Target just the last added entries to a matrix

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Vance Blake
Vance Blake 2019-8-30
评论： Vance Blake 2019-9-1
Hey, I am working with matrices and I wanted to know if there is a standard way/code to target just the last added entries to a matrix. I used the general syntax for appending data points to a matrix (A= [A;B]) and I wanted to know if there was an analogous general method for targetting the last added entries into a matrix. For example, I append 4 sets of coordinates to a matrix and now I want to call just those 4 sets from the matrix and use them for next step in my code. Any help is greatly appreciated.
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Guillaume
Guillaume 2019-8-30
There is always another way, but your question is not particularly clear.
A(end-n:end, :)
will give you the last n+1 rows of the matrix.
Note that growing a matrix by appending rows in a loop is slow. If you know beforehand how many rows the final matrix is going to be, then preallocating the matrix is a lot more efficient.
Vance Blake
Vance Blake 2019-8-30
Yeah I know that growing dynamically is slow (matlab kindly reminds me every time i run my code) but unfortunately I don't know how big the matrix will be beforehand. But Im not appending the sets of coordinates to the matrix during the loop but after it has been completed.
@Guillaime does your solution only work with a loop ??

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Guillaume
Guillaume 2019-8-30
As commented:
A(end-n:end, :)
will give you the last n+1 rows of the (2D) matrix.
does your solution only work with a loop
Why do you think that? There's no loop involved in that line. It's basic matrix indexing using the end keyword.
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Guillaume
Guillaume 2019-9-1
Have you read the documentation page I linked?
This is really basic indexing. As everything in matlab, the name of the variable is irrelevant. You can call them whatever you want or replace the variable by a hardcoded constant
A(end-n:end, 1) %returns the last n+1 rows of A
A(end-10:end, 1) %returns the last 11 rows of A
somename(end-someothername:end, 1) %returns the last someothername+1 rows of somename
Vance Blake
Vance Blake 2019-9-1
My bad didnt notice the link but thanks for the help.

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