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Matlab Training Session 5: Importing Data

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fid = fopen( filepath') absolute directory filename ... fscanf(fid,format,size) Format ... Need to set file pointer to beginning using rewind(fid) ... – PowerPoint PPT presentation

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Title: Matlab Training Session 5: Importing Data


1
Matlab Training Session 5 Importing Data
2
  • Course Outline
  • Weeks
  • Introduction to Matlab and its Interface (Sept.
    27 2005)
  • Fundamentals (Operators)
  • Fundamentals (Flow)
  • Importing Data
  • Functions and M-Files
  • Plotting (2D and 3D)
  • Statistical Tools in Matlab
  • Analysis and Data Structures
  • Classes on ? Nov 1st ?, 8th and 15th have been
    cancelled for Society for Neuroscience (SFN) 35th
    annual meeting.
  • Course Website
  • http//www.queensu.ca/neurosci/Matlab Training
    Sessions.htm

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 outargsfuncname(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,2ones(1,length(x))) return elseif
nargin 2 if(length(x)length(n))
y NaN e 1 return end
for(i1length(x)) for(j1n(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 y
x.n e 0 end
7
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
  • 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 it C\Andrew\Project_1\file.da
    t
  • 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
  • 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
  • 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
  • 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
Getting Data
  • fgetl returns the next line of the file as a
    character array
  • You may need to convert these to numbers

gtgt fid1 fopen(test1.txt) gtgt a_str
fgetl(fid1) a_str 1 2 gtgt a_num
str2num(a_str) a_num 1 2
21
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

22
Realistic File
  • Powerful function textread can be used to input
    almost any text file
  • Handles
  • Opening the file
  • Ignoring Header Information
  • Accepting Column Labels
  • Will work for most applications

23
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 tad delimitted

24
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 9/5Deg_C 32

25
Getting Help
  • Help and Documentation
  • Digital
  • Accessible Help from the Matlab Start Menu
  • Updated online help from the Matlab Mathworks
    website
  • http//www.mathworks.com/access/helpdesk/help/tech
    doc/matlab.html
  • Matlab command prompt function lookup
  • Built in Demos
  • Websites
  • Hard Copy
  • Books, Guides, Reference
  • The Student Edition of Matlab pub. Mathworks Inc.
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