Cell Arrays

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Misc

Final Exam

Monday, May 8, 8:00 AM.

Comprehensive, but emphasizing material since second hour exam (e.g., vectorization, strings, cell arrays, etc.).

Designed for around 1 1/2 hours, you’ll have 2 1/2.

Rules and format otherwise similar to hour exams, especially open-references rule.

I’ll bring donuts and cider.

SOFIs

2 completed so far (thank you to the two).

Questions?

Cell Arrays

Sections 14.1 through 14.3

Suppose you wanted to make a list of pets, with each pet described by its name (a string), species (another string), and age (a number).

Relevant ideas or questions from reading:

• Every array is at least 0-by-0.
• Arrays are rectangular, i.e., they have the same number of columns in all rows.
• Strings aren’t a separate class.
• Databases can be made from records of fields.
• You generally can’t mix different data types in an array.
• Cell arrays are unique to Matlab, and different cells in the same array can hold different kinds of data, e.g., some cells can hold strings, others numbers, etc.
• Cell arrays are accessed like other arrays.
• Cell arrays must still be rectangular.
• Use braces to create cell arrays, as in

    variable = { data1_1, data1_2, ..., data1_n; data2_1, data2_2, ... data2_n; ... }

• You can index cell arrays, but don’t misuse braces or confuse them with parentheses.

Solution:

>> pets = { 'Pet1', 'Dog', 2; 'Fido', 'Dog', 10 }
pets = 
    'Pet1'    'Dog'    [ 2]
    'Fido'    'Dog'    [10]

Comments:

• The essential thing about cell arrays that this demonstrated was that you create them much like you create regular arrays from lists of their elements, except that you enclose the elements of a cell array in curly braces.

Now how would you pick information out of the pets cell array, e.g., the name of the second pet?

>> pets(2,1)
ans = 
    'Fido'
>> pets{2,1}
ans =
Fido

Notice the different ways Matlab displays the two results. The first with the indentation and quotation marks around strings indicates that what you’re seeing is a cell from a cell array. The second, unindented and without quotation marks, like strings normally display, indicates that what you’re seeing is the value in the cell.

To understand the difference between cells and values, think of cells as memory locations that tell the computer where to find values, possibly of different sizes than the values referenced by other cells. Normally, all values in an array must be the same size to allow indexing, but this idea of cells as fixed-size references to variable-size values allows Matlab to get around that restriction.



Last Exercise

See handout for details.

Next

Reading Matlab data from and writing it to files.

Read textbook section 4.12.

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