Engineering H192 - Computer Programming
MATLAB: Structures and File I/O
Lecture 21
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Lect 21
P. 1
Engineering H192 - Computer Programming
MATLAB Relational Operators
• MATLAB supports six relational operators.
Less Than
Less Than or Equal
Greater Than
Greater Than or Equal
Equal To
Not Equal To
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<
<=
>
>=
==
~=
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Lect 21
P. 2
Engineering H192 - Computer Programming
MATLAB Logical Operators
• MATLAB supports three logical operators.
not
and
or
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~
&
|
% highest precedence
% next highest precedence
% next highest precedence
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Engineering H192 - Computer Programming
MATLAB Selection Structures
• An if - elseif - else structure in MATLAB.
Note that elseif is one word.
if
expression1
% is true
% execute these commands
elseif expression2
% is true
% execute these commands
else
% the default
% execute these commands
end
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Lect 21
P. 4
Engineering H192 - Computer Programming
MATLAB Repetition Structures
• A for loop in MATLAB
for x = array
for x=1: 0.5 : 10
% execute these commands
end
• A while loop in MATLAB
while expression
while x <= 10
% execute these commands
end
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Lect 21
P. 5
Engineering H192 - Computer Programming
Scalar - Matrix Addition
EDU» a=3;
EDU» b=[1, 2, 3;4, 5, 6]
b=
1 2 3
4 5 6
EDU» c= b+a
% Add a to each element of b
c=
4 5 6
7 8 9
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Lect 21
P. 6
Engineering H192 - Computer Programming
Scalar - Matrix Subtraction
EDU» a=3;
EDU» b=[1, 2, 3;4, 5, 6]
b=
1 2 3
4 5 6
EDU» c = b - a %Subtract a from each element of b
c=
-2 -1 0
1 2 3
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P. 7
Engineering H192 - Computer Programming
Scalar - Matrix Multiplication
EDU» a=3;
EDU» b=[1, 2, 3;4, 5, 6]
b=
1 2 3
4 5 6
EDU» c = a * b % Multiply each element of b by a
c=
3 6 9
12 15 18
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Lect 21
P. 8
Engineering H192 - Computer Programming
Scalar - Matrix Division
EDU» a=3;
EDU» b=[1, 2, 3;4, 5, 6]
b=
1 2 3
4 5 6
EDU» c = b / a
% Divide each element of b by a
c=
0.3333 0.6667 1.0000
1.3333 1.6667 2.0000
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Lect 21
P. 9
Engineering H192 - Computer Programming
Reading Data from Files
• The load command does not always succeed in
reading an input file. It only works when all lines
in the file have the same ASCII format. Files with
header material do not qualify, nor do binary files.
• ASCII files that cannot be input with the load
function can be opened and input with MATLAB
functions that are similar to C language functions
we have been using. The MATLAB functions
include fopen, fgets, fscanf, and sscanf.
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Engineering H192 - Computer Programming
Opening Data files
• The command to open a file for input is:
infile_id = fopen('filename','r');
• The command to open a file for output is:
outfile_id = fopen('filename','w');
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Engineering H192 - Computer Programming
Inputing Data from Open Files
• After the file is opened for reading we can input
one line at a time as a text string. The command
to read one line from the file as a string is:
line = fgets (infile_id);
• If the remainder of the data in the file all has the
same format we can read the rest of the file and
store it in an array with the command:
myarray = fscanf(infile_id,...
'%*s%*s%*s%f');
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Lect 21
P. 12
Engineering H192 - Computer Programming
Inputting Data from Open Files
• Data that has been read in as a string using fgets
can be parsed using the function sscanf.
• The syntax to input a line and then extract and
discard three columns from the string and save the
fourth column of floating point data in an element in
the array myarray would be:
line = fgets(infile_id);
myarray(k)=sscanf(line,'%*s%*s%*s%f');
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Lect 21
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Engineering H192 - Computer Programming
Note on Indexing of Array Elements
• Indexing in C/C++
• Indexing in MATLAB
float myarray[20];
int i;
% No declarations
% are necessary
for (i = 0; i < 20; i++)
{
myarray[ i ] = 0.0;
}
for i = [1:20]
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myarray( i ) = 0.0;
end
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Engineering H192 - Computer Programming
Printing Data to Files
• When a file has been opened for output, the
command to output one line to the file as a string
is:
fprintf(outfile_id,'%s',line);
• Lines of other data can be output by inserting the
appropriate format string and variable names. For
example:
fprintf(outfile_id,'%f%f\n',a,b);
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Engineering H192 - Computer Programming
Printing to the Screen
• Data can also be printed to the screen with the
fprintf function. To do so, we omit the file
idenification (outfile_id).
• For instance, to print a line of text to the screen
we could use:
fprintf('%s',line)
and to print other data we might use:
fprintf('%f %f\n', a , b)
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Lect 21
P. 16