as most programming languages, including for loops and while loops if/else structures Syntax for all of these involves the use of a heading, a set of indented statements, and the word end no parentheses or brackets involved

18 Selection structures - examples x = randi([-10,10],1); if x >= 0 disp('x is positive'); end if x >= 0 disp('x is positive'); else disp('x is negative'); end

19 Selection structures - examples if x > 0 disp('x is positive'); elseif x < 0 disp('x is negative'); else disp('x is zero'); end

20 Switch statements Like C, MATLAB has a switch/case construct Cases can be single or multiple; if multiple, enclose the case values (separated by commas) in curly brackets Case values can be scalars or strings No break statements necessary no fallthrough (unlike C) Instead of default, use otherwise Example on next slide

21 Switch statement (from MATLAB help docs) method = 'Bilinear'; switch lower(method) case {'linear','bilinear'} disp('method is linear') case 'cubic' disp('method is cubic') case 'nearest' disp('method is nearest') otherwise disp('unknown method.') end Method is linear

22 Loops In MATLAB, count-controlled loops are generally written as for loops, while indeterminate loops ( event-controlled ) are written as while loops Examples of each type appear on the next couple of slides

23 Count-controlled loop: find the sum of the first n integers function summer = thesum(n) summer = 0; for i = 1:n summer = summer+i; end

24 Event-controlled loop: determine the number of terms required for the sum of a series (10k 2 4k + 2) to reach a maximum value function numterms = f(max) total = 0; ct = 0; while total < max ct = ct + 1; total = 10*ct^2-4*ct total; end disp('the number of terms is: ') disp(ct) disp('the sum is: ') disp(total)