Target just the last added entries to a matrix

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.

[Rik]

The best strategy is to use B directly, instead of trying to get it back from an aggrageted form.

[Vance Blake]

okay that was my fallback option if there wasnt antoher way. thanks for the help.

[Guillaume]

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]

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 ??

Answer 1

[Guillaume]

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.

[Vance Blake]

I use n as a variable for the number of points. will this indexing work despite that or can i replace n with a different variable ??

[Guillaume]

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]

My bad didnt notice the link but thanks for the help.