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Matlab Types - Structure

Matlab TypesCompleted

Structure

In MATLAB, structure and cell are called heterogeneous containers: special arrays that, unlike a regular array, can hold elements of different types.

  • A structure works like a data container: it holds multiple values of possibly different types under named fields
  • A single structure is itself a 1×1 structure array
  • Example: |200
  • The dot . is the structure delimiter

Properties

  • Identity test functions
    • isstruct
    • isfield(S,F) — checks whether F is the name of one of S’s fields
      • The result can be true only when S is a struct and F is a char vector
  • fieldnames returns a cell whose elements are the names of every field of a struct

Creation

  • Assignment: struct.field = x
    • Connect the structure name and field name with a dot .
    • To create a larger structure array, use array indexing: struct(i).field = x
      • Every structure in a structure array shares the same fields
      • Unassigned fields default to an empty array
  • Function struct

Example using assignment to build a structure array:

>> lover.name = 'Josh';
>> lover.age = 21;
>> lover
lover = 
  struct with fields:
    name: 'Josh'
     age: 21
>> lover(2).score = [90 80 95]
lover =
  1x2 struct array with fields:
    name
    age
    score
>> lover(2)
ans =
  struct with fields:
     name: []
      age: []
    score: [90 80 95]
>> lover(1)         
ans =
  struct with fields:
     name: 'Josh'
      age: 21
    score: []
>> whos
  Name       Size            Bytes  Class     Attributes
  ans        1x1               424  struct
  lover      1x2               568  struct

Access

A structure array uses indexing to reach the structures inside and their fields; see Matlab Array - Indexing. The basic syntax is struct(I1).field(I2).

  • I1 indexes into the structure(s) of the structure array struct
    • You may pass an index spanning multiple structures only when no field access follows, otherwise it raises an error

    • You can index into part of a field only when you refer to a single element of a structure array

  • I2 indexes into the elements of the field field of those structures

Example:

>> lover(2).score([1,3]) 
ans =
    90    95
>> lover(:).score(:)     
Expected one output from a curly brace or dot indexing expression, but there were 2 results.
>> lover(:)         
ans =
  2x1 struct array with fields:
    name
    age
    score

Dynamic Fields

When the field name is itself a variable, use dynamic fields: wrap the variable in parentheses ().

>> S.(char(100+rand*10)) = [] % use a random value as the field name
S =
  struct with fields:
    m: []
>> S.(char(100+rand*10)) = []
S =
  struct with fields:
    m: []
    e: []
 
>> S1.a = 1; S1.b = 2; F = 'ab';
>> S1.(F(randi(2))) % access a randomly chosen field
ans =
     1

Removal

rmfield deletes selected fields from a structure and returns the trimmed structure.

  • Syntax: rmfield(S,fieldname)
    • fieldname may be a char vector or a cell array of char vectors

Nesting

A field of a structure may itself be a structure, producing a nested structure. Construction and access follow the same rules as for an ordinary structure.

>> S.A.a = rand(2)
S =
  struct with fields:
    A: [1x1 struct]
>> S.A(2).a = rand(2)
S =
  struct with fields:
    A: [1x2 struct]
>> S.A(1).a(2,1)           
ans =
    0.8491
  • Dot-separated nesting reads naturally as a path: S.layer1.layer2...

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