change equal values in a column to nan

I have matrix with several columns and rows. Most of the columns are like (1.02 1.05 1.03 1.04 1.04 1.04 1.04 .. 1.04). No I would like to search in my matrix in each column the point from where the following numbers in the column are equal change the second until the last value to nan.

As an example I would like to change (1.02 1.05 1.03 1.04 1.04 1.04 1.04 .. 1.04) to (1.02 1.05 1.03 1.04 nan nan nan ... nan).

But there are some columns that don't have a point from where the column has the same value until the end.

Thank you for your help.

Answer 1

[John Chilleri]

Hello,

Try this:

% Given matrix A
for i = 1:size(A,2) % go through each column
   for j = 2:size(A,1) % through elements in column
      if (sum(A(j-1,i) == A(j:end,i)) == size(A,1)-j+1)
         A(j:end,i) = NaN;
         break;
      end
   end
end

If you have any questions please ask! This will only replace the values if they're forever equivalent (if it repeats 2,2,2,3, it wont replace the 2,2,2).

Hope this helps!

[FC93]

Thank you for your help. This was exactly what I was looking for.

[Jan]

The comparison

if (sum(A(j-1,i) == A(j:end,i)) == length(A(j:end,i)))

is more expensive than required. The length of A(j:end,i) is size(A,1)-j+1. The explicit creation of A(j:end,i) only to determine its length, consumes time.

A simplification:

   for j = size(A,1)-1:1 % through elements in column
      if A(j,i) == A(j+1,i)
         A(j+1,i) = NaN;
      else
         break;
      end
   end

[John Chilleri]

Edited code to include Jan Simon's size(A,1)-j+1, thanks!