structs
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(1) General: Structure Arrays
      
    + special kind of array
    + holds arrays inside elements   
    + arrays may be different types!
    + addresses elements by names called FIELDS
    + SAs usually 1-D but can be multi-dim
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(2) Creating

   + Just make assignments
   + to store values use the $.$ operator
     syntax: name.field=value

     >> A.fn = 'ben'; 
     >> A.ln = 'stein';
     >> A.id = 4321

     A = 
        fn: 'ben'
        ln: 'stein'
        id: 4321

   + Note:
     - MATLAB will display a reference if the field is not 1-D
     - Try $A.x = [1:3;3:5]$ and then displaying $A$

       >>  A.x = [1:3;3:5]

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(3) Retrieve values

    Use name.field
 
    >> A.fn

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(4) Fields can be any other data structure, including another SA

    Create a new SA

    >> B.tests = [99 88]
    >> B.projects = [10 10 10];
    >> B

    B = 
          tests: [99 88]
       projects: [10 10 10]

    Create new field of A called $scores$
    Assign scores in $B$ to $A$

    >> A.scores = B

    A = 
       fn: 'ben'
       ln: 'stein'
       id: 4321
        x: [2x3 double] 
   scores: [1x1 struct]

   Notes:
   + $scores$ and $x$ values NOT 1-D, so ML uses pointers
   + But, values are still there!
   + Access particular value
     name.(substructure access) means name.name.field

     >> A.scores.tests
     ans =
           99    88
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(5) Additional Things

    + Useful help:
      help struct
      help rmfield
      help getfield
      help setfield

    + Preallocation

      >> SA1 = struct('x',0,'y',1)

      SA1 = 
        x: 0
        y: 1

    + Removing fields
      - ML's SAs have a feature not common to other languages
      - Entering $rmfield$ will remove the NAME and CONTENTS of a field

        >> A = rmfield(A,'x')

          A = 
             fn: 'ben'
             ln: 'stein'
             id: 4321
         scores: [1x1 struct]
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(6) Array of Structures

    + can create an array of SAs by "adding" another SA
      - start with initial SA and make another
      - give 1 or more fields a value
      - ML automatically creates a new SA
      - empty fields represented as empty array $[]$

        >> A(2).fn = 'gene';
        >> A(2)

        ans = 

             fn: 'gene'
             ln: []
             id: []
              x: []
         scores: []

    + What's an array of SAs???
 
        >> A

         A = 
            1x2 struct array with fields:
            fn
            ln
            id
            x
            scores

    + to get individual SAs, enter $A(1),A(2)$
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