Functions 1 ENGR 1181 MATLAB 14 Userdefined Functions

Functions 1 ENGR 1181 MATLAB 14

User-defined Functions in Real Life Using available built-in functions in our calculators and MATLAB is great, but often we encounter situations for which there is no pre-defined function. Like adding a quadratic equation solver program to our calculators, we can create our own functions in MATLAB to perform all sorts of calculations.

Today's Learning Objectives § After today’s class, students will be able to: • Demonstrate the proper use of functions in their programs. • Explain how functions can be used in multiple places and as an organizational tool. • Describe purpose and structure of a function definition line.

What is a function? § MATLAB has built-in functions that we have been using all semester such as sum, length, mean, and others. § We will now learn how to create our own functions; these are called user-defined functions. § It is also referred to as a subroutine or a procedure in other programming languages.

What is a function? § MATLAB functions can be used to make programs easier to read and reduce the lines of code. § Allows multiple people to work on the same project seamlessly • Ex: One person's function can be referenced in another person's script file.

What is a function? § Just like a built-in function, a user-defined function can be used within the command window or your script file. § In order to do this, the function must be saved in your active directory.

Opening a function file Open a function file (not a script file)

Basic function structure function[output variable(s)] = function_name(input variables) typical code as compared to a script file end **Function definition line Required if using nested functions, optional otherwise **Must follow this format or Mat. Lab will think it’s a script file and/or you will get an error

Saving a function file function[fun, outputs] = eng_fun(math, science, physics) Function files MUST be saved as the name it is called out as in the function definition line as highlighted above. Also, it must be saved as a. m file. For example, this function file would be saved as: eng_fun. m

Example: One input & output Let’s calculate how many Oreo® cookies are left in a box using a function file: 1. Open a new function file 2. Complete the function definition line Function[oreos_left]=oreos(hrs_of_HW) 3. Save file – oreos. m

Example: One input & output Now that we have the function file defined, we can carry out the calculations to determine how many Oreo® cookies we will eat: eat_yum=ceil(1/2*exp(hrs_of_HW)); %calculates the number of oreos % Typical oreos in a package is 30 package=30; oreos_left= package-eat_yum; %This line calculates the output

Example: One input & output Let’s test our file now; be sure to save your function file. Go to your command window and type this: oreos(hrs_of_HW) What happens? >> oreos(hrs_of_HW) Undefined function or variable 'hrs_of_HW'.

Example: One input & output Now try this in the command window: oreos(3) What happens? How about this: ans = 19 x=3; box=oreos(x)

Example: Two inputs & one output Let’s modify our oreo code to account for stress of the number of midterms that week: function[oreos_left]=oreos_rev(hrs_of_HW, exams) SAVE YOUR FILE! oreos_rev. m Modify our equations: eat_yum=ceil(1/2*exp(hrs_of_HW)); package=30; bonus_yum=exams*3; oreos_left= package-eat_yum-bonus_yum;

Example: Two inputs & one output Run from the command window where you did 1 hour of homework and have 2 midterms this week: HW=1; %Hours of homework MT=2; %Number of midterms this week box=oreos_rev(HW, MT) box = 22

Create a script file using oreos_rev. m Script file clc clear hrs=input(‘How many hours of homework have you done today? ’); midterms=input(‘How many midterms do you have this week? ’); box=oreos_rev(hrs, midterms); fprintf(‘n. You have %i oreos left, you better do more homework!’, box) Output How many hours of homework have you done today? 2 How many midterms do you have this week? 1 You have 23 oreos left, you better do some more homework!

Functions with multiple inputs & outputs Suppose we want to calculate the stress and deflection of a cantilever beam like the one in lab, let’s create a function file for this: Function definition line function [stress, deflection] = beam_lab(w, t , F, L, E)

Functions with multiple inputs & outputs § Write a function file to calculate stress and deflection • Hint: When using more than one output, you must assign the function to multiple outputs. Ex: [s, d]=beam_lab(inputs) § Then write a script file that calls the function file and uses these values. w =. 05 meters t =. 01 meters F = 100 newtons L = 1 meter E = 70 x 109 N/m 2 What do you get? stress = 200, 000 N/m 2 deflection = 0. 1143 m

Functions Example function v = freefall(t, m, cd) %freefall: bungee velocity with second-order drag %v=freefall(t, m, cd) computes the free-fall velocity of an object with %second-order drag %input: %t=time (s) %m=mass(kg) %cd = second-order drag coefficient(kg/m) %output: %v=downward velocity (m/s) g = 9. 81; %acceleration due to gravity v=sqrt(g*m/cd)*tanh(sqrt(g*cd/m)*t); Command Window Output: >>freefall(12, 68. 1, . 25) ans = 50. 6175 Now try changing the argument values

Important Takeaways § User-defined functions in MATLAB improve readability and reduces overall amount of code. § They can be used to accomplish calculations or perform subroutines within a program. § Function files must be saved in the same directory as the script file that uses them.

Preview of Next Class § Functions 2 • Having to share many function files along with your script file can be tedious • MATLAB offers a way to condense all this into a single file

What’s Next? § Review today’s Quiz #14 § Open the in-class activity from the EEIC website and we will go through it together. § Then, start working on MAT-14 homework. § Prepare for the next class by reading about MATLAB Functions 2.