Course Outline
Weeks:
1. Introduction to Matlab and its Interface (Jan 13 2009) 2. Fundamentals (Operators)
3. Fundamentals (Flow)
4. Importing Data
5. Functions and M-Files 6. Plotting (2D and 3D)
7. Statistical Tools in Matlab 8. Analysis and Data Structures
Course Website:
Week 5 Lecture Outline
Importing Data
Functions in Matlab
• In Matlab, each function is a .m file
– It is good protocol to name your .m file the same as
your function name, i.e. funcname.m
• function
outargs
=
funcname
(
inargs
);
Mini-Project
• Raising any number of numbers to the n
thpower
• Inputs:
– A vector of numbers to be raised (N1…Nm) – A vector of powers (P1…Pm)
• Outputs:
– A vector of raised values (N1P1 … NmPm)
– An error flag: 1 if error in calculation, 0 if successful
• Caveats:
– If only one input is provided, the function should square each entry, so output = (N12…Nm2) and error flag is 0
– If the length of N and P are not the same,this is an error, return anything in the output vector and a 1 in the error flag
Solution
Complexfunction [y, e] = raise(x,n) y = ones(1,length(x)); if nargin == 1[y e] = raise(x,2*ones(1,length(x))); return
elseif nargin == 2
if(length(x)~=length(n)) y = NaN;
e = 1; return end
for(i=1:length(x)) for(j=1:n(i))
y(i) = y(i)*x(i); end
end e = 0; return end
Simple
function [y, e] = raise(x,n) if nargin == 1
[y e] = x.^2; return
elseif nargin == 2
if(length(x)~=length(n)) y = NaN;
e = 1; return end
Importing Data
• Basic issue:
– How do we get data from other sources into
Matlab so that we can play with it?
• Other Issues:
– Where do we get the data?
– What types of data can we import
load
• Command opens and imports data from a
standard ASCII file into a matlab variable
• Restrictions
– Data must be constantly sized
– Data must be ASCII
load
• Consider the simple file below
• Create in notepad and save as test1.txt and
text2.txt
• In matlab, set the path to the correct place (ie.
where the file is) and type load(‘test1.txt’)
• Now, type x = load(‘test1.txt’)
load
• Now the file is no longer simple because not
every row has the same amount of characters
• Create in notepad and save as test2.txt and
text2.txt
• type y = load(‘test2.txt’)
– Error!
load
• Now type in the same thing from test1.txt into
Excel and save the workbook as test1.xls
• type y = load(‘test1.xls’)
– What happens?
• Forcing the issue with Excel data
load
• Works for simple and unstructured code
• Powerful and easy to use but limited
• Will likely force you to manually handle
simplifying data which is prone to error
File Handling
• f* functions are associated with file opening,
reading, manipulating, writing, …
• Basic Functions of Interest for opening and
reading generic files in matlab
– fopen
– fclose
– fseek/ftell/frewind
– fscanf
fopen
• Opens a file object in matlab that points to the
file of interest
• fid = fopen(‘filepath’)
– absolute directory + filename
• If file of interest is C:\Andrew\Project_1\file.dat • fid = fopen(‘C:\Andrew\Project_1\file.dat’)
– relative path + filename
• If your matlab path is set to c:\Andrew\Project_1 • fid = fopen(‘file.dat’)
• fid is an integer that represents the file
fclose
• When you are done with a file, it is a good idea
to close it especially if you are opening many
files
What is a File?
• A specific organization of data
• In matlab it is identified with a fid
• Location is specified with a pointer that can be
moved around
file_name
fid
Moving the Pointer
• We already know how to assign a fid (fopen)
• To find where the file is pointing:
– x = ftell(fid)
• To point somewhere else
– fseek(fid,offset,origin)
• Move pointer in file fid by offset relative to origin
– Move pointer by a fixed number of bytes – Origin can be beginning, current, end of file
• To point to the beginning
Getting Data
• Why move the pointer around?
– Get somewhere in the file from where you want data
• [data] = fscanf(fid
,format,size)
• Format
– You have to tell matlab the type of data it should be
expecting in the text file so that it can convert it
• ‘%d’, ‘%f’, ‘%c’, ‘%s’
• Size
– You can specify how to organize the imported data
• [m,n] – import the data as m by n, n can be infinite
Lets Try It
• Open text1.txt using the fopen command
– Remember to save the fid, we will need it
• Create a variable with the data of text1.txt
• Now create another variable y with the data of text1.txt in it by using fscanf (do not simply copy x)
– What happens here?
– Need to set file pointer to beginning using rewind(fid)
• Now use the size option to import the data with 5 rows and 2 columns
• Try the same thing with test2.txt
Lets Try It
• Open text1.txt using the fopen command
– Remember to save the fid, we will need it fid = fopen('test1.txt)
• Create a variable with the data of text1.txt [x] = fscanf(fid,'%f%f')
• Now create another variable y with the data of text1.txt in it by using fscanf (do not simply copy x)
[y] = fscanf(fid,'%f%f')
– What happens here?
Lets Try It
• Now use the size option to import the data with 5 rows and 2 columns
[data2] = fscanf(fid,'%f%f',[5,2])
- Careful this is the same format as the original data but not the same organization!!
frewind(fid)
[data3] = fscanf(fid,'%f%f',[2,5]) data3’
- now the data is formatted correctly • Try the same thing with test2.txt
Getting Data
• fgetl returns the next line of the file as a
character array
• You may need to convert these to numbers
>> fid1 = fopen(‘test1.txt’); >> a_str = fgetl(fid1)
a_str = 1 2
Realistic File
• A realistic file of data will have header information,
labeled columns and other information embedded within
it.
– See PDXtemp.dat
• Option 1: Manually go through deleting this information
and import using load of fopen commands.
Realistic File
• Powerful function textread can be used to
input almost any text file
• Handles (input variables):
– Opening the file
– Ignoring Header Information
– Accepting Column Labels
Realistic File
• Powerful function textread can be used to
input almost any text file
Usage:
[var1 varN] =
Summary
• Lots of options to load files
– load for basics
– fscanf for complex
– textread for most things
– xlsread for Excel worksheets
Mini-Project
• Using the textread function, import the full data located in
PDXtemp.dat with the stated names and correct data
types
• Take the resulting temperatures in Fahrenheit and
convert to Celsius
– Make use of Matlab help to learn about and implement the textread function
Mini-Project
Solution
% Assume that the textfile is saved in the matlab work directory % which is automatically in the path
% read data from PDXtemp.txt into 4 variables (assume PDXtemp.txt is in path)
[month, high_F, low_F, avg_F] = … textread('PDXtemp.txt','%4c%f%f %f','headerlines', 6);
% convert each temperature variable to celcius
Getting Help
Help and Documentation
Digital
1. Accessible Help from the Matlab Start Menu
2. Updated online help from the Matlab Mathworks website:
http://www.mathworks.com/access/helpdesk/help/techdoc/matlab.html
3. Matlab command prompt function lookup 4. Built in Demo’s
5. Websites
