repnan_documentation.m

%% |repnan| documentation
% This function replaces |NaN| values in a 1D array by replacing them with
% interpolated or neighboring values. Interpolation schemes assume data are
% evenly spaced.  This function does not extrapolate. 
% 
%% Syntax
% 
%  x = repnan(x); 
%  x = repnan(x,method); 
% 
%% Description 
% 
% |x = repnan(x)| returns |x| sans NaNs. 
% 
% |x = repnan(x,method)| specifies a method for replacing the original |x|'s
% NaNs. Methods can be 
% 
% * |'linear'| (default) performs linear interpolation over missing values
% in |x|, 
% * |'nearest'| performs nearest-neighbor interpolation,  
% * |'spline'| performs spline interpolation using not-a-knot end conditions,
% * |'pchip'| is shape-preserving piecewise cubic interpolation,
% * |'cubic'| same as |'pchip'|,
% * |'v5cubic'| cubic convolution via Matlab v5 algorithm
% * |'next'| replaces |NaN| values in |x| with the next non-NaN value in
% |x|, 
% * |'previous'| replaces |NaN| values in |x| with the previous non-NaN value in
% |x|, 
% 
%% Examples
% You measured some data in the lab, but the ground connection on your
% sensor's cable was spotty, so you've got some missing data entries. 
% Here's the data you have: 

t = 1:.5:10; 
x = round(10*sin(t));
x([3 7 8 9 16:18]) = NaN

plot(t,x,'o-','linewidth',2,'color',[0.8039 0.6902 0.3294])
box off
axis([0 10 -10 10.5])
xlabel('time (s)') 
ylabel('signal amplitude (V)')
text(0,0,' measured data','color',[0.8039 0.6902 0.3294],'fontweight','bold')
hold on

%% 
% There are a number of ways to take care of those |NaN| values. One of the
% simplest workarounds is to linearly interpolate between valid |x| values.
% If no interpolation method is specified, |repnan| interpolates linearly: 

xlin = repnan(x);
plot(t,xlin,'-','color',[0.6902 0.4039 0.7569])
text(0,-1,' linear interpolation','color',[0.6902 0.4039 0.7569])

%% 
% Cubic interpolation might be a better choice for this application:

xcubic = repnan(x,'pchip'); 
plot(t,xcubic,'-','color',[0.5922 0.7333 0.7490])
text(0,-2,' cubic interpolation','color',[0.5922 0.7333 0.7490]) 

%% 
% Or better yet, spline: 

xspline = repnan(x,'spline'); 
plot(t,xspline,'color',[0.7529 0.3216 0.2863]) 
text(0,-3, ' spline interpolation','color',[0.7529 0.3216 0.2863]) 

%%
% For some applications, you might want the nearest neighbor: 

xnear = repnan(x,'nearest'); 
plot(t,xnear,'color',[0.3765 0.2588 0.3725]) 
text(0,-4, ' nearest neighbor','color',[0.3765 0.2588 0.3725]) 

%% 
% Sometimes you may want to replace missing data with the most recent 
% or previous valid value in |x|:

xprev = repnan(x,'previous'); 
plot(t,xprev,'color',[0.3373 0.3804 0.2353]) 
text(0,-5,' previous neighbor','color',[0.3373 0.3804 0.2353]) 

%% 
% And other times you may want to replace missing data with the next valid
% value in |x|: 

xnext = repnan(x,'next'); 
plot(t,xnext,'color',[0.4941 0.8078 0.3686]) 
text(0,-6,' next neighbor','color',[0.4941 0.8078 0.3686])

%% Author Info
% 
% Written by <http://www.chadagreene.com Chad A. Greene> of the
% University of Texas Institute for Geophysics
% (<http://www.ig.utexas.edu/people/students/cgreene/ UTIG>), October 31, 2014.