How can I get a matrix B of the indexes in a matrix A that satisfy the following —i representing index— A(i-1)>0 && A(i+1)<0?

How can I get a matrix B of the indexes in a matrix A that satisfy the following —i representing index— A(i-1)>0 && A(i+1)<0?

Im ploting an oscillating graph, and would like to get indexes of rate of change closest to 0, in order to get the frequency.

Answer 1

[Star Strider]

‘Im ploting an oscillating graph, and would like to get indexes of rate of change closest to 0, in order to get the frequency.’

You might find Curve fitting to a sinusoidal function (link) helpful.

If you want to find the indices of the zero-crossings of your data, this little utility function could help:

zci = @(v) find(v(:).*circshift(v(:), [-1 0]) <= 0);                    % Returns Approximate Zero-Crossing Indices Of Argument Vector

Answer 2

[Roger Stafford]

B = find(A(1:end-2)>0 & 0>A(3:end)) + 1;

Note that the "+1" is necessary here. For example. if A(1)>0 and A(3)<0, we would want to get an index of 2 included in B, but without the "+1" 'find' would record it as an index of 1.