8.6: Arrays of Structures
- Page ID
- 136686
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Why Use an Array of Structures?
A single structure is useful for storing information about one item. But in real applications, we usually have many items. A professor has many students. A store has many parts. A lab has many sensors. A dealership has many cars.
When you need to store many similar records, you can create an array of structures. Each element in the array is one structure, and every structure has the same fields. The following example creates an array of 3 packages with the fields item_no, cost, price, and code:
packages(1) = struct('item_no', 123, 'cost', 19.99, ...
'price', 39.95, 'code', 'g');
packages(2) = struct('item_no', 456, 'cost', 5.99, ...
'price', 49.99, 'code', 'l');
packages(3) = struct('item_no', 587, 'cost', 11.49, ...
'price', 33.95, 'code', '3');
The variable packages is now a structure array with three elements. Each element represents one package sold by the electronics shop.
Accessing Data in a Structure Array
If you type the name of a structure array, MATLAB displays the size of the array and the field names. Each element is a structure.
packages(2)
To access one field of one structure, specify the element number first and then the field name.
packages(2).code
This returns the code field from the second package.
To collect the same numeric field from all structures, use square brackets around the comma-separated output:
costs = [packages.cost]
This creates a numeric vector containing the cost values from all structures in the array.
packages.cost produces a comma-separated list. To store numeric values from all structures into one vector, use square brackets: costs = [packages.cost].
Preallocating a Structure Array
When a program creates an array one element at a time, MATLAB may need to repeatedly find more memory as the array grows. For very small examples, you may not notice a speed difference. For large programs, repeated resizing can slow the code down.
One simple way to preallocate a structure array is to assign the last element first.
packages(3) = struct('item_no', 0, 'cost', 0, 'price', 0, 'code', '');
packages(1) = struct('item_no', 123, 'cost', 19.99, 'price', 39.95, 'code', 'g');
packages(2) = struct('item_no', 456, 'cost', 5.99, 'price', 49.99, 'code', 'l');
packages(3) = struct('item_no', 587, 'cost', 11.49, 'price', 33.95, 'code', '3');
Another common method is to use repmat to repeat a template structure.
template = struct('item_no', 0, 'cost', 0, 'price', 0, 'code', '');
packages = repmat(template, 1, 3);
packages(1) = struct('item_no', 123, 'cost', 19.99, 'price', 39.95, 'code', 'g');
packages(2) = struct('item_no', 456, 'cost', 5.99, 'price', 49.99, 'code', 'l');
packages(3) = struct('item_no', 587, 'cost', 11.49, 'price', 33.95, 'code', '3');
Preallocation is a good programming habit because it helps MATLAB manage memory more efficiently.