Why is sum(X, 1) the sum of the columns in MATLAB?

Question

>> X = [0 1 2
        3 4 5]

>> sum(X, 1)

ans =

     3     5     7

sum(X, 1) should sum along the 1st dimensio

Answer 1

In my opinion, it is perfectly consistent with everything else.

sum(A,dim) sums along the direction of dimension dim.

Rows are counted "down", so sum(A,1) sums "down". Columns are counted "to the right", so sum(A,2) sums "to the right".

Another way to look at this is that sum(A,dim) collapses dimension dim to 1 by taking the sum. Thus, a 4x3 array summed along dimension 1 collapses the first dimension, leading to a 1x3 array.