cs211 lesson 24

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Quote:

Some succeed because they are destined to; most succeed because they are

determined to. Unknown

Lesson Objectives:

Be able to read (input) and write (output) data using the loadand saveMATLAB functions

Understand how to write data to a binary file using fwrite

Understand how to read data from a binary file using fread

Understand the syntax differences between issuing commands and callingfunctions

Lesson:

I. MATLAB Concepts

A. Saving/Loading Workspace Variables

savewrites (outputs) the contents of one or more workspace variables to aspecified file in binary format.For example:

save('mydata.mat', 'Alpha', 'Beta')

stores the contents of the variables Alphaand Betato a file called

mydata.mat

loadreads (inputs) variable data values from a specified file.For example:

load('mydata.mat', 'Beta')

creates a variable called betain the current workspaceand initializes its value

from the data in the file mydata.mat .

B. Binary File I/O - fwriteand fread

fwrite(File_id, Variable, 'Precision') writes (outputs) the

contents of a variable to a file in binary format. A file must be opened withfopen() before executing any fwrite() commands.

The precision (data type) of the values written to the file are controlled by thethird argument. Possible data types include:

'schar' signed character (1 byte)

'uchar' unsigned character (1 byte)'char' UNICODE character (2 bytes)

'int8' 8-bit integer


11 Lesson 24 http://cse.unl.edu/~sincovec/Matlab/Lesson 24/Binary/CS211 Lesson 24...

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'uint8' unsigned 8-bit integer




'float32' fractional number with 7 significant digits

'float64' same as double

'double' fractional number with 15 digits of accuracy

For example,

File_id = fopen('mydata.dat', 'w');

fwrite(File_id, Alpha, 'double')

fclose(File_id);

stores the contents of the variable Alphato a file called "mydata.dat" in

double precision format (8 bytes per value).

fread(File_id, Number_of_elements,

'File_precision=>Memory_precision') reads (inputs) the binary data stored

in a file into a variable. The data type of each element is converted as specified by the

third argument. The possible precision options are shown in the table above. A file must

be opened with fopen()before executing any fread() commands.

IMPORTANT: You must know two (2) things about the binary data in the file before

you can read the data:

1) what binary format (precision) was used to store the data

2) the exact number of values in the file to read.

For example,

File_id = fopen('mydata.dat', 'r');

Alpha = fread(File_id, 5, 'int32=>double')

fclose(File_id);

reads 5, 32-bit integers from the file mydata.datand stores the values as

double precision numbers into the variable Alpha.

Usefreadandfwrite when:You need to store data in different data types (e.g., int8, int16, etc.) to save

memory and/or disk space for large data sets.

You need to store less than a full vector or matrix to a file. For example:

File_id = fopen('MyApp.dat', 'w');

fwrite(File_id, Alpha(2:end,3:5), 'double');

fclose(File_id);

IMPORTANT: A binary file is a series of bits. The bits will be misinterpreted if you

write the data in one format and then read the data back using a different format. It is

your job as the programmer to remember the structure of data in a binary file. If youread a binary file incorrectly you will end up with "garbage values" as a result.

Consider the following example of what NOT to do!

A = rand(1,5); % 5 random fractions -- all double


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precision

disp(A);

File_id = fopen('test.dat', 'w');

fwrite(File_id, A, 'double'); % writes 5 double

precision values

fclose(File_id);

File_id = fopen('test.dat', 'r');A = fread(File_id, 40, 'int8=>int8'); % reads 40

8-bit integers

fclose(File_id);

disp(A);

C. Commands vs. Functions (unrelated to file I/O)

Many MATLAB "commands" have a command lineformat and a function

format.

Commandsuse the format:

commandName arg1 arg2 arg3 ...

Functionsuse the format:

functionName(arg1, arg2, arg3, ...)

For example - the help system defines the "load" command as shown below.

The first 7 lines describe the functionformat, while the last line describes thecommandformat.

load

load('filename')

load('filename', 'X', 'Y',

'Z')

load('filename', '-regexp',

exprlist)

load('-mat', 'filename')

load('-ascii', 'filename')S = load(...)

function

format

load filename -regexp expr1

expr2 ...command

format

The arguments to commandsand functionssometimes require differentformats. For example, both of the lines below will load the contents of a

MATLAB binary data file into a variable called alpha. Note that the function call

format requires quotes around the variable name while the commandformatdoes not.

load('MyData.mat', 'Alpha'); % function call

load 'MyData.mat' Alpha % command


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It is recommended that you always use the function callformat of MATLAB

built-in functions to avoid confusion. In addition, in many cases, the function

callformat also allows for greater control over how a particular MATLABprocedure performs its work.

II. Good Programming Practices

Use save() to write the entire contents of one or more variables from a program (actually

workspace) to a file.

Use load() to read the entire contents of one or more variables from a file into a

program.

Use fwrite() to write the partial contents of one or more variables from a program to a

file, or to save data to a file in a different data type.

Use fread() to read the partial contents of one or more variables from a file into a

program, or to load data from a file and change its data type while it is being read.

If you write data in a binary format to a file, you should typically write the number ofvalues "in front of" the actual data. This allows the code that reads the data to know howmany values are stored in the file. (See the example in the algorithms section below.)

III. Algorithms

Suppose you need to store a series of "values", each of a different size and data type, toa binary file. To be able to read the data from the binary file, the size of each "value" must

be known. Therefore, when you write binary data to a file, typically you need to write thesize of the data size as well.

For example, the following code writes a series of "values" related to cadets to a binaryfile.

Cadets = { { 'Fred Smith ', '1234567890', [96 87 76 89] }, ...

{ 'Mary Taylor', '5678901234', [100 94 89 93] }, ...

{ 'Red Brown ', '8901234567', [95 100 85 92] } };

File_id = fopen('Cadets.dat', 'w');

for j = 1:length(Cadets)fwrite(File_id, length(Cadets{j}{1}), 'int16'); % writes

data "size"

fwrite(File_id, Cadets{j}{1}, 'char'); % writes

cadet name

fwrite(File_id, length(Cadets{j}{2}), 'int16'); % writes

data "size"

fwrite(File_id, Cadets{j}{2}, 'char'); % writes

cadet ID

fwrite(File_id, length(Cadets{j}{3}), 'int16'); % writesdata "size"

fwrite(File_id, Cadets{j}{3}, 'int8'); % writes

cadet grades

end


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fclose(File_id);

The following code reads the binary file created by the code above.

File_id = fopen('Cadets.dat', 'r');

Number_cadets = 0;

while true

% Read the number of characters in the cadet name

N = fread(File_id, 1, 'int16=>double');

if isempty(N)

break; % The file is empty - stop reading the file

end

% Read the cadet name

Number_cadets = Number_cadets + 1;

Cadets{Number_cadets}{1} = fread(File_id, N, 'char=>char');

% Read the cadet ID


Cadets{Number_cadets}{2} = fread(File_id, N, 'char=>char');

% Read the cadet grades


Cadets{Number_cadets}{3} = fread(File_id, N, 'int8=>double');

end

fclose(File_id);

for J = 1:length(Cadets)

for K = 1:length(Cadets{J})

disp(Cadets{J}{K})

end

end

Lab Work: Lab 24

References: Chapman Textbook: sections 8.2, 8.5


cs211 lesson 24

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