Extract every element except every n-th element of vector

Question

Given a vector

A = [1,2,3,...,100]

I want to extract all elements, except every n-th. So, for n=5, my output should be

B = [1,2,3,4,6,7,8,9,11,...]

I know that you can access every n-th element by

A(5:5:end)

but I need something like the inverse command. If this doesn't exist I would iterate over the elements and skip every n-th entry, but that would be the dirty way.

Answer 1

You can eliminate elements like this:

A = 1:100;
removalList = 1:5:100;
A(removalList) = [];

Answer 2

Use a mask. Let's say you have

A = 1 : 100;

Then

m = mod(0 : length(A) - 1, 5);

will be a vector of the same length as A containing the repeated sequence 0 1 2 3 4. You want everything from A except the elements where m == 4, i.e.

B = A(m ~= 4);

will result in

B == [1 2 3 4 6 7 8 9 11 12 13 14 16 ...]

Answer 3

Or you can use logical indexing:

n = 5; % remove the fifth
idx = logical(zeroes(size(A))); % creates a blank mask
idx(n) = 1; % makes the nth element 1
A(idx) = []; % ta-da!

About the "inversion" command you cited, it is possible to achieve that behavior using logical indexing. You can negate the vector to transform every 1 in 0, and vice-versa.

So, this code will remove any BUT the fifth element:

negatedIdx = ~idx;
A(negatedIdx) = [];

Answer 4

why not use it like this?

say A is your vector

A = 1:100
n = 5
B = A([1:n-1,n+1:end])

then

B=[1 2 3 4 6 7 8 9 10 ...]