MATLAB Programming Techniques, Extracting Portions of a table

Hello,

in the course "Programming Techniques" theses lines show how you can extract portions of a table:

https://matlabacademy.mathworks.com/R2024a/portal.html?course=mlpr#chapter=2&lesson=4&section=2

t2 = t1(6:15,[1 5 end-1:end])

or

t2 = t1(6:15,["A" "E" "N" "O"])

Why do I need

end-1:end

in the first version?

Lisa

Answer 1

[Piyush Kumar]

Hi @Elisabeth,

The end keyword in MATLAB is a reference to the last element or position in an array, matrix, or table. You can learn more about the end keyword from this documentation link.

In the second version, "N" and "O" must be the last 2 columns which can be referenced by "end-1" and "end" too. These are just 2 ways to perform same task.

Answer 2

[John D'Errico]

Why do you need it? Well, I don't see the question in the course, and I am not enrolled in the course, so...

But what does that construct do? When you are indexing, the end keyword indicates the last element of a vector or array, in whatever dimension you are looking at. I'll try it out in an example.

V = primes(20)

V = 1x8

2 3 5 7 11 13 17 19

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What does V(end) do?

V(end)

ans = 19

Ah, do you see? It returns the last element. How about this one?

V(end-1:end)

ans = 1x2

17 19

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end-1 is the next to last element, so that returns the final two elements. And this one?

V([1, 3, end-1:end])

ans = 1x4

2 5 17 19

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So the frst element, the third one, and the final two elements.

When it is an array you are working with, TRY IT!

A = magic(7)

A = 7x7

30 39 48 1 10 19 28 38 47 7 9 18 27 29 46 6 8 17 26 35 37 5 14 16 25 34 36 45 13 15 24 33 42 44 4 21 23 32 41 43 3 12 22 31 40 49 2 11 20

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A([2 4],[1 end-1:end])

ans = 2x3

38 27 29 5 36 45

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It extracted from rows 2 and 4. And in terms of the columns, it took columns 1 6 and 7. So the 1st column, and the last two columns, since A was a 7x7 array.