MATLAB diff function with matrices of unequal dimensions

I read MATLAB diff.m readme, still couldn't figure out the following:

 a = [1 2 3]' b = [3 2 1 4; 1 1 1 5; 5 5 5 6] diff([a b]') = 2 -1 2 -1 0 0 -1 0 0 3 4 1 

what rule is MATLAB applying here? does MATLAB apply different rule if one of the matrices (i.e. a or b) is logical matrix? or both a and b are logical matrix?

2 Answers

MATLAB applies the same rule regardless of the input matrices. Run your code line-by-line in the command window and see.

a and b are like this:

>> a = [1 2 3]' a = 1 2 3 >> b = [3 2 1 4; 1 1 1 5; 5 5 5 6] b = 3 2 1 4 1 1 1 5 5 5 5 6 

then [a b]':

>> [a b]' ans = 1 2 3 3 1 5 2 1 5 1 1 5 4 5 6 

Now apply the diff rule on this as follows:

[ row 2 - row 1 ] [ row 3 - row 2 ] [ row 4 - row 3 ] [ row 5 - row 4 ] 

you will get

>> diff([a b]') ans = 2 -1 2 -1 0 0 -1 0 0 3 4 1 

a = [1 2 3] 

So,

[a b] = [ 1 3 2 1 4 2 1 1 1 5 3 5 5 5 6 ] 

and thus

[a b]' = [ 1 2 3 3 1 5 2 1 5 1 1 5 4 5 6 ] 

and then diff takes the differences along the first dimension whose size is not 0 (i.e. down each column). This gives the result

diff([a b]') = [ 2 -1 2 -1 0 0 -1 0 0 3 4 1 ].