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MATLAB Functions

MATLAB Functions. ENGR 1181 Presentation by Cody Allison. Today’s Topics. What is a function? Basic function structure Function Examples. What is a function?. MatLab has built-in functions that we have been using all semester such as sum, length, mean, and many others

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MATLAB Functions

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  1. MATLAB Functions ENGR 1181 Presentation by Cody Allison

  2. Today’s Topics • What is a function? • Basic function structure • Function Examples

  3. What is a function? • MatLab has built-in functions that we have been using all semester such as sum, length, mean, and many 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

  4. 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: one person's function can be referenced in another person's script file.

  5. 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.

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

  7. 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 MatLab will think it’s a script file and/or you will get an error

  8. 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

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

  10. Example: One input & output Now that we have the function file defined, we can carry out the calculations to determine how many oreos 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

  11. 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'.

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

  13. 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) 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; SAVE YOUR FILE! oreos_rev.m

  14. 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

  15. Create a script fileusing 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(‘\nYou have %ioreos 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!

  16. 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)

  17. 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 109N/m2 What do you get? stress = 200,000 N/m2 deflection = 0.1143 m

  18. Questions?

  19. 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 Functions Example

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