1. Matlab Training Session 5: Importing Data

2. Course Outline
Weeks:
Introduction to Matlab and its Interface (Jan )
Fundamentals (Operators)
Fundamentals (Flow)
Importing Data
Functions and M-Files
Plotting (2D and 3D)
Statistical Tools in Matlab
Analysis and Data Structures
Course Website:

3. Week 5 Lecture Outline Importing Data A. Week 4 Review
B. Simple Import
C. Handling Files
D. Mini-Project

4. 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);
Function
input
output

5. Mini-Project Raising any number of numbers to the nth power 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
Make sure to comment and document the function

6. Solution Complex function [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;
end
for(i=1:length(x))
for(j=1:n(i))
y(i) = y(i)*x(i);
e = 0;
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;
end
y = x.^n
e = 0;

7. Importing Data Basic issue: Other Issues:
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
Easily or Not

8. 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
No other characters

9. 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’)
1 2
3 4
5 2
4 8
16 32
test1

10. 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!
1 2
3 4 5
4 8
16 32
test2

11. 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
test1
1 2
3 4
5 2
4 8
16 32

12. 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
More complex functions are more flexible

13. 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
fgetl

14. 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
Can open multiple files and matlab will assign unique fids

15. fclose
When you are done with a file, it is a good idea to close it especially if you are opening many files
fclose(fid)

16. 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
fid
file_name
Pointer

17. 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
frewind(fid)

18. 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
Be careful because matlab will mangle your data and not tell you

19. 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
It works and fills in the blanks. This is powerful but dangerous

20. 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?
Need to set file pointer to beginning using frewind(fid)

21. 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
It works and fills in the blanks. This is powerful but dangerous

22. 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
>> a_num = str2num(a_str)
a_num = [1 2]

23. 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.
Option 2: Have matlab delete and format available data on the fly

24. 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
Will work for most applications

25. Realistic File
Powerful function textread can be used to input almost any text file
Usage:
[var1 varN] = textread(‘filename’,’format’,args)

26. Summary Lots of options to load files load for basics
fscanf for complex
textread for most things
xlsread for Excel worksheets
Also saving Excel sheets as tab delimitted

27. 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
Deg_F = 9/5*Deg_C + 32

28. 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
high_C = (5/9).*(high_F - 32);
Low_C = (5/9).*(low_F - 32);
avg_C = (5/9).*(avg_F - 32);

29. Getting Help Help and Documentation Digital Hard Copy
Accessible Help from the Matlab Start Menu
Updated online help from the Matlab Mathworks website:
Matlab command prompt function lookup
Built in Demo’s
Websites
Hard Copy
Books, Guides, Reference
The Student Edition of Matlab pub. Mathworks Inc.