replace missing values with nan

Question

I have the following variables:

x = [0 1 2 2 3 4 5 6 7 8 9];
y = [0 1 2 nan 3 4 5 6 7 8 9];

I would like to pass 'y' through an equation to give 'y2', for example:

y2 = y.*2;

Note this is just an example. The real equation I have is more complicated. The 'real' equation does not allow nans to be within the vector (as one value depends on the last).

If I can't have nans passing through the equation, however, I can type

y2 = y(~isnan(y)).*2;

y2 =

     0     2     4     6     8    10    12    14    16    18

This removes the nan and then performs the calculation.

How can I make 'y2' to be back to the same length as 'x' i.e. with a nan as the fourth value?

Something like:

y2 =

     0     2     4   NaN     6     8    10    12    14    16    18

The reason I'm doing this is that I want to plot 'y2' against 'x' and thus they must be the same size.

I realize that I can do

x2 = x(~isnan(y))

and then just plot 'x2' against 'y2' but I would like to find a way of doing it the way I specify above.

Answer 1

Store the NaN indices somewhere and then use those to input the new values into the right places and then plug back in the NaN values too.

Code

%%// Define function
func1 = @(x)x*2;

%%// Input data - y
y = [0 1 2 nan 3 4 5 6 7 8 9];

%%// Store NaN indices
nan_ind = isnan(y)

%%// Process data on the function
y2(~nan_ind) = func1(y(~nan_ind))
y2(nan_ind) = NaN