Lect%2019P.%201

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MATLAB Script and Function Files

• Lecture 19

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MATLAB Script Files

• A MATLAB script file (Called an M-file) is a text
  (plain ASCII) file that contains one or more
  MATLAB commands and, optionally, comments.
• The file is saved with the extension ".m".
• When the filename (without the extension) is
  issued as a command in MATLAB, the file is
  opened, read, and the commands are executed as if
  input from the keyboard. The result is similar
  to running a program in C. The following slide
  is the contents of an M-file. Note that there
  are no prompts (gtgt).

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MATLAB Script Files

• This is a MATLAB script file.
• It has been saved as g13.m".
• load g13.dat Load data file
• voltage g13( , 4) Extract volts vector
• time .0051length(voltage) Create time
  vector
• plot (time, voltage) Plot volts vs time
• xlabel ('Time in Seconds') Label x axis
• ylabel ('Voltage') Label y axis
• title ('Bike Strain Gage Voltage vs Time')
• grid Put a grid on graph

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MATLAB Script Files

• The preceding file is executed by issuing a
  MATLAB command
• gtgt g13
• This single command causes MATLAB to look in the
  current directory, and if a file g13.m is found,
  open it and execute all of the commands. The
  result, in this case, is a plot of the data from
  g13.dat.
• If MATLAB cannot find the file in the current
  working directory, an error message will appear.

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MATLAB Script Files

• When the file is not in the current working
  directory, a cd or chdir command may be issued to
  change the directory.
• gtgt cd a\ Make a\ the current working
  directory
• gtgt g13

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MATLAB Demo

• MATLAB Example use of ginput
• clear, clf, hold off
• axis (0, 10, 0, 10)
• hold on
• plot (1, 2, 2, 1, 1,2, 2, 3, 3, 2)
• text (1, 1.6, 'Click inside box to quit')
• while 1
• x, y, buttun ginput (1)
• if buttun 1, plot (x, y, 'r'), end
• if buttun 2, plot (x, y, 'oy'), end
• if buttun 3, plot (x, y, 'g'), end
• if x gt 1 x lt 2 y gt 2 y lt 3, break end
• end
• hold off

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MATLAB Function Files

• A MATLAB function file (called an M-file) is a
  text (plain ASCII) file that contains a MATLAB
  function and, optionally, comments.
• The file is saved with the function name and the
  usual MATLAB script file extension, ".m".
• A MATLAB function may be called from the command
  line or from any other M-file.

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MATLAB Function Files

• When the function is called in MATLAB, the file
  is accessed, the function is executed, and
  control is returned to the MATLAB workspace.
• Since the function is not part of the MATLAB
  workspace, its variables and their values are not
  known after control is returned.
• Any values to be returned must be specified in
  the function syntax.

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MATLAB Function Files

• The syntax for a MATLAB function definition is
• function val1, , valn myfunc (arg1, ,
  argk)
• where val1 through valn are the specified
  returned values from the function and arg1
  through argk are the values sent to the function.
• Since variables are local in MATLAB (as they are
  in C), the function has its own memory locations
  for all of the variables and only the values (not
  their addresses) are passed between the MATLAB
  workspace and the function.

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MATLAB Function Files

• It is OK to use the same variable names in the
  returned value list as in the argument. The
  effect is to assign new values to those
  variables. As an example, the following slide
  shows a function that swaps two values.

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Example of a MATLAB Function File

• function a , b swap ( a , b )
• The function swap receives two values, swaps
  them,
• and returns the result. The syntax for the
  call is
• a, b swap (a, b) where the a and b in the
  ( ) are the
• values sent to the function and the a and b in
  the are
• returned values which are assigned to
  corresponding
• variables in your program.
• tempa
• ab
• btemp

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Example of a MATLAB Function File

• To use the function a MATLAB program could assign
  values to two variables (the names do not have to
  be a and b) and then call the function to swap
  them. For instance the MATLAB commands
• gtgt x 5 y 6 x , y swap ( x , y )
• result in
• x
• 6
• y
• 5

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MATLAB Function Files

• Referring to the function, the comments
  immediately following the function definition
  statement are the "help" for the function. The
  MATLAB command
• gtgthelp swap displays
• The function swap receives two values, swaps
  them,
• and returns the result. The syntax for the call
  is
• a, b swap (a, b) where the a and b in the ( )
  are the
• values sent to the function and the a and b in
  the are
• returned values which are assigned to
  corresponding
• variables in your program.

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MATLAB Function Files

• The MATLAB function must be in the current
  working directory. If it is not, the directory
  must be changed before calling the function.
• If MATLAB cannot find the function or its syntax
  does not match the function call, an error
  message will appear. Failure to change
  directories often results in the error message
• Undefined function or improper matrix assignment.

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MATLAB Function Files

• When the function file is not in the current
  working directory, a cd or chdir command may be
  issued to change the directory.
• gtgt cd a\ Make a\ the current working
  directory

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MATLAB Function Files

• Unlike C, a MATLAB variable does not have to be
  declared before being used, and its data type can
  be changed by assigning a new value to it.
• For example, the function factorial ( ) on the
  next slide returns an integer when a positive
  value is sent to it as an argument, but returns a
  character string if the argument is negative.

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MATLAB Function Files

• function n factorial (k)
• The function n factorial(k) calculates and
• returns the value of k factorial. If k is
  negative,
• an error message is returned.
• if (k lt 0) n 'Error, negative argument'
• elseif klt2 n1
• else
• n 1
• for j 2k n n j end
• end