User-defined Functions

Transcription

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2 User-defined Functions >> x = 19; >> y = sqrt (x); sqrt is a built-in function Somewhere there is a file called sqrt.m that contains all the code to compute sine It would be a pain to write that yourself and then have to repeat it whenever you needed it

3 Function Concept So far: Have used Matlab s built-in functions; for example sin(x), exp(x), sort(x, descend )... Function: Reusable script Sometimes called a subprogram or subroutine Useful as a building block for larger programs Useful for avoiding duplication of code Computes an output(s) from an input(s)

4 A function is a MATLAB program that can accept inputs and produce outputs. Some functions don't take any inputs and/or produce outputs. A function is called by another program or function. That program can pass it input and receive the function's output.

5 Building Block Concept: These are calls to functions % Main Program y = func1 (x); result = func2 (a, b); a = func3 (n, m, q); func1 func2 func3 Each function goes in its own file.

6 Function Format A function goes in its own file. deg2rad.m: function r = deg2rad (deg) Special first line % a function to convert degrees to radians % input variable is deg % output variable is r r = deg * pi()/180.; end end statement

7 Function Format A function goes in its own file. deg2rad.m: function r = deg2rad (deg) % a function to convert degrees to radians % input variable is deg % output variable is r input argument(s) r = deg * pi()/180.; end output argument(s)

8 Usage: Syntax is just like built-in functions can be called from the command window or from a script file

9 Naming a function file Function name Made up of letters, digits, underscores NO spaces, NO dashes Follows same rules as variable names Avoid using names of built-in functions, or names of variables already defined (by user or MATLAB).

10 Input arguments: Used to transfer data into the function from the workspace Workspace variables are unavailable within the function Any necessary variables must be brought in Can be zero or more input arguments For multiple inputs: Separate them by commas Order is important Can be scalars, vectors, or arrays Examples of built-in functions with inputs: sum(x) plot(x,y)

11 Outputs: Used to transfer results back into the workspace from the function For multiple outputs: Separate them by commas in brackets Order is important Can be zero or more output arguments Output variables must be assigned Examples of built-in functions with outputs: y = sum(x) [value,location] = max(x)

12 Examples of Function Definition Lines also see page One input, one output: function [area_square] = square(side) 2. Brackets are optional for one output: function area_square = square(side) 3. Multiple inputs, one output: function [volume_box] = box(height,width,length) 4. One input, two outputs: function [area_circle,circumf] = circle(radius) 5. No output: function sqplot(side)

13 Comments are suggested in lines 2, 3 Words in line 2 are searched when lookfor is employed Comments in lines 2, 3, are returned when help name is executed

14 Variables inside a function are different from those outside. % Main Program r = ; radius = 10; a1 = area (radius); function a = area (r) a = pi() * r^2; end So when you write a function, you don t have to care about the variables in the calling program.

15 A tip for designing a function 1. Decide how you want the function to look to the calling program. Say you want a function to compute the volume of a cylinder given the radius and the height. Maybe you want it to look like this: v = cyl_volume (r, h) 2. Then the first line of the function looks like this: function v = cyl_volume (r, h) 3. And the file is called cyl_volume.m

16 Exercise Write a function called cyl_vol that will take two input arguments, the radius and the height of a cylinder, and return one argument, the volume of the cylinder. The volume of a cylinder is: V = πr 2 h Test it by entering this at the command line: >> cyl_vol (1, 1) % should be >> cyl_vol (2,2) % should be

17 A function may have no input arguments and no output list. For example, the function clear

18 Using Functions Once the function M-file has been saved, the function is now available for use from: The command window From a script file From another function You cannot execute the function by using the run button from the function menu bar.

19 Exercise Write a function definition line, named drop, that returns 2 outputs, dist and vel, when passed 3 input arguments, g, vo, and t function [dist,vel] = drop(g,vo,t); Write the function body where vel and dist are calculated from the following equations: vel dist gt v 1 2 gt 2 o v o t

20 function [dist,vel] = drop(g,v0,t); % Computes the distance traveled and the % velocity of a dropped object, % as functions of g, v0, and t. vel = g*t + v0; dist = 0.5*g*t.^2 + v0*t; end Execute your function with the following values 1. g = 32.2, v0 = 10, t =5 2. g = 32.2, v0 = 10, t = 0:10 (continued )

21 Function Example (continued) The input variables need not be assigned values outside the function prior to the function call: [feet_dropped,speed] = drop(32.2, 10, 5) The inputs and outputs may be arrays: [feet_dropped,speed]=drop(32.2, 10, [0:5]) This function call produces the arrays feet_dropped and speed, each with six values corresponding to the six values of time in the array time.

22 Function Example (continued) [feet_dropped,speed]=drop(32.2, 10, [0:5]) plot ([0:5], feet_dropped, o- );

23 Typical Errors Too few inputs

24 Too many inputs Too many outputs Wrong input type funny result

25 Usually, functions do not print anything out. Since they are designed to do work on behalf of a calling program, it is left up to the calling program whether to print something or not. Of course that doesn t apply while you are debugging.