MATLAB: Replacing all values except NaN with column number

Question

I'm trying to convert

53  96  87
48  94  NaN
49  95  NaN
51  98  NaN
52  99  NaN
55  100  NaN

to

1  2  3
1  2  NaN
1  2  NaN
1  2  NaN
1  2  NaN
1  2  NaN

The best I can do is this, which of course fills each column (including the NaNs).

for a = 1:size(y,2)
for b = a
  y(:,a) = b;
end
end

If anyone could point me toward a function that could work that would be very helpful. I've tried ~isnan(y) and others but they only allow one input argument.

Answer 1

E.g.,

y = floor(rand(6,3)*100);
y(2:6,3) = nan; % test data
z = repmat(1:size(y,2),size(y,1),1);
z(isnan(y)) = nan;