A. Introduction to non-homogeneous arrays
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Up to this point in this course, all arrays have held values of a single data type. That is, an array contained all double precision values, or all character values, or all integer values, but never a mixture of these different data types.
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Now we introduce two new data structures that can store values of possibly different data types in a single structure. They are:
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cell arrays, which access elements within them using indexes (subscripts)
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structure arrays, which access elements within them using field names
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These two data structures can be nested, meaning you can have a cell array that contains structure arrays that contains other structure arrays that contain other cell arrays, etc. . You can create arbitrarily complex data relationships by nesting cell and structure arrays.
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You typically organize data into cell arrays if you need to access the various element values using a numerical subscript (or index).
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You typically organize data into structure arrays when the various values are best accessed using a descriptive name (called the field name).
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You will better understand when to use one type over the other after you have had some experience using them.
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Structure arrays are described in the next lesson, lesson 22.
B. Introduction to Cell Arrays
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A cell array is a MATLAB array that can be used to store values of different data types.
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Each element of a cell array is called a cell.
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All cells of a cell array are 4-byte addresses (called pointers) to other data structures.
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Each cell points to a distinct array, and each distinct array can possibly be of a different size and contain different types of data.
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Cell arrays are often used to store arrays of strings because this allows each string to be a unique length.
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For example, the statement
A = { 'Wayne', 'Sam', [5 8 2 9; 4 3 1 6] };
creates the memory shown in the diagram below. The cell array A contains three items: a 1-by-5 character array (a string), a 1-by-3 character array (a string), and a 2-by-4 double array. This example cell array will be used throughout the rest of this lesson to demonstrate cell array syntax and manipulation.
C. Cell array syntax
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When specifying cells in a cell array, you have two choices and each choice does a completely different operation. Study this carefully!
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Use braces { } - which is called content indexing, to retrieve the data a cell points to.
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Given a cell array name with a context index, you can treat it just like a variable name for the data it points to and use any additional notation that is consist with the data it references.
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For example:
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A{1} is the entire 1-by-5 character array containing 'Wayne'
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A{1}(3) is the single character 'y' -- that is, the 3rd value in the 1-by-5 character array 'Wayne'
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A{2} is the entire 1-by-3 character array containing 'Sam'
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A{2}(1) is the single character 'S' -- that is, the 1st value in the 1-by-3 character array 'Sam'
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A{3} is the entire 2-by-4 double array, [5 8 2 9; 4 3 1 6]
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A{3}(2,3) is the single value 1.0, which is the element on row 2 and column 3 of the array
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A{1}(2,4) would generate an error message because cell 1 does not point to a 2-by-4 array
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Note the use of braces { } to access a cell and the use of parentheses ( ) to access individual elements in the arrays each cell points to.
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Use parentheses ( ) - which is called cell indexing, in the rare case where you need to do something to the cell pointer.
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The only potential usage for cell indexing in CS211 will be to delete cells from a cell array. For example, the statement
A(2) = []; % note the use of parentheses
deletes the second cell in the A cell array. This shifts all cells with a larger index by one position. The diagram below shows what A would look like if this "delete" statement was executed.
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Note the difference between the above statement and the statement
A{2} = []; % note the use of braces
which does not modify the cell array, but it deletes the data that was being pointed to by cell 2. The diagram below shows what A would look like if this "delete" statement was executed.
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D. How to create cell arrays
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Creation method 1: enclose a series of values within braces { }
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For example:
A = { 'Wayne', 'Sam', [5 8 2 9; 4 3 1 6] };
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Creation method 2: use the cell() function to pre-allocate an entire cell array and then assign each cell to its desired value.
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For example:
A = cell(1,3);
A{1} = 'Wayne';
A{2} = 'Sam';
A{3} = [5 8 2 9; 4 3 1 6];
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Cell arrays will grow dynamically if you use an index that is larger than the current size of the cell array.
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It is more efficient to pre-allocate cell array cells with the cell() function than to make them grow dynamically. For example, to create a cell array of 40 strings, pre-allocate the cells before adding the strings:
Num_squads = 40;
Squad_names = cell(Num_squads,1);
for Squad = 1:Num_squads
Squad_names{Squad} = input(['Enter squad ' num2str(Squad) ' name: '], 's');
end
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You can use the cellstr() function to convert a 2-D char array to a cell array of strings. For example:
Cell_array = cellstr(['John'; 'Jim '; 'Jo ']);
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You can use the char() function to convert a cell array of strings to a 2-D char array. For example:
Char_array = char({'John', 'Jim', 'Jo'});
E. How to display/print cell arrays
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The disp() function will display the contents of all cells in a cell array (or just a summary of a cell's data if the amount of data is large).
For example, disp(A) displays
'Wayne' 'Sam' [2x4 double]
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The celldisp() function will display the contents of each cell of a cell array.
For example, celldisp(A) displays
A{1} =
Wayne
A{2} =
Sam
A{3} =
5 8 2 9
4 3 1 6
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Typically when you write a program that requires formatted output, print the contents of cells using fprintf().
For example, the statements:
fprintf('%s had scores of %d %d %d %d\n', A{1}, A{3}(1,:));
fprintf('%s had scores of %d %d %d %d\n', A{2}, A{3}(2,:));
produces the output:
Wayne had scores of 5 8 2 9
Sam had scores of 4 3 1 6
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The cellplot() function is useful for visualizing the structure of a cell array. It creates a visual plot of a cell array, as shown in the example plot of the cell array A below:
