Question

I’m looking at some Matlab code (don’t have the software). I’m wondering what the following does:

d=[0 diff(fM)]

where fM is itself an array.

Answer 1

Help documentation for diff in Octave (I suppouse Matlab equivalent works the same as with any other Matlab/octave functions)

help diff
 `diff' is a function from the file /usr/share/octave/3.2.4/m/general/diff.m

 Function File:  diff (X, K, DIM)
 If X is a vector of length N, `diff (X)' is the vector of first
 differences  X(2) - X(1), ..., X(n) - X(n-1).

 If X is a matrix, `diff (X)' is the matrix of column differences
 along the first non-singleton dimension.

 The second argument is optional.  If supplied, `diff (X, K)',
 where K is a non-negative integer, returns the K-th differences.
 It is possible that K is larger than then first non-singleton
 dimension of the matrix.  In this case, `diff' continues to take
 the differences along the next non-singleton dimension.

 The dimension along which to take the difference can be explicitly
 stated with the optional variable DIM.  In this case the K-th
 order differences are calculated along this dimension.  In the
 case where K exceeds `size (X, DIM)' then an empty matrix is
 returned.

So basically, diff returns a vector with n - 1 entries, the code posted is creating a new array adding a zero at the beginning so the new diff vector remain the same size as the input vector (maybe the resulting vector is going to be used in another routine that assumes the input vector length for calculations, it’s difficult without the rest of the code), hope it helps