CS99 Fall 2002
Lecture 12
11/18
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(0) Announcements
    + Prelim 2 reminder
    + Final Project reminder
    + missing work issue
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(1) Topics
    + review of function basics
    + MATLAB function syntax and rules
    + Scope
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(2) Function Review
    + functions do tasks
    + functions may or may not take input values
    + functions may or may not return values
    + functions help you program by reducing redundancy
      and breaking programs into smaller problems
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(3) Syntax of function M-files

    FUNCTION [output_args] = name(input_args)
    H1 comment line accessed by LOOKFOR
    comments accessed by HELP
    statements (optional)
    RETURN     (optional)

    + name: name of function
      - similar rules for naming as variables
      - see help pages for additional rules
    + input_args: names of variables used inside function
      - visible only to function
      - not used from "outside", nor visible to "outside"
        why? each function has a separate workspace with its own names!
      - may have zero or many inputs
    + output_args: names of variables to return to caller
      - the value(s) of the outputs are "sent" back to the expression that 
        invoked the function
      - may have zero or many outputs
    + keywords: function, return

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(4) Handy Rules (from MASTERING MATLAB):

    + you should give the function file and its the name the same name
    + function names are used up to 31 chars
    + begin names with lowercase letter and follow other variable naming rules
    + the H1 line is FUNCTION DECLARATION LINE (for LOOKFOR)
    + the following comments are for HELP
    + BODY of function are the statements
    + code inside function stops when reaching the last statement or RETURN
      (RETURN acts like a BREAK because you can exit function "early")
    + use ERROR to abort execution:

      example) if length(val) > 1
                  error('wrong!')
               end
      see HELP WARNING to flag the user but continue with execution


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(5) Scope

    + range or location where a name is visible (or not)
    + MATLAB partitions memory into WORKSPACES
      - each workspace has its own names (variables, scripts, functions)
      - the command window provides one workspace
      - each function has a different workspace
    + variables of a workspace are not shared in other workspaces
      - variables are called LOCAL VARIABLES when not shared]
      - use GLOBAL to get around this, but usually discouraged

  example) Your own function for adding, add.m:

  +-----------------------------------------+     
  | function x = add(a,b)                   |                             
  | % ADD(A,B) returns the sum of A and B   |	  a and b are __________
  | % ADD(A,B) adds A and B together.       |	  
  | % This function is just a simple demo.  |      x is _________________
  | x = a+b;                                |
  +-----------------------------------------+

   At command window prompt:

      >> add(1,2)
      ans = 

      >> add(1,2) - add(1,2)
      ans =

      >> add(1,2)*sqrt(16)
      ans =

      >> x = add(1,2); y = add(3,4); z = add(x,y)
      z   =

  example) nested functions

      >> z = add( add(1,2) , add(3,4) )
      z   =

  example) design function to swap values

      In file called swap.m:
      +------------------------------+
      | function [x,y] = swap(a,b)   |
      |    x = b;                    |
      |    y = a;                    |
      +------------------------------+

      In command window:
      >> a=1;b=2;
      >> [a b] = swap(a,b)
      >> a


      >> b
 

      Even shorter version: 
      +--------------------------------+
      | function ______ = swap(______) |
      +--------------------------------+  
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