Select nearest non zero value in column

Hi, I have a matrix of which the last column consists mainly of zeros and an occasionally non zero value. In a For loop I use each loop the next row for calculations, but for the last column I want to select the closest non zero value in that column. How should I do that? Thanks in advance!

[madhan ravi]

example?

[Mabel Gray]

 a = rand(11,4)
 b = [0 0 0 2 0 0 0 0 0 3 0]'
 c = [a b]

For the first six loops, the 2 should be used for the calculation and the last four loops, the 3 should be used for the calculations. It would be optimal if for the seventh loop, the average of 2 and 3 would be selected for the calculation, since it is has a similar distance to the two values.

[Kevin Chng]

Can't get what you are saying.

[the cyclist]

Is it correct that given the input

b = [0 0 0 2 0 0 0 0 0 3 0]'

we could create the output

b2 = [2 2 2 2 2 2 2.5 3 3 3 3]'

then you would be all set?

[Kevin Chng]

My solution is coming from hard code. I would like to see any other shorter solution.

   b = [0 0 0 2 0 0 0 0 0 3 0]';
    ind = find(b>0);
    if numel(ind)>1
    indaverage2(1) = 1;
    for i=1:1:numel(ind)-1;
         indaverage1(i+1) = (ind(i)+ind(i+1))/2;
         indaverage2(i+1) = floor((ind(i)+ind(i+1))/2);
    end
    k=1;
    indaverage2(end+1) = numel(b);
    for i=1:1:numel(ind)
          valueforcalculation(indaverage2(i):indaverage2(i+1)) = b(ind(i)) ;
    end
    for i=1:1:numel(indaverage1)-1
    if floor(indaverage1(i+1))==indaverage1(i+1)
        valueforcalculation(indaverage1(i+1)) = (b(ind(i))+b(ind(i+1)))/2;    
    end  
    end
    elseif numel(ind)==1
         valueforcalculation(1:numel(b)) = b(ind);
    elseif numel(ind)==0
         valueforcalculation(1:numel(b)) = 0;
    end

Answer 1

[Bruno Luong]

b = [0; 0; 0; 2; 0; 0; 0; 0; 0; 3; 0]

nearestfun = @(b) interp1(find(b),b(b~=0),(1:length(b))','nearest','extrap');
closest = 0.5*(nearestfun(b) + flip(nearestfun(flip(b))))

[Bruno Luong]

A variant

b = [0; 0; 0; 2; 0; 0; 0; 0; 0; 3; 0]

nearestfun = @(b) interp1(find(b),b(b~=0),(1:length(b))','nearest','extrap');
closest = median([nearestfun(b),flip(nearestfun(flip(b)))],2)

[michael fechter]

Just had a similar problem of a frequency logger, skipping a few entries when an infinite value is recorded. works fine for what I was about to do hard coding.

Thanks!

Answer 2

[the cyclist]

Assuming my comment above is correct, then here is a somewhat ugly solution:

b = [0 0 0 2 0 0 0 0 0 3 0]';

nonZeroIdx = find(b);

b2 = interp1(nonZeroIdx,b(nonZeroIdx),1:numel(b),'nearest','extrap');

for ni = 1:numel(nonZeroIdx)-1
    isOddIdx = mod(nonZeroIdx([ni ni+1]),2);
    if not(xor(isOddIdx(1),isOddIdx(2)))
        b2(round((nonZeroIdx(ni)+nonZeroIdx(ni+1))/2)) = (b(nonZeroIdx(ni))+b(nonZeroIdx(ni+1)))/2;
    end
end

The first part is easy ... using nearest-neighbor interpolation will get most of the values correct.

But you need the for loop (I think) to get the elements that are halfway between non-zero values, and that is ugly. There might be a better way.

[Guillaume]

Wouldn't it make more sense to perform a linear interpolation and then round to the nearest 0.5? Granted you may get several consecutive values at 0.5 instead of just the middle one but wouldn't that be more appropriate?

[the cyclist]

I had the same thought. Hard to know, without more context from OP.

Answer 3

[the cyclist]

Here is a slightly slicker way:

b = [0 0 0 2 0 0 0 0 0 3 0]';

nonZeroIdx = find(b);

b_extended = [b(nonZeroIdx(1)); b; b(nonZeroIdx(end))];

nonZeroIdx = [1; nonZeroIdx+1; numel(b_extended)];

b_lo = interp1(nonZeroIdx,b_extended(nonZeroIdx),1:numel(b_extended)-1,'nearest','extrap');
b_hi = interp1(nonZeroIdx,b_extended(nonZeroIdx),2:numel(b_extended),  'nearest','extrap');

out = (b_lo+b_hi)/2;
out = out(1:end-1);