Lecture 4 MatLab

1
Lecture 4 - MatLab

• January 25, 2001
• CVEN 302

2
Lectures Goals

• Programming flow controls
• Misc. functions
• Program Design
• Organization
• Setup
• Test
• Debug
• Document

3
switch flow control

• switch(relation expression)
• case 1(value)
• block of statements
• case 2(value)
• block of statements
• case 3 (value)
• block of statements
• otherwise
• end
• example cond4.m

• The switch command
• can be used for multiple
• case of the if command.

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for flow control

• for(index )
• block of statements
• end
• example
• x4,5,6,7 sumx0
• for k 1length(x)
• sumx sumx x(k)
• end

• The for command is
• used to do a series of
• steps of redundant
• process.

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for flow control

• example
• (counter) startstepfinal
• value size value
• k 12n (1,3,5,n)
• m 0pi/15pi (0,Pi/15,,Pi)
• k n -11 (n,n-1,,1)

• The for command can
• have various forms of
• step sizes.

6
while flow control

• while(expression)
• block statements
• end
• Example
• x4
• while (x-1)gt 0.01
• x sqrt(x)
• disp(x)
• end

• The while command
• can do redundant process
• by checking on a logical
• or relational operation or
• combination of the two.

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Escape commands

• break is an escape from a loop, which will go
  the end of the loop and start the next step in
  the program.
• return is an escape from a program or
  subroutine or a function and return to the main
  program.

8
Misc. Functions

• disp(expression) - displaces the expression to
  the screen.
• format (long,e,short, ...) determines the format
  of numerical values.
• global variable makes a variable available in
  any function or program as shared memory.

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Design Steps for a Program

• Design the code to do the problem
• Input types
• Output types
• Write up the code with modified comments.
• Test the code with a simple set of test examples.
• Document the code

10
Design of the M-files

• Remain consistent in how you put your
• program together. Break it into parts
• Input information - comments and variables
  definition and options verify input
  parameters.
• Primary computational task
• Output options (plot information, ascii,
  filenames, etc.)

11
Program Design

• Visual blocking show the layout of the program.
• Whitespace in the text of the program.
• Meaningful variable names
• Documentation

12
Visual Blocking

• Which is easier to read?
• n length(x)
• if mean(x) lt0
• y(1)0 y(n) 0
• for k2n-1
• y(k) x(k1)-x(k-1)
• end
• else
• y(1)x(1) y(n) x(n)
• for k2n-1
• y(k)0.5( x(k1) x(k-1))
• end
• end

• How do you visualize your
• code?
• n length(x)
• if mean(x) lt0
• y(1)0 y(n) 0
• for k2n-1
• y(k) x(k1)-x(k-1)
• end
• else
• y(1)x(1) y(n) x(n)
• for k2n-1
• y(k)0.5( x(k1) x(k-1))
• end
• end

13
Whitespace layout

• Which line is easier to read?
• y(k)0.5(x(k1)x(k-1)) or
• y(k) 0.5(x(k1)x(k-1)) or
• y(k) 0.5( x(k1) x(k-1) ) or
• y(k) 0.5 ( x(k1) x(k-1) )

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Variable Names
Use variable names, which have meaning so you can
read the code and understand it.
Variable typical code meaningful
code inside diameter d5 ins_diam
5 thickness t0.02 thickness
0.02 inside radius rd/2 ins_radius
ins_diam / 2 outside radius r2rt
out_radius ins_radius thickness
15
Documentation

• Documentation is critical for any code that is
  not going to be used and immediately discarded.
  Documentation takes the form of comment
  statements that describe the input and output
  parameters of a function as well as the steps
  performed in the analysis.

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Documentation Recommendations

• If a program is incorrect, it matters little what
  the documentation says.
• If documentation does not agree with the code it
  is not worth much.
• Consequently, code must largely document itself.
  If it cannot, rewrite the code rather than
  increase the supplementary documentation. Good
  code needs fewer comments than bad code does.

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Documentation Recommendations

• Comments should provide additional information
  that is not readily obtained from the code
  itself. Comments should never parrot the code.
• Logical variable names and labels, and layout
  that emphasizes logical structure, help make a
  program self-documenting.
• Kernighan and Plaugher, The Element of
  Programming Style, 1978

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Input

• Function definition
• Summary is what the program does.
• Synopsis run program
• Input definition of terms
• Output what type of output
• Notes

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Input Example

• function rho H2Odensity(T,units)
• H2Odensity Density of saturated liquid water
• Synopsis rho H2Odensity
• rho H2Odensity(T)
• rho H2Odensity(T,units)
• Input T (optional) temperature at which
  density is
• evaluatedDefault T 20 C If units F
• then T is degrees F
• units (optional) units for input
  temperature,
• Default C, units C for Celsius
• units F for Fahrenheit
• Output rho density, kg/m3,if unit C
  or lbm/ft3 if
• unit F
• Notes Use 4th order polynomial curve fit of
  the data in

20
Organization of Numerical Method

• Start with an abstract or verbal description of a
• computational task, it will be necessary to
  develop a
• sequence of smaller tasks that can be implemented
• sequentially.
• Stepwise refinement is designed to break up a
  large
• task into a set of smaller tasks. The same
  procedure
• is called top-down design or divide and
  conquer.

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Implementation and Testing

• Break the problem into small modules/segments of
• code, which you can test.
• Testing
• It is important to verify that m-files are
  working correctly before they are used to produce
  results that inform other decisions. The best
  check on any calculation, be it manual or
  automatic, is by an independent test. One which
  you have to designed.

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Debugging

• Searching for and removing bugs is an inevitable
  part of programming. Bugs are caused by a number
  of factors from outright programming blunders to
  the improper application of numerical method.

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Defensive Programming

• Do not assume that the input data are correct.
  Test them.
• Guard against the occurrence of an impossible
  condition that would cause a failure in a
  calculation.
• Provide a catch or default condition for a
  sequence of if elseif else constructs.
• Provide diagnostic error messages that will help
  the user determine why the function failed to
  complete it calculation.

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Debugging Tools

• type and dbtype commands
• type prints out the file error
• dbtype will print out the file with line
  numbers. dbtype startstop will print out
  lines from start to stop.
• Error function - error(message) will print out
  the message and halt execution.

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Debugging Tools

• pause command temporarily suspends the
  execution of the m-file. Useful way to print a
  warning message without necessarily aborting an
  analysis.
• keyboard command keyboard command suspends
  execution of an m-file and gives control to user.
  It gives the user access to internal variables
  of the function containing command.

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Debug example

• The example program
• keyboard stops a program to check internal
• variables. It is a powerful tool.
• Kgtgt is the prompt and you can use it to check
  numbers out.
• Kgtgt return will return to the executable
  program.