YOu may have a look on this example code:
% signal
x = linspace(0,2*pi)' ;
y = sin(x) ;
% Get the x index for yi
yi = randsample(y,1) ;
% Do interpolation
xi = interp1(y,x,yi) ;
% GEt the index
idx = knnsearch([x y],[xi yi]) ;
value = [xi yi]
iwant = [x(idx) y(idx)]
