Tutorial IV: Function Files
Last updated 5/05/05 by G.G. Botte
Department of Chemical Engineering
ChE-1800: Approaches to Chemical Engineering Problem Solving
MATLAB Tutorial IV
Function Files
(last updated 5/05/05 by GGB)
Objectives:
These tutorials are designed to show the introductory elements for any of the topics
discussed. In almost all cases there are other ways to accomplish the same objective, or higher
level features that can be added to the commands below. Read Chapter 6 of the textbook to learn
more about function files
Any text below appearing after the double prompt (>>) can be entered in the Command
Window directly or in an m-file.
______________________________________________________________________________
The following topics are covered in this tutorial;
Introduction
Creating Function m_files
Executing Function files
Solved Problems (guided tour)
Proposed Problems
______________________________________________________________________________
Introduction:
Function m_files are used to calculate a value of a dependent variable or variables, at specified
values of independent variables.
You may have noticed that when you use the built_in Matlab functions, such as 'sum' or 'sin', you
don't see the specific Matlab commands that are executed and any variables used internally by
the function are not recognized by the Command Window. We can take advantage of this feature
to perform calculations or sequences of commands that appear often in working a larger
problem. We do this by creating our own functions.
Functions are created in an m_file using the Matlab Editor, just like a script m_file. The major
difference between a script m_file and a function m_file lies in the interaction between the
m_file and the Command Window. Remember a script m_files was able to see all variables
stored in memory, and all variables created in the m_file were stored in memory. A function
m_file interacts with the Command Window only through its input and output. Intermediate
variables that are not passed to or from the function cannot be accessed by the Command
Window, and the Command Window cannot use variables created in the function file unless they
are specifically passed to the Command Window as output.
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Creating Function m_files
A function is going to accept input that you control, and return output values that you request.
The structure of a function file must be as follows:
(1)
The first executable line of the file must begin with the word function.
This is followed by the output arguments. If more than one variable is to be returned they
must be in an array (square brackets). This is followed by an equal sign, and the name of
the function with the input arguments in parentheses, separated by commas. See example:
function [output1,output2,output3, etc]=function_name(input1,input2,input3, etc)
The word function must be
the first word, and must be
typed in lower-case letters
List of output variables
returned by the function.
Variables are returned as
a line vector, therefore,
they can be accessed
using the order of the
vector
The name of
the function
List of input arguments
typed inside parentheses
The input and output argument names can be the same or different from those used in the calling
statement for the function. In either case they are merely matched up by location in the input
argument list and the output array.
(2)
After the first executable line any comments, variable definitions etc. should be listed
using the % sign. These comments will be displayed when you type >> help
function_name at the MATLAB prompt.
Same guidelines for developing quality programs must be followed in function files. It is
a good practice to define all the variables that the function uses. That way, when the user wants
to learn more about the function the description of the variables used is shown in the command
screen by using the command help function_name. The minimum comments that should be
written in a function file are:
1. Description of the function
2. Author of the function, date, and last modified dates
3. Call syntax. This is the syntax needed to run the function
4. Input variables including the units
5. Output variables including the units
(3)
The commands which perform the calculations.
Example 1:
To practice, let us work on the following example. Develop a function file that will be able to
convert temperature from Celsius to Fahrenheit. We are going to call the function Fahrenheit.
That is, our function should be able to transform oC to oF, therefore, the input of the function
should be temperature (TC) in oC, and the output of the function should be temperature (TF) in
o
F.
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Tutorial IV: Function Files
Last updated 5/05/05 by G.G. Botte
Step 1: Write the script file of the function
Function name
Comments that
should be written
in a function file
Step 2: Save the function. The file name should be the function name with the extension “m”,
that is, function_name.m. See example
File name is
the function
name
Executing a function
A function can be executed from the Command Window, from a script file, or from another
function. To use a function file, you must be in the directory where the function file was saved.
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Example 2:
Execute the function Fahrenheit from the command window. If you use the command help you
can see the description of the function that we just created
Execute the function to convert 25 oC to oF:
Input
variable
Example 3: Develop a program that will convert temperature from Celcius to Fahrenheit using
the function that was developed. This example allows illustrating how to execute the function
from a script file.
This is the script file
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Tutorial IV: Function Files
Last updated 5/05/05 by G.G. Botte
This is what you will see when you execute the file
SOLVED PROBLEMS
1. Write a program in Matlab to convert temperatures from Celsius to Fahrenheit or from
Fahrenheit to Celsius. The user must have the choice of leaving the program at anytime. Use the
menu option to make selections. Your code must use functions to convert from Celsius to
Fahrenheit and from Fahrenheit to Celsius.
Solution:
1. Follow the “tips for solving problems”
2. Write a flowchart diagram (see H-2). In the space given below draw your flowchart
diagram
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3. Write the code in Matlab. See the solution given below.
4. Execute the program for different temperatures, example
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Tutorial IV: Function Files
Last updated 5/05/05 by G.G. Botte
2. Write a function in Matlab that calculates the local maximum or minimum of a quadratic
function of the form: f (x) = ax 2 + bx + c . For the function name and arguments use
[x,y]=maxmin(a,b,c). The input arguments are the constants a, b, and c, and the output arguments
are the coordinates x and y of the maximum or the minimum.
Use the function to determine the maximum or minimum of the following functions:
a) f (x) = 3x 2 − 18x + 48
b) f (x) = −5x 2 + 10x − 3
Solution:
We know that the minimum or maximum of a function is found by determining the value of x
that makes the first derivative of the function equal to zero. Therefore,
f (x) ' = 2ax + b = 0
x min =
1. Follow the “tips for solving problems”
2. Develop the function file
−b
2a
3. This is how the function is called from the command window. Remember that the solution is a
vector
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PROPOSED PROBLEMS
The following chemical reaction describes the neutralization of sodium hydroxide by sulfuric
acid.
H2SO4 + 2NaOH → Na 2SO4 + 2H2O
write a function file which calculates the volume and mass of 0.75M H2SO4 needed to neutralize
an NaOH solution. The density of the sulfuric acid solution is 1045 g/l. Let the user specify the
volume and concentration of the NaOH in a main m-file. Pass this information to the function
file and return the volume and mass of sulfuric acid from the function file. The output should be
in the main file.
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