# extracting values from a matrix or vector

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Sofia Santos on 17 Feb 2021
Commented: Image Analyst on 17 Feb 2021
Hi!
I'm new at this and a need a litle help.
How can extract non-consecutive numbers from a vector?
For example, a = [11 22 33 44 55]
I want to extract only the 22 and the 55, b = a (...?...)
thanks

Fangjun Jiang on 17 Feb 2021
b=a([2,5]), second and fifth element

Image Analyst on 17 Feb 2021
Consecutive in what way? Consecutive in value or index location? None of those numbers are consecutive in their values. Why those two that you chose? What criteria are you using to decide upon the second and fifth indexes?
% To extract odd numbered indexes:
b = a(1 : 2 : end);
% To extract even numbered indexes:
b = a(2 : 2 : end);
In general you can give "a" a vector of actual indexes, like [2, 4, 5] (this is called linear indexing) OR you can give a vector of logicals (or doubles that can be evaluated as logicals) that is the same size as the array, like [false, true, false, true, true] or [0, 1, 0, 1, 1].
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Image Analyst on 17 Feb 2021
This (indexing and extraction) will be covered in the basic course: