Matlab Trimming at Bounds

Question

I have a matrix A:

NaN NaN NaN NaN NaN NaN NaN 10 1 8 7 2 5 6 2 3 49 NaN NaN NaN NaN NaN NaN

I was wondering if there was a way to detect when the NaNs first turn to numbers and turn the 1st 2 points to NaNs, such as the 10 & 1 both to NaN.

Then find when the numbers turn to NaNs and turn the last two number points, 3 and 49 to NaNs.

Originally I was thinking of using the following, but I was wondering if this was the best way:

i= 2;
while i < 1440
    if isnan(A(i)) < isnan(A(i-1))     //Transitioning from NaN to numbers
        A(i:i+2) = NaN;
        i = i+ 4;
    elseif isnan(A(i)) > isnan(A(i-1))   //Transitioning from numbers to NaNs 
        A(i-2:i) = NaN;
        i = i + 1;
    else
        i = i + 1;
    end
end

but was wondering if there was any other way I could optimize it?

Answer 1

First I assume that your vector A is organized with NaN's at the start and end and a continous set of numerics in the middle, as in

A = [NaN ... NaN, contiguous numeric data, NaN ... NaN]

First, I suggest locating the numeric data and working from there, as in,

flagNumeric = ~isnan(A);

Now flagNumeric will be a true for the entries that are numeric and false for NaN's.

So the first numeric will be at

firstIndex = find(flagNumeric,1,'first');

and the last numeric at

 lastIndex = find(flagNumeric,1,'last');

You can then use firstIndex and lastIndex to change the first and last numeric data to NaN's

 A(firstIndex:firstIndex+1) = NaN;
 A(lastIndex-1:lastIndex) = NaN;

Answer 2

% Set the first two non-NaN numbers to NaN
first = find(isfinite(A), 1, 'first');
A(first:first+1) = NaN;

% Set the last two non-NaN numbers to NaN
last = find(isfinite(A), 1, 'last');
A(last-1:last) = NaN;

Of course, the above code will break in special cases (e.g. when last == 1), but these should be straightforward to filter out.

Answer 3

Here's a slightly simpler version based on the same assumption as Azim's answer:

nums = find(~isnan(A));
A( nums([1 2 end-1 end]) ) = NaN;