Indexing with Parentheses
I have talked about indexing a bunch of times in the past. Recently I have visited quite a few customers who still get tripped up a bit sometimes. So I thought I'd try again.
Contents
Arrays
What are arrays? In MATLAB, they are containers that hold information. And they are "regular". By that, I mean that they have a uniform layout, equal numbers of elements in each row, each column, each page, etc. as you march along the dimensions. How do you get information into them, and out from them?
Where People Trip
One of the common places I see people trip is accessing information from cell arrays (and sometimes similarly for structures).
Function Calls
Parentheses mean multiple things in MATLAB. For example, you enclose the input arguments to a function between parentheses, like this:
image(magic(7))
axis square
Numeric Arrays
Another use of parentheses is to create a subset of an array. Let's suppose we have a matrix (2-D) and we want to create a new matrix from some particular rows and columns.
Anumeric = magic(5) Amine = Anumeric([1 4],[2 5])
Anumeric = 17 24 1 8 15 23 5 7 14 16 4 6 13 20 22 10 12 19 21 3 11 18 25 2 9 Amine = 24 15 12 3
We now have the 2x2 matrix defined by rows 1 and 4 and columns 2 and 5 from the original matrix A.
Cell Arrays
Similarly, we create a new cell array from an existing one, but indexing with parentheses for the rows and columns we want.
Cellofstuff = {Amine, 'loren', 'shure'; 17, 51, 153} Cmine = Cellofstuff([1 2],[1 3])
Cellofstuff = [2x2 double] 'loren' 'shure' [ 17] [ 51] [ 153] Cmine = [2x2 double] 'shure' [ 17] [ 153]
Tables
And tables work similarly. First I'm going to load in data from a MAT-file, into a struct rather than into separate variables.
Structpatient = load('patients.mat');
Next convert it to a table.
Tpatient = struct2table(Structpatient);
whos T*
Name Size Bytes Class Attributes Tmine 5x6 4641 table Tpatient 100x10 59738 table
The table Tpatient is longer than I want to display, but I can easily look at a piece of it. How? By indexing with parentheses again to create a smaller table.
Tmine = Tpatient(1:5, [1 2 5 6 8 end])
Tmine = Gender LastName Smoker Systolic Height SelfAssessedHealthStatus ________ __________ ______ ________ ______ ________________________ 'Male' 'Smith' true 124 71 'Excellent' 'Male' 'Johnson' false 109 69 'Fair' 'Female' 'Williams' false 125 64 'Good' 'Female' 'Jones' false 117 67 'Fair' 'Female' 'Brown' false 122 64 'Good'
Structures
I most commonly see user code with scalar structures, by which I mean a structure with fields that contain other MATLAB stuff. But, since this is MATLAB, structs can also be arrays themselves. I think about them as cell arrays with one 'extra' dimension of indices being names, the rest being the usual indexing.
Sdata(1).FirstName = 'Loren'; Sdata(1).LastName = 'Shure'; Sdata(1).Height = 150; Sdata(2).FirstName = 'Miss'; Sdata(2).LastName = 'Piggy'; Sdata(2).Height = 45; Sdata(3).FirstName = 'Big'; Sdata(3).LastName = 'Bird'; Sdata(3).Height = 275;
To create a struct with only Sesame Street characters, I use parentheses again.
Ssesame = Sdata(1,2:3)
Ssesame = 1x2 struct array with fields: FirstName LastName Height
or equivalently
Ssesame = Sdata(2:3)
but NOT
Ssesame = Sdata(2:3,1)
Follow Up
I plan to follow this up with discussions of other sorts of indexing. Hope you find this helpful. You can let me know here.
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- Indexing
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