A question regarding one of my previous questions

Dear all,

In my previous question

http://www.mathworks.de/matlabcentral/answers/44968-data-frequency-conversion-problem

I had

A = {

    1    '1-2 2004'      0.256       0.385
    1    '3-4 2004'      0.268      3.0394
    1    '5-6 2004'     0.0504      0.6475
    1    '7-8 2004'    14.0985    148.2583
    1    '9-10 2004'     0.1128      1.1506
    1    '11-12 2004'        NaN    148.2583
    1    '1-2 2005'        NaN    148.2583
    1    '3-4 2005'     2.5852     34.0146
    1    '5-6 2005'      0.322      3.2846
    1    '7-8 2005'    14.0985    148.2583
    1    '9-10 2005'     2.5852         NaN
    1    '11-12 2005'     0.2938       2.854
    2    '1-2 2004'      0.256       0.385
    2    '3-4 2004'      0.268      3.0394
    2    '5-6 2004'     0.0504      0.6475
    2    '7-8 2004'    14.0985    148.2583
    2    '9-10 2004'     0.1128      1.1506
    2    '11-12 2004'        NaN    148.2583
    2    '1-2 2005'        NaN    148.2583
    2    '3-4 2005'     2.5852     34.0146
    2    '5-6 2005'      0.322      3.2846
    2    '7-8 2005'    14.0985    148.2583
    2    '9-10 2005'     2.5852         NaN
    2    '11-12 2005'     0.2938       2.854
    3    '1-2 2004'      0.256       0.385
    3   '3-4 2004'      0.268      3.0394
    3    '5-6 2004'     0.0504      0.6475
    3    '7-8 2004'    14.0985    148.2583
    3    '9-10 2004'     0.1128      1.1506
    3    '11-12 2004'        NaN    148.2583
    3    '1-2 2005'        NaN    148.2583
    3    '3-4 2005'     2.5852     34.0146
    3    '5-6 2005'      0.322      3.2846
    3    '7-8 2005'    14.0985    148.2583
    3    '9-10 2005'     2.5852         NaN
    3    '11-12 2005'     0.2938       2.854}

These are bimonthly data and I wanted to obtain estimated monthly averages The solution that was given was

 data       = inpaint_nans(cell2mat(A(:,3:4)),2);
% Partition interpolation in blocks (first column)
blocks     = [A{:,1}];
unBlocks   = unique(blocks);
% Preallocate
interpData = cell(numel(unBlocks),1);
% Interpolate each block
for b = unBlocks
    idxBlock      = b == blocks;      % index the block
    n             = nnz(idxBlock)*2;  % counts its length
    interpData{b} = interp1((1:2:n)', data(idxBlock,:),(1:n-1)');
end

The only thing that I do not understand is why (1:n-1)' and not (1:n)'

thanks

[salva]

Will I be correct if i use (1:n)' instead of (1:n-1)'

thanks again

Answer 1

[Walter Roberson]

The code is wrong, for the same reason that I pointed out in one of your previous Questions.

http://www.mathworks.com/help/techdoc/ref/interp1.html

If Y is a vector, it must have the same length as x

Your Y is not the same length as your x.

Whether you use (1:n-1) or (1:n) is not going to matter as the call is going to error out anyhow.

[salva]

I think that Y has the same length as x. In bith cases the size is 12 by 1

[Walter Roberson]

In that csse, n-1 is correct. 1:2:n with n being even, ends at n-1 not at n. 1, 3, 5, 7... never even, so there is no input past 23 that would allow you to interpolate point 24 without using extrapolation.