EGR 115 Introduction to Computing for Engineers User-Defined Functions1 – Part 1 Wednesday 22 Oct...

EGR 115 Introduction to Computing for Engineers

User-Defined Functions1 – Part 1

Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers

http://www.erau.edu/

Lecture Outline


• User Defined Functions Intro & variable passing

Slide 2 of 17


User Defined FunctionsIntro & Variable Passing


• What are functions in programming? “Small” Self Contained Computing Units

o Dedicated to a Single Computing TaskGives rise to reusable code

o Isolation from unintended side effectsSelf-contained blocks of code

o MATLAB: Pass by value rather than pass by referencePass the contents (i.e., value) array rather than the address

(i.e., reference) of the start of the arrayo Independent test & exclusion from debug once fully verified

Divide and conquer!! o More organized code compartmentalized by function

MODULARITY!!!

Slide 3 of 17




• What are the main benefits of using functions?1. Independent testing of sub-tasks

o Verification of a small piece of code in isolation is easier than verification of a massive software task

2. Reusable Codeo Once verified & documented a piece of code (e.g., factorial

routine) can be used by many software tasks3. Isolation from unintended side effects

o Data used by function is isolated to that function (scope of data) and will not damage or be damaged by other code in your software project

Slide 4 of 17




• General Structure of a Function in MATLAB

function [out1, out2, …] = fn_name (in1, in2, …)% Function Description:% …% INPUTS:% …% OUTPUTS:% …% NOTES/COMMENTS:Some code;…end % end fn_name

out1 and out2 are the outputs.

in1 and in2 are the inputs.

These are local copies: MATLAB only passes by "value" not by "reference."

MUST be saved in a file called

fn_name.m

ALWAYS add comments to describe & document the function

The “end” statement is optional but good practice

Slide 5 of 17




• An Example of a simple function Develop a function to swap two values

function [out1, out2] = swap2(in1, in2)% Function Description:% This function swaps in1 and in2% INPUTS:% in1, in2% OUTPUTS:% out1, out2% NOTES/COMMENTS:% None out1 = in2; % Body of the functionout2 = in1; end % end swap2

swap2.m

Slide 6 of 17

http://mercury.pr.erau.edu/~bruders/teaching/2014_fall/EGR115/MATLAB_code/user_functions/





• Types of Functions Functions that just accomplish a task that may not even

require any inputs (e.g., clock) or outputs (e.g., pause)o Optional Arguments

Functions that accepts one value and return one value.o Single input / single output

Functions that calculate and return more than one value.o Multiple inputs and/or outputs

Slide 7 of 17




• First Simple example: Develop a function that requires NO input values and NO

output values and prints “Hello World!!” to the command window

• Second Simple example: Develop a function that accepts YourLastName as an input

argument/value and has NO output values and prints “Hello YourLastName!!” to the command window

• Third Simple example: Same as 2nd example, but, also provides a single output

corresponding to the number of letters in YourLastName

Slide 8 of 17


User Defined FunctionsIntro & Variable Passing – Example #1


function [] = fn_ex1()% Function Description:% This function prints Hello World!!% INPUTS:% none% OUTPUTS:% none% NOTES/COMMENTS:% None disp('Hello World!!'); % Body of the function end % end fn_ex1

fn_ex1.m

Slide 9 of 17






function [] = fn_ex2(LastName)% Function Description:% This function prints Hello LastName!!% INPUTS:% LastName - A string of characters% OUTPUTS:% none% NOTES/COMMENTS:% None disp(['Hello ',LastName,'!!']); % Body of the function end % end fn_ex2

fn_ex2.m

Slide 10 of 17





function [cnt] = fn_ex3(LastName)% Function Description:% This function prints Hello LastName!! and computes the number of% letters in LastName% INPUTS:% LastName - A string of characters% OUTPUTS:% cnt - The number of letters in LastName% NOTES/COMMENTS:% None disp(['Hello ',LastName,'!!']); % Body of the functioncnt = length(LastName);

end % end fn_ex3

fn_ex3.m

Slide 11 of 17





• Variable Passing Options Pass-by-Value: Used by default in MATLAB

o Upon Function invocation, a copy of the actual arguments are made, and they are copied into the dataspace of the function using Dummy argument names

o Helps to prevent unintended side effectso Function can modify dummy arguments and the data will

remain unchanged in the outside calling code Pass-by-Reference: Not supported by MATLAB

o Other option, included for completenesso Passes the address (i.e., location) of the array NOT the contentso Used by many other programming languages, when specifiedo Changes to data in function will modify values outside function

Slide 12 of 17


User Defined FunctionsIntro & Variable Passing – Further Example


• Develop a function to convert from polar coordinates (r, theta) to rectangular coordinates (x, y) Call the function “my_polar2rect”

o Be careful about potentially using a MATLAB function name Assume that the angle theta is provided in units of deg x, y, and r are in units of meters Two inputs of type double Two outputs of type double

Slide 13 of 17




function [x, y] = my_polar2rect(r, theta)% Function Description:% Function to convert from polar to rectangular coordinates% % INPUTS:% r - radius (in meters)% theta - angle (in deg)%% OUTPUTS:% x - x-coordinate (in meters)% y - y-coordinate (in meters)%% NOTES/COMMENTS:% Note that the angle is provided in deg NOT radians theta = theta * pi / 180; % Convert from deg to radians (rad) x = r * cos(theta);y = r * sin(theta); end % end my_polar2rect

my_polar2rect.m

Slide 14 of 17





%% COURSE: EE 115 Introduction to Computing for Engineers% DESCRIPTION: Main program to test polar2rect function% AUTHOR: Stephen Bruder% DATE: 10/19/2014% COMMENTS: None clc; % Clear the command windowclear all; % Remove all variables from workspaceclose all; theta = 0:5:5*360; % 5 full rotations (deg)N = length(theta); % Number of elements in the angle array (dimless)r = 0:1/N:1; % Grow the radius from 0 to 1 (m)x = zeros(1,N); % Initialize x (m)y = zeros(1,N); % Initialize y (m) for i=1:N [x(i), y(i)] = my_polar2rect(r(i), theta(i)); scatter(x, y, 'ro'); axis([-1.1 1.1 -1.1 1.1]); pause(0.01);end

test_my_polar2rect.m

Slide 15 of 17





• Resultsmy_polar2rect.m

test_my_polar2rect.m

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Slide 16 of 17




Next Lecture


• User-Defined Functions2 Data visibility (Global, ...)

Slide 17 of 17


EGR 115 Introduction to Computing for Engineers User-Defined Functions1 – Part 1 Wednesday 22 Oct...

Documents

Transcript of EGR 115 Introduction to Computing for Engineers User-Defined Functions1 – Part 1 Wednesday 22 Oct...