Swap k elements in an n x1 vector

Hi I have a vector which is a random permutation of [1:n]. Say

x=randperm(n)

I want to create a vector that is equal to x, but different in at most c positions. For example, if c=2 and

x=[1 2 3 4 5 6]

I want to create a vector that looks like

x=[3 2 1 4 5 6]

Any suggestions?

[Stephen23]

Do you require a strict derangement of the swapped values (i.e. swapped values cannot be randomly returned to the same location) or is it permitted that values might be returned to the same location?

Answer 1

[Rik]

Create a logical vector with at most c true elements. Then permute the values at those positions and use logical indexing to store them back into the original array.