How to take the middle of a list?

Question

William 2023-6-15

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评论： Dyuman Joshi 2023-6-17

So I am trying to find an efficient way to take the middle numbers of a list so I can match it to another list.

For example:

a = [1 2 3 4 5 6 7 ];
b = [ 8 9 10 ];

I would want to reduce to a = [ 3 4 5 ] so it has the same ammount of values as b. The ideas I have came up with are really specific and don't work well when numbers are changed. So if anyone has any advice it would be greatly appreciated!

Thanks,

William

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Answer 1

John D'Errico 2023-6-15

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"really specific" issues probably means that it makes no sense to ask for the middle 3 elements of a VECTOR (Python has lists, MATLAB just has vectors) when the vector has an even length.

That is, what are the middle 3 elements of the vector V?

V = 1:6
V = 1×6
     1     2     3     4     5     6

Likewise, what are the middle 4 elements of the vector 1:5?

As long as the parity of vector lengths is the same, then it is pretty easy.

middle = @(V,m) V(((numel(V) - m)/2 + 1):((numel(V) + m)/2));

Now it is easy to do, as long as the length of V and m have the same parity, thus both are either even or odd.

middle(1:6,2)
ans = 1×2
     3     4
middle(1:6,4)
ans = 1×4
     2     3     4     5

Odd parities for both also work.

middle(1:7,1)
ans = 4
middle(1:7,3)
ans = 1×3
     3     4     5
middle(1:7,5)
ans = 1×5
     2     3     4     5     6

And it complains when there are mixed parities.

middle(1:3,2)
Warning: Integer operands are required for colon operator when used as index.
ans = 1×2
     2     3

A well written function would be smarter, and test those parities in advance, and then bounce out with an error message. I'll let you do that part.

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John D'Errico 2023-6-15

Actually, I suppose MATLAB does have the concept of a comma separated list, but it is not really anything you use very often.

William 2023-6-15

That's kinda what I had thought

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Answer 2

Image Analyst 2023-6-15

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Try this:

a = [1 2 3 4 5 6 7 8 9 10 1 2 3 8 9 10 88 83 99];
b = [ 8 9 10 ];
% Find starting indexes where b is in a:
indexes = strfind(a, b)
indexes = 1×2
     8    14

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Image Analyst 2023-6-15

Well we don't really know what you meant since you didn't explicitly describe when you said you wanted to take a few elements (a sub-vector) and "match it to another list". What does that mean exactly? I thought it meant that you wanted to find where the values in one small vector occurred/matched those in another longer vector. I guess not. Sorry I misinterpreted what you want. If you still need help, explain a lot better what you want.