MatLab09.pdf

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LABORATORIUM MECHATRONIKI, DIAGNOSTYKI
I BEZPIECZEŃSTWA TECHNICZNEGO
INSTYTUT POJAZDÓW
WYDZIAŁ SAMOCHODÓW I MASZYN ROBOCZYCH
POLITECHNIKA WARSZAWSKA
ul. Narbutta 84, 02-524 Warszawa
Tel. (22) 234-8117 do 8119
e-mail :msekretariat@mechatronika.net.pl
http://www.mechatronika.net.pl
Laboratorium Inżynierii Oprogramowania
Matlab Tutorial
Data Files
Matlab does not allow you to save the commands that you have entered in a session, but it
does allow a number of different ways to save the data. In this tutorial we explore the
different ways that you can save and read data into a Matlab session.
1.
Saving and Recalling Data
2.
Saving a Session as Text
3.
C Style Read/Write
Saving and Recalling Data
As you work through a session you generate vectors and matrices. The next question is how
do you save your work? Here we focus on how to save and recall data from a Matlab session.
The command to save
all
of the data in a session is
save.
The command to bring the data set
in a data file back into a session is
load.
We first look at the
save
command. In the example below we use the most basic form which
will save all of the data present in a session. Here we save all of the data in a file called
"stuff.mat." (.mat is the default extension for Matlab
data.)
>> u = [1 3 -4];
>> v = [2 -1 7];
>> whos
Name Size
u
v
1x3
1x3
Bytes Class
24 double array
24 double array
Grand total is 6 elements using 48 bytes
>> save stuff.mat
>> ls
stuff.mat
The
ls
command is used to list all of the files in the current directory. In this situation we
created a file called "stuff.mat" which contains the vectors
u
and
v.
The data can be read
back in to a Matlab session with the
load
command.
This tutorial was originally written by
Kelly Black.
Modified by
Jędrzej Mączak.
It is licensed under a
Creative Commons Attribution-ShareAlike 2.5 License
[Wpisz tekst]
>> clear
>> whos
>> load stuff.mat
>> whos
Name Size
u
v
1x3
1x3
Bytes Class
24 double array
24 double array
Grand total is 6 elements using 48 bytes
>> u+v
ans =
3
2
3
In this example the current data space is cleared of all variables. The contents of the entire
data file, "stuff.mat," is then read back into memory. You do not have to load all of the
contents of the file into memory. After you specify the file name you then list the variables
that you want to load separated by spaces. In the following example only the variable
u
will
be loaded into memory.
>> clear
>> whos
>> load stuff.mat u
>> whos
Name Size
u
1x3
Bytes Class
24 double array
Grand total is 3 elements using 24 bytes
>> u
u=
1
3 -4
Note that the
save
command works in exactly the same way. If you only want to save a
couple of variables you list the variables you want to save after the file name. Again, the
variables must be separated by a space. For an example and more details please see the help
file for
save.
When in matlab just type in
help save
to see more information. You will find
that there are large number of options in terms of how the data can be saved and the format
of the data file.
Saving a Session as Text
Matlab allows you to save the data generated in a session, but you cannot easily save the
commands so that they can be used in an
executable file.
You can save a copy of what
[Wpisz tekst]
happened in a session using the
diary
command. This is very useful if you want to save a
session for a homework assignment or as a way to take notes.
A diary of a session is initiated with the
diary
command followed by the file name that you
want to keep the text file. You then type in all of the necessary commands. When you are
done enter the
diary
command alone, and it will write all of the output to the file and close
the file. In the example below a file called "save.txt" is created that will contain a copy of the
session.
>> diary save.txt
... enter commands here...
>> diary
This will create a file called "save.txt" which will hold an exact copy of the output from your
session. This is how I generated the files used in these tutorials.
C Style Read/Write
In addition to the high level read/write commands detailed above, Matlab allows C style file
access. This is extremely helpful since the output generated by many home grown programs
is in binary format due to disk space considerations. This is an advanced subject, and we do
not go into great detail here. Instead we look at the basic commands. After looking at this
overview we highly recommend that you look through the relevant help files. This will help
fill in the missing blanks.
The basic idea is that you open a file, execute the relevant reads and writes on a file, and
then close a file. One other common task is to move the file pointer to point to a particular
place in the file, and there are two commands,
fseek
and
ftell
to help.
Here we give a very simple example. In the example, a file called "laser.dat" is opened. The
file identifier is kept track of using a variable called
fp.
Once the file is opened the file
position is moved to a particular place in the file, denoted
pos,
and two double precision
numbers are read. Once that is done the position within the file is stored, and the file is
closed.
fp = fopen('laser.dat','r');
fseek(fp,pos,'bof');
tmp = fread(fp,2,'double');
pos = ftell(fp);
fclose(fp);

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