EGR 106 Week 5 More on Files

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EGR 106 Week 5 More on Files

• Debugging errors in scripts
• Interacting with data files
• Functions
• Concept
• Examples and applications
• Textbook chapter 4.4-4.6.1, 4.7-4.8 and chapter
  6.1-6.7, 6.10

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Debugging Errors

• Syntax errors

Note red text bad news
But tells you where
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• Run-time errors inf or NaN results

Note black text okay, just a warning
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• Logical errors in your program hard to find
• Example quadratic equation solver
• But x22x1 (x1)2 ? x 1
• Use the built-in debugger

Missing parentheses around 2a and around b
sqrt( ..)
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MATLABs Debugger
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• The red dot at line 6 indicates a breakpoint
• The green arrow shows which line will next be
  evaluated

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Matlab Data Files (not in the text)

• Types
• .asv auto save
• ascii regular text files
• .mat Matlabs proprietary format (multiple
  variables)

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• Saving data
• save filename
• save filename array1 array2
• save filename ascii
• Reading in (loading) data
• load filename
• load filename array1 array2
• load filename ascii

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Function concept

• So far
• Have used Matlabs built-in functions
• Have started writing scripts
• Function computes an output from an input
• Reusable script
• Sometimes called a subprogram
• Building block for larger programs
• Example converting degrees to radians

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Example Function DEG2RAD
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Usage

• Syntax is just like a built-in functions
• Application is independent of the variable names
  within the function (x,y)
• Executed by typing name (input)

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Rules for Functions

• First line of the file must be of the form
• function outputs name(inputs)

List of any variables that the function needs
Identifies a function file
List of function result variables
Name of the function and the file (name.m)
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• inputs
• Used to transfer data into the function from the
  workspace
• Workspace variables are unavailable within the
  function
• Any necessary variables must be brought in
• For multiple inputs
• Separate them by commas
• Order is important
• Examples of built-in functions with inputs
• sum(x) plot(x,y)

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• outputs
• Used to transfer results back into the workspace
  from the function
• For multiple outputs
• Separate them by commas in brackets
• Order is important
• Output variables must be assigned
• Examples of built-in functions with outputs
• y sum(x)
• value,location max(x)

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• Note brackets on the left only make sense for
  functions
• value,location max(x) is okay
• value,location 1, 2 is not
• Default output is the first one
• value,location max(x)
• value max(x)

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• Comments in lines 2,
• Words in line 2 are searched when lookfor is
  employed
• Comments in lines 2, 3, are returned when help
  name is executed

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Variables Local vs Global

• Usually, once created, variables are available in
  the workspace until cleared
• Functions create their own workspace with their
  own local variables distinct from those in the
  original workspace
• Functions cannot modify variables within the
  original workspace except through outputs
• Exception global variables can span both
  workspaces and be manipulated in both

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Example Functions

• Consider a script to compute the area of a
  triangle (from week 2)
• First as a script

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Now as a function
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• Usage doesnt depend on knowing parameter names
  internal to the program

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• Could add the perimeter calculation as a second,
  optional output

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• My plotting function

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• Can stack functions in one file (all must be
  functions)
• Useful for mailing and testing

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Typical Errors
Too few inputs
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Too many inputs
Too many outputs
Wrong input type funny result
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Application f(x) 2e-xsin(x)e-x/2sin(2x)

• Find its minimum and the first root beyond zero

x ?
f(x) ?
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Solution using Matlab

• Construct a function m-file for f(x)

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• To find the root fzero('fun',a)
• To find the minimum fminbnd('fun',a,b)