1. EGR 115 Introduction to Computing for Engineers User-Defined Functions1 – Part 1 Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers

2. Lecture Outline Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers User Defined Functions  Intro & variable passing Slide 2 of 17

3. User Defined Functions Intro & Variable Passing Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers What are functions in programming?  “Small” Self Contained Computing Units o Dedicated to a Single Computing Task  Gives rise to reusable code o Isolation from unintended side effects  Self-contained blocks of code o MATLAB: Pass by value rather than pass by reference  Pass the contents (i.e., value) array rather than the address (i.e., reference) of the start of the array o Independent test & exclusion from debug once fully verified  Divide and conquer!! o More organized code compartmentalized by function  MODULARITY!!! Slide 3 of 17

4. User Defined Functions Intro & Variable Passing Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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 Code o Once verified & documented a piece of code (e.g., factorial routine) can be used by many software tasks 3.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

5. User Defined Functions Intro & Variable Passing Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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

6. User Defined Functions Intro & Variable Passing Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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 function out2 = in1; end % end swap2 swap2.m Slide 6 of 17

7. User Defined Functions Intro & Variable Passing Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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

8. User Defined Functions Intro & Variable Passing Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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 2 nd example, but, also provides a single output corresponding to the number of letters in YourLastName Slide 8 of 17

9. User Defined Functions Intro & Variable Passing – Example #1 Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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

10. User Defined Functions Intro & Variable Passing – Example #2 Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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

11. User Defined Functions Intro & Variable Passing – Example #3 Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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 function cnt = length(LastName); end % end fn_ex3 fn_ex3.m Slide 11 of 17

12. User Defined Functions Intro & Variable Passing Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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 effects o 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 completeness o Passes the address (i.e., location) of the array NOT the contents o Used by many other programming languages, when specified o Changes to data in function will modify values outside function Slide 12 of 17

13. User Defined Functions Intro & Variable Passing – Further Example Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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

14. User Defined Functions Intro & Variable Passing – Further Example Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers 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

15. User Defined Functions Intro & Variable Passing – Further Example Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers % 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 window clear all; % Remove all variables from workspace close 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

16. User Defined Functions Intro & Variable Passing – Further Example Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers Results my_polar2rect.m test_my_polar2rect.m Slide 16 of 17

17. Next Lecture Wednesday 22 Oct 2014 EGR 115 Introduction to Computing for Engineers User-Defined Functions2  Data visibility (Global,...) Slide 17 of 17