The question:
"Some people are standing in a row in a park. There are trees between them which cannot be moved. Your task is to rearrange the people by their heights in a non-descending order without moving the trees. People can be very tall!"
Example
For
a = [-1, 150, 190, 170, -1, -1, 160, 180],
the output should be
solution(a) = [-1, 150, 160, 170, -1, -1, 180, 190].
We basically need to scan through this list and see where there a downstep (unless it is to a -1) and then find a way to re-order non -1 elements so that the list is non-ascending. I basically don't know where to even begin on this question. Would anyone give this a go in base R and talk me through your thinking?
a = c(-1, 150, 190, 170, -1, -1, 160, 180)
a[a>0] = sort(a[a>0])
a
[1] -1 150 160 170 -1 -1 180 190
Explanation:
a>0 is boolean mask to get the non-trees values:FALSE TRUE TRUE TRUE FALSE FALSE TRUE TRUE
If you apply it to a you get :
#a[a>0]
150 160 170 180 190
An thus you can order the values with sort(a[a>0]) and set back to the orignal vector using the same aproach with the boolean mask.
a[a>0] = sort(a[a>0])