Reading and Writing Data Files - PowerPoint Presentation

Slide1

Reading and Writing Data Files

In MatlabSlide2

*.MAT Files

*.MAT files are binary Matlab files that can hold your data.

E.g.

% Data to save

x = [1 2; 3 4];

% Method 1: Binary *.mat files

save

x

% saves the variable x in

x.mat

save

MyData

x

% saves the variable x in

MyData.mat

save(

'

MyOtherData

'

,

'x'

);

% saves the variable x in

MyOtherData.matSlide3

Saving Multiple Variables in *.MAT Files

% Data to save

x = [1 2; 3 4];

y = 5;

save

MyDatas

x

y

% saves the variables x & y in

MyDatas.mat

save(

'

MyOtherDatas

'

,

'

x'

,

'y

'

);

% saves the variable x & y in

MyOtherDatas.matSlide4

Loading Variables from *.MAT Files

clear

all

load

MyData.mat

whos

load

MyOtherDatas.mat

whos

d = load(

'

MyOtherDatas.mat

'

);

dSlide5

Pros and Cons of *.MAT Files

Pros:

Convenient if the variables will be manipulated at a later time in Matlab.

Compact (saves memory).

Binary file format makes it difficult for a novice to accidentally view confidential data.

Cons:

Not so great for sharing with colleagues who are not using Matlab.

Not editable with any other software.Slide6

xlswrite

x = [1 2; 3 4];

% Data to be written

FileName

=

'MyExcelFile.xlsx'

;

% Excel file name

xlswrite

(

FileName,x

);

% Write the data

FileName

=

'MyExcelFile2.xlsx'

;

% Specify that

we

want to write on Sheet 2

xlswrite

(FileName,x,

'Sheet2'

);

FileName

=

'MyExcelFile3.xlsx'

;

% Specify that we want to start our matrix

on cell B,2

xlswrite

(FileName,x,

'Sheet1'

,

'B2'

);Slide7

xlsread

FileName

=

'MyExcelFile3.xlsx'

;

[

Numbers,Text,Raw

] =

xlsread

(FileName,

'Sheet1'

,

'B2:C3'

)Slide8

Pros and Cons of Excel Files

Pros:

Easy to share data with friends.

Data can be easily viewed and edited.

Data from people who like Excel can easily be imported into Matlab.

Cons:

There’s nothing you can do in Excel that you can’t do in Matlab, but Matlab can do much more.Slide9

Custom Files

FileName

=

'MyFile.txt'

;

% Specify a file name

fid =

fopen

(

FileName

,

'w+'

);

% Open file for writing

% Write to the filefprintf(fid,

'%i

%0.2f %s\n',5,2.7183,'hello world'

); % Close the file

fclose(fid);Slide10

fopen

Opens a file for reading or writing.

Usage:

FileID

=

fopen

(

FileName,Permission

)

FileName

may be any file name with any extension (not just *.txt)

E.g.

FileName

= ‘MyFile.dat’;

FileName = ‘MyFile.m’;Permission may be any of the following:‘r’

Open file for reading

‘w’

Open file for writing; discard existing contents

‘a’

Open or

create file for writing; append data to end of file

'r+'

Open (do not create) file for reading and writing

'w+'

Open or create file for reading and writing; discard

existing contents

'a+'

Open or create file for reading and writing; append data to end of file

'W'

Open file for writing without automatic flushing

'A'

Open file for appending without automatic flushingSlide11

fprintf

Prints data to a file.

Usage:

fprintf

(

FileID,Format,Data

);

FileID

is the file identifier returned by

fopen

.

Format specifies the data formatting and may include any of the following:

Format Specifier

Used For

‘%a.bf’Floating point digits with a digits before the decimal place and b digits after (e.g. ‘%1.2f’ = 3.14)‘%s’Strings (e.g. ‘hello world’)‘%i’Integers (e.g. 5)

‘\b’Backspace

‘\n’New line‘\t’Horizontal tab

‘’’’Single quotation mark‘%%’

Single % character‘\\’

Single \ characterSlide12

fprintf (continued)

If

FileID

= 1, then

fprintf

prints to the command window.

E.g.

fprintf

(1,

'%s %

i

\

n'

,

'Display a number',5)Display a number 5

File identifier

We’re writing a string

The string will be followed by an integer, and then a carriage return

The string we will print

The integer we will printSlide13

fclose

Closes the file.

Usage:

fclose

(

FileID

)Slide14

File Reading

FileName

=

'MyFile.txt'

;

% Specify a file name

%

Open the file for reading

fid =

fopen

(

FileName

,

'r');

% Display the contents of the file

while true ThisLine

= fgetl(fid

); %

Get the next line of the file and return it as a string

if

~

ischar

(

ThisLine

)

break

end

disp

(

ThisLine

)

end

% Close the file

fclose

(fid);Slide15

fgetl

Gets a line from a file

Usage:

TheLine

=

fgetl

(

FileID

)

Where

FileID

is the file identifier returned by

fopen

.

TheLine is the next unread line of the file excluding the line terminator (e.g. excluding \n).Repeated calls to fgetl result in each successive line of the file being read until the end of the file is reached, at which time TheLine = -1.Slide16

Saving Data

% Open a file for writing

fid =

fopen

(

'MyDataFile.

dat

'

,

'a+'

);

% Write a comment

fprintf

(fid,'%% SubjectName = ''%s'';\

n'

,'Harry Potter');

fprintf(fid,

'%% Age = %i;\n'

,11);

% Write some dataData = rand(5,2);

fprintf

(fid,

'%0.2f %0.2f\

n'

,Data

');

% Close the data file

fclose

(fid);Slide17

Loading the Data in Matlab

% Load the data

SavedData

= load(

'MyDataFile.dat'

)

Comments are ignored

Only the data is loaded into MatlabSlide18

Evaluating the Comments

FileName

=

'MyDataFile.dat'

;

fid

=

fopen

(

FileName

,

'r'

);

while true

ThisLine =

fgetl(fid);

if ischar

(ThisLine) && ~

isempty(ThisLine

)

if

ThisLine

(1)==

'%'

eval

([

'

Vars

.'

,

ThisLine

(3:end)]);

end

else

break

end

end

fclose

(fid);Slide19

eval

Evaluates a string and makes the evaluated variables available in the current script or function.

E.g.

eval

(

‘x = 1;’

)

Creates a variable x and sets it equal to 1.

eval

(

'

SubjectName

= ''Harry Potter'''

);

Creates a variable called SubjectName and sets it equal to ‘Harry Potter’.Slide20

Pros and Cons of Custom File Formats

Pros:

New file types can be created that anyone snooping around your computer will not know how to open.

This keeps subjects data a little safer.

Data can be easily loaded or read back in via Matlab

Files may be saved in *.txt format if you want other people to be able to read and edit them.

Cons:

The syntax is more complicated than for the other data saving methods.