changesign.m

% function [I X] = changesign(a)
% X is indexed zerocrossing (may not be an element of a)
% I is the integer index of 'a' greater than X
% CAUTION: a must be monotonic !

function [I, X] = changesign(a)

a = rowvec(a);
[~, sz1] = size(a);

if sz1 > 1
    if min(a) >= 0
        I = 1;
        X = a(1);
    elseif max(a) < 0
        I = sz1;
        X = a(I);
    else
        b(1:(sz1-1)) = a(2:sz1);
        tst = a(1:(sz1-1)).*b;
        [X, Itmp] = min(tst);
        I = Itmp + 1;
    end
else
    X = 0;
    I = 1;
end