matlab input-output statements nafees ahmed asstt. professor, ee deptt dit, dehradun

MATLAB Input-Output Statements

Nafees AhmedNafees AhmedAsstt. Professor, EE DepttDIT, DehraDun

Introduction

Data Input: Three methods1. Assign data to variables through an assignment statement 2. Input data from Keyboard 3. Read data from a file stored in computer memory

Data Input

• 1. Assign data to variables through an assignment statement

>>x=2;>>y=34.2;>>name=“Raheel Sufyaan”;

or

>>x=2; y=34.2; name=“Raheel Sufyaan”;

Data Input

• 2. Input data from Keyboard (Interactive input)I. Input Function II. Keyboard Command III. menu function IV. pause command

Data Input

I. Input Function : See different version of this function

>>r=input(‘Enter radius in meters:’)Enter radius in meters:56 %r will become integer

r = 56Enter radius in meters: ’ninety six’ %r will become string , Note: ‘ ‘ compulsory

r = ninety six

>>r=input(‘what is your father’s name:’, ’s’) %r string,

what is your father’s name: Nafees Ahmed % Note: ‘ ‘ not required

r =

Nafees Ahmed

Data Input

II. ‘Keyboard’ & ‘return’ command‘keyboard’ command when included in a script file returns control to the keyboard at the point where command occurs. The command window prompt is prefixed by the letter ‘k’. This command is used for

1. Check the intermediate results in the program 2. Make changes in variables value if required 3. Add new MATLAB commands in the program

Exp: Make a .m file for the following code and run it

A=10;B=16;X=A+B;Y=X/2;Keyboard %check the value of Y and change Y=10, after finishing write

%‘return’ command, it will execute rest program Z=A-B

Data Input

III. ‘menu’ function for pictorial selection. Syntax is I=menu(‘title’, ‘option1’, ‘option2’,……..)

Exp: Make a .m file for the following code and run it

i=menu('Select your favorite name','Teenu','Sumbul','Humera');

if i==1

disp('Teenu is your wife')

elseif i==2

disp('Sumbul is yr sis-in-low')

elseif i==3

disp('Humera is yr sis-in-low')

else

disp('Bewakoof kuch to select karo')

end

Data Input IV. ‘pause’ command

‘pause’ command temporarily halts the current computation and waits for the user to give a command to resume the computations. Pressing any key resumes the computation. pause(k) command stops the computation for k seconds.

Exp: Make a .m file for the following code and run it P=[1 2; 3 4]; Q=[5 5; 7 8];

disp('The matrix P is given as:')

disp(P)

disp('Program is paused, please press any key to display matrix Q')

pause

disp('The matrix Q is given as:')

disp(Q)

Z=P+Q;

disp('Program is paused, please press any key to display sum of P & Q')

pause

disp(Z)

disp('Again program is pausing for 5 sec')

pause(5)

Reading and storing file data

• There are many functions which are used to get data from the files.

1.’load’ function: To load variables/data in workspace from the disk file

Different syntax are

load %load matlab.mat (default) file

Load(‘filename’) %load complete file whose name is given

Load(‘filename’, ’x’, ’y’, ’z’) %load only variables x, y, z of given file

Load filename x, y, z %same as above

Note:-

1. No extension with file name means .mat file

2. If file name has extension other than (.mat) it will be treated as ASCII data file

3. Load function is use only if data file contains numeric data


Exp:

>>edit abc.txt

Write following and save it

1 3 4 5 7

6 7 8 0 9

>>load abc.txt %it will load this file in workspace with a variable %name abc, to check it write abc

>>abc

1 3 4 5 7

6 7 8 0 9

If we want to change the variable name write following command

>>test=load (‘abc.txt’) %now data ‘abc.txt’ will store in variable ‘test’


2.’save’ function: To save variables/data from workspace to disk file


save %save all workspace data in matlab.mat file

save(‘filename’) %save complete workspace data in specified file name

save(‘filename’, ’x’, ’y’, ’z’) %save only variables x, y, z in a file

save filename x, y, z %same as above

Note:-

1. No extension with file name means .mat file

2. If file name has extension other than (.mat) it will be treated as ASCII data file

3. To save variables to a directory other than current directory, full pathname of the file has to be specified.


3.’dlmread’ function: To read numeric text data separated by character other than space.(dlm=>delimiter, by default delimiter is comma )


data=dlmread(‘filename’) %read numeric data from ASCII delimited file named filename data=dlmread(‘filename’, delimiter) %read numeric data from ASCII delimited with other delimiter

i.e.

var_name=dlmread(‘abc.txt’, ‘;’) % this will read the data from file abc.txt till it encounters ‘;’ delimiter.

For example see next slide


4.’dlmwrite’ function: To write numeric text data separated by character other than space. (dlm=>delimiter, by default delimiter is comma )


dlmwrite(‘filename’,A) %write matrix ‘A’ on a specified file name with default delimiter (,)

dlmwrite(‘filename’, A, ‘D’) % write matrix ‘A’ on a specified file name with default delimiter ‘D’

i.e.

>>A=[1 2 3; 4 5 6; 7 8 9]

>>dlmwrite(‘abcd.txt’, A,‘D’) % this will write matrix A on file abcd.txt with delimiter ‘D’. Open this file & See.

Now try

>>B=dlmread(‘abcd.txt’,’D’)


5.’textscan’ and ‘textread’ functions: If a text file contains numeric as well as other characters. The file need to be opened for read operation using ‘fopen’ command & then read using ‘textscan’ command.

Different syntax are fid=fopen(‘abc.txt’, ’r’) %Open abc.dat file for reading

var_name=textscan(fid, ‘%f %c …….’) %here %f for float, %c for char etc

Note: var_name will be cell array

The command textread can be used to read the data into separate variables.

[a, b, c, d]=textread(‘abc.txt”, ‘%f %c …….’)

i.e.

[name, types, a, b, c]=textread(‘abc.txt’, ’%s %s %f %f %f’)

Output commands

1. Output may be formatted or unformatted • Default format :

>>a=7

a= 7

>>b=1.01

b= 1.0100 %4 digit after decimal

>>2100.34

c= 2.1003e+003 %2.1003x103

• Format command to change default format

>>x=pi

x = 3.1416

>>format long %similarly we can have other format also see help

x =

3.141592653589793

Output commands

2. ‘disp’ function

>>A=[1 2; 3 4]; % to display variable

>>disp(A)

1 2

3 4

>>disp(‘The largest number is;’) %to display a message

The largest number is

Low-level input-output functions

• There are a number of low-level file input-output function in MATLAB taken from ANSI Standard C Library. These are

1.File opening and closing functions

2.Binary input-output functions

3.Formatted input-output functions

Low-level input-output functions 1. File opening and closing functions ‘fopen’ function: To perform any operation on file the first step

is open it. Syntax are

• fid=fopen( filename, permission_code)where

fid=file id permission_code = read or write or append

• [fid, message] = fopen(filename, permission_code)where

message = error msg if file doesn’t open properly otherwise it is empty string

• [fid, message] = fopen(filename, permission_code, format)

where format = optional string specifying the numeric

format of the data in the file

Low-level input-output functions Standard file identifier values

Note: As more files are opened fids are allotted in continuation and if a particular file is closed, the fid assigned to it is released.

fid Meaning

-1 Fid opening fails

0 Standard input (keyboard)

1 Standard output (monitor screen)

2 Standard error

3 onwards Assign to file opened successfully

Low-level input-output functions Permission code

Note: As more files are opened fids are allotted in continuation and if a particular file is closed, the fid assigned to it is released.

Permission text string

Meaning

r Open a file(default) for reading, if not exist error occurs

r+ Open a file(default) for reading & writing, if not exist error occurs

w Delete the contents of an existing file or create a new file, and open it for writing.

w+ Delete the contents of an existing file or create new file, and open it for reading and writing.

a Create and open a new file or open an existing file for writing, appending to the end of the file.

a+ Create and open new file or open an existing file for reading and writing, appending to the end of the file.

Low-level input-output functions Other use of ‘fopen’ function:

• fids=fopen(‘all’)where

fids=a vector showing file ids of all opened files

• [filename, permission_code, format] = fopen(fid)

This function provieds filename, permission_code & format for open file specified by fid

Low-level input-output functions ‘fclose’ function: After performing operations on file the last

step is to close it. Syntax are

• status = fclose( fid)where

status= 0 if successfully closed =-1 otherwise

• status = fclose(‘all’)

It closes all open files except for standard output(fid = 1) & standard error (fid = 2)

Note: Standard input, output & error can’t be opened or closed with fopen & fclose functions


2. Formatted input-output functions: These are used to print, scan or get data in desired format. Different functions are

i. fprintf function ii. fscanf functioniii. fgetl functioniv. fgets function

Low-level input-output functions i. ‘fprintf ‘ function: Syntax is

fprintf(fid, format, data)

Exp: Create a file having following code and rut it.

x = 0:.1:1; y = [x; exp(x)];fid = fopen('exp.txt','w');fprintf(fid,'%6.2f %12.8f\n', y);fclose(fid);Now open the file ‘exp.txt’ and see the data

Note: 1. Here %6.2f . it always starts with % sign 6 = total width of field 2 = no of digits after decimal f = fixed point 2. fprintf(‘%6.2f %12.8f\n’, y ) will print on monitor, here fid =0 i.e monitor


In general ‘format’ may be written as %a.be

Or %+a.be % + = always print with sign character (+ or -)Or %-a.be % - = left justified Or %0a.be % pad zeros in starting

Here a = field width b = digits after decimal pointe = conversion character (%c for char, %s for string, %d for int, %e for exponential, %f for fixed point etc)

Note:1. '%6.2f %12.8f\n‘ here note the space b/w to formats2. \n means new line 3. ‘fprintf ‘ displays only real portion of complex value


ii. ‘fscanf ‘ function: Syntax is

fscanf(fid, format)And

[A, count] = fscanf(fid, format, sizeA)Where

A = variable to store data count = no of values reads from the files

sizeA = size of matrix A

Exp: Read the data of previously created file ‘exp.txt’. Create a file having following code and rut it.

fid = fopen('exp.txt');A = fscanf(fid, '%g %g', [2 inf]); % A is 2 row x inf column matrix A=A’ % transpose to get the

original matrix fclose(fid);


iii. ‘fgetl ‘ function: Reads the next line of the specified file, removing the newline characters. Syntax is

statement=fgetl(fid)

Exp: Read and display the file fgetl.m one line at a time:fid = fopen('fgetl.m'); tline = fgetl(fid); while ischar(tline) disp(tline) tline = fgetl(fid); end fclose(fid);


iv. ‘fgets ‘ function: Reads the next line of the specified file, including the newline characters Syntax is

statement=fgets(fid)

Exp: Read and display the file fgetl.m one line at a time:fid = fopen('fgetl.m'); tline = fgets(fid); while ischar(tline) disp(tline) tline = fgets(fid); end fclose(fid);


3. Binary input-output functions: The formatted input-output functions works with human readable text in a file. This file is also called an ASCII text file. These files require more disk space as compared to binary files.Different functions for binary files system are

i. ‘fwrite’ function ii. ‘fread’ function


i. ‘fwrite’ function: Syntax is

count=fwrite(fid, array, ‘percision’)

Wherecount = no of values (data) written to file

fid = file id

array = array of values to be written

precision = format of storing the values (char, uchar, short, int, long, float, double)


Exp: Please make a .m file of following code and run it.

clear all %clear all variable in workspace

fid=fopen('check.txt','r');

[A, count1]=fread(fid, [2,3], 'double') %A is ‘double’ 2x3 matrix

[B, count2]=fread(fid, [1], 'int') %B is an int constant

[C, count3]=fread(fid, [inf], 'char') %C is a char string

C=C' %transpose of C

C=char(C) %Convert numeric values into character string

fclose(fid


i. ‘fread’ function: Syntax is

[variable, count]=fread(fid, size, ‘percision’)

Wherevariable= values to be read from file

size = dimensions of data to be read

Note: To read binary data from a file correctly, it must be know how it was written


Exp: Please make a .m file of following code and run it. a=[2 3 6; 3 5 7];b=105;c='IPL has started'; fid=fopen('check.txt',‘r');

[A, count1]=fread(fid, [2,3], 'double') %A is ‘double’ 2x3 matrix

[B, count2]=fread(fid, [1], 'int') %B is an int constant

[C, count3]=fread(fid, [inf], 'char') %C is a char string

fclose(fid)

matlab input-output statements nafees ahmed asstt. professor, ee deptt dit, dehradun

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