ECE 102 Engineering Computation

Transcription

1 ECE 102 Engineering Computation Phillip Wong MATLAB Function Files Nested Functions Subfunctions Inline Functions Anonymous Functions

2 Function Files A basic function file contains one or more function definitions that are stored in an M-file (text file). Properties of a function file: It can be invoked by any script or other function file that resides in the same directory or is in the current search path. It does not share its variable workspace with scripts or other function files, unless global variables are used. 1

3 Example: Write a MATLAB program that asks the user for a positive integer N, sums up the integers from 0 to N, and displays the result. Input: positive number N Output: summed value from 0 to N Algorithm: 1. Get N 2. R = N 3. Display R 2

4 R k = (k-1)+k Start Process: Prompt user R 0 = 0 R 1 = 0+1 = R 0 +1 Input N R 2 = = R 1 +2 R = 0, k = 0 R 3 = = R 2 +3 : k <= N? T R = R + k R k = R k-1 +k F k = k + 1 Display R End 3

5 % Non-modular program ver. 1 while true N=input('Enter N: '); if N > 0 break; R = 0; k = 0; while k <= N R = R + k; k = k + 1; fprintf('sum = %d\n', R) % Non-modular program ver. 2 while true N=input('Enter N: '); if N > 0 break R = 0; for k = 0:N R = R + k; fprintf('sum = %d\n', R) 4

6 ms_main.m (functions in three, separate M-files) mysum.m % Main script N = prompt_n; Nsum = mysum(n); fprintf('sum = %d\n', Nsum) prompt_n.m function N = prompt_n % Ask user for N and validate function R = mysum(n) % mysum adds # s from 0 to N % N is the max value to sum R = 0; for k = 0:N R = R + k; % function while true N=input('Enter N: '); if N > 0 break % function 5

7 Nested Functions A nested function is a function that is defined within the body of another function. Properties of a nested function: It can only be invoked by its parent function or by other nested functions belonging to that parent. Where it is defined within the parent function does not matter. It shares the variable workspace of its parent. 6

8 Example: Nested function function myfun a = 3; fprintf('1: a = %d\n', a) function zap (x) w = 7; fprintf('zap: x = %d\n', x) fprintf('zap: a = %d\n', a) fprintf('zap: w = %d\n', w) a = x; % function zap Output: 1: a = 3 zap: x = 99 zap: a = 3 zap: w = 7 3: a = 99 4: w = 7 zap(99) fprintf('3: a = %d\n', a) fprintf('4: w = %d\n', w) % function myfun 7

9 Subfunctions A function file can contain one primary function andpossibly multiple subfunctions. The primary function has the same name as the M-file. A subfunction is defined at the same level as the primary function. It is not nested within the primary. Properties of a subfunction: It can only be invoked by the primary function or by other subfunctions within the same M-file. It does not share the workspace of the primary function or other subfunctions. 8

10 Example: Bad subfunction version of nested example function myfun a = 3; fprintf('1: a = %d\n', a) zap(99) fprintf('3: a = %d\n', a) fprintf('4: w = %d\n', w) % primary function function zap (x) w = 7; fprintf('zap: x = %d\n', x) fprintf('zap: a = %d\n', a) fprintf('zap: w = %d\n', w) a = x; % subfunction This will fail with an error message because this w is local to myfun and has not been defined yet. This will fail with an error message because a is local to myfun and is not known inside zap. 9

11 Example: Subfunction Write a MATLAB program that calculates and plots the following piece-wise math function: f ( x) = 1 cos( x) log( x 3π + 1) 1 0 x < 0 x < 3π x 3π Version 1: Single script no functions 2: Functions in separate M-files 3: All functions in a single M-file 10

12 fpw_script.m (single script no functions) % Initialize & calculate x = [-10:0.1:20]; y = []; for k = 1:length(x) u = x(k); if u < 0 y(k) = 1; elseif 0 <= u && u < 3*pi y(k) = cos(u); else y(k) = log10(u - 3*pi + 1) - 1; % Show our results on a graph plot(x, y) axis([-10, 20, -2, 2]) xlabel('x') ylabel('y') title('piece-wise Function') 11

13 fpw_main.m (functions in separate M-files) piecewise.m (this contains two functions) % Main script % Initialize & calculate x = [-10:0.1:20]; % Domain y = piecewise(x); % Range % Show our results draw_graph(x, y) draw_graph.m function draw_graph ( x, y ) plot(x, y) axis([-10, 20, -2, 2]) xlabel('x') ylabel('y') title('piece-wise Function') % function function y = piecewise ( x ) for k = 1:length(x) u = x(k); if u < 0 y(k) = 1; elseif 0<= u && u < 3*pi y(k) = cos(u); else y(k) = fun_eqn(u); % primary function % =========================== function y = fun_eqn ( x ) y = log10(x - 3*pi + 1) - 1; % subfunction 12

14 fpw.m (all functions in a single M-file) function fpw x = [-10:0.1:20]; % Domain y = piecewise(x); % Range draw_graph(x, y) % primary function % =========================== function draw_graph ( x, y ) plot(x, y) axis([-10, 20, -2, 2]) xlabel('x') ylabel('y') title('piece-wise Function') % subfunction function y = piecewise ( x ) for k = 1:length(x) u = x(k); if u < 0 y(k) = 1; elseif 0<= u && u < 3*pi y(k) = cos(u); else y(k) = fun_eqn(u); % subfunction % =========================== function y = fun_eqn ( x ) y = log10(x - 3*pi + 1) - 1; % subfunction 13

15 Variable Classes in Functions Functions can work with three classes of variables: Local variable Defined inside a function Known only inside the function where it is defined Changing its value will not affect a variable with the same name outside the function When the function s, its value becomes undefined 14

16 Global variable Defined in a script or primary function using global keyword Example: global var Declared in a function using global keyword Changing its value inside a function will alter the value outside the function Persistent variable Defined in a function using persistent keyword Example: persistent var When the function s, the variable still exists and its value is retained between calls 15

17 Example: Write a program to control an LED that is connected to the LabJack's DAC0. Version 1: Single script no functions 2: Organized into functions single M-file 3: Load ljud_constants in the subfunction 4: Use global variables in the subfunction 16

18 VERSION 1: Single script no functions % test1 % This program flashes an LED that is connected to the LabJack's DAC0. ljud_loaddriver % Loads LabJack UD Function Library ljud_constants % Loads actual LabJack constants % Open LabJack and reset pin assignments [Error ljhandle] = ljud_openlabjack(lj_dtu3,lj_ctusb,'1',1); Error = ljud_eput (ljhandle, LJ_ioPIN_CONFIGURATION_RESET, 0, 0, 0); % Output 2V, 2.5V, 3V, 3.5V for 1.5 sec each voltage = [ ]; dtime = 1.5; % Sets DAC0 to a sequence of voltage values for k = 1:length(voltage) Error = ljud_eput(ljhandle,lj_ioput_dac,0,voltage(k),0); pause(dtime) Error = ljud_eput(ljhandle,lj_ioput_dac,0,0,0); % Reset DAC0 to 0 volts 17

19 VERSION 2: Organized into functions stored in single M-file function test2 ljud_loaddriver % Loads LabJack UD Function Library ljud_constants % Loads actual LabJack constants % Open LabJack and reset pin assignments [Error ljhandle] = ljud_openlabjack(lj_dtu3,lj_ctusb,'1',1); Error = ljud_eput (ljhandle, LJ_ioPIN_CONFIGURATION_RESET, 0, 0, 0); % Output 2V, 2.5V, 3V, 3.5V for 1.5 sec each set_dac(ljhandle,[ ],1.5) % primary function function set_dac (ljhandle, voltage, dtime) % Sets DAC0 to a sequence of voltage values. for k = 1:length(voltage) Error = ljud_eput(ljhandle,lj_ioput_dac,0,voltage(k),0); pause(dtime) This will fail with an error message because LJ_ioPUT_DAC is unknown in set_dac s workspace. Error = ljud_eput(ljhandle,lj_ioput_dac,0,0,0); % Reset DAC0 to 0 volts % subfunction 18

20 VERSION 3: Load ljud_constants in the subfunction function test3 ljud_loaddriver % Loads LabJack UD Function Library ljud_constants % Loads actual LabJack constants % Open LabJack and reset pin assignments [Error ljhandle] = ljud_openlabjack(lj_dtu3,lj_ctusb,'1',1); Error = ljud_eput (ljhandle, LJ_ioPIN_CONFIGURATION_RESET, 0, 0, 0); % Output 2V, 2.5V, 3V, 3.5V for 1.5 sec each set_dac(ljhandle,[ ],1.5) % primary function function set_dac (ljhandle, voltage, dtime) % Sets DAC0 to a sequence of voltage values. ljud_constants % Loads actual LabJack constants for k = 1:length(voltage) Error = ljud_eput(ljhandle,lj_ioput_dac,0,voltage(k),0); pause(dtime) This works but is inefficient because all the constants are loaded into set_dac s workspace. Error = ljud_eput(ljhandle,lj_ioput_dac,0,0,0); % Reset DAC0 to 0 volts % subfunction 19

21 VERSION 4: Use global variables in the subfunction function test4 ljud_loaddriver % Loads LabJack UD Function Library global LJ_ioPUT_DAC % Defines this as global so other functions can access it ljud_constants % Loads actual LabJack constants % Open LabJack and reset pin assignments [Error ljhandle] = ljud_openlabjack(lj_dtu3,lj_ctusb,'1',1); Error = ljud_eput (ljhandle, LJ_ioPIN_CONFIGURATION_RESET, 0, 0, 0); % Output 2V, 2.5V, 3V, 3.5V for 1.5 sec each set_dac(ljhandle,[ ],1.5) % primary function function set_dac (ljhandle, voltage, dtime) % Sets DAC0 to a sequence of voltage values. global LJ_ioPUT_DAC % Declares this global value to access for k = 1:length(voltage) Error = ljud_eput(ljhandle,lj_ioput_dac,0,voltage(k),0); pause(dtime) This gives access to the only constant needed by set_dac. Error = ljud_eput(ljhandle,lj_ioput_dac,0,0,0); % Reset DAC0 to 0 volts % subfunction 20

22 Inline Function An inline functionis a quick way to evaluate a math equation at the command line or in a script. Syntax: fin = inline('equation') finis the name assigned to the inline function. equation is a string. Arguments can be passed to the inline function. 21

23 Example: F = inline('x^2 + 3*x 5'); F(3) 13 g = inline('x.*exp(-x.^2/5)'); g(0.5) g([0.1,0.5]) fun = inline('x/(x+y)'); z = 4 * fun(1,2) fun(g(0.5)/f(3), z)

24 Anonymous Function An anonymous functionis a more robust and modern version of the inline function. The function is anonymous because it is not named. Instead it returns a function handle. Syntax: fan list) math expression fan contains the function handle. input list contains the input arguments. 23

25 Example: F x^2 + 3*x 5; F(3) 13 g x.*exp(-x.^2/5); g(0.5) g([0.1,0.5]) fun x/(x+y); z = 4 * fun(1,2) fun(g(0.5)/f(3), z)