How to get sum of eath two elements with the vector functions? I want the same result as:
{x+y}':[1 2 3 2 1]
Why this approach gives something different than first one?
sum':[1 2 3 2 1]
sum is not the same as {x+y}.
sum has rank 1 meaning it takes one input and sums the elements of that input.
It can sum an atom:
q)sum 1
1
a uniform list
q)sum 1 2
3
or a list of lists
q)sum(1 2;3 4)
4 6
{x+y} is rank 2 meaning it requires two inputs.
q){x+y}[1;2]
3
q){x+y}[1 2;3 4]
4 6
Giving it an atom, a single list, or a list of lists leads to projections
q){x+y}1
{x+y}[1]
q){x+y}1 2
{x+y}[1 2]
q){x+y}(1 2;3 4)
{x+y}[(1 2;3 4)]
Since each-prior (':) creates binary pairs from the input and attempts to apply a rank 2 function, it works as intended on your rank 2 function {x+y}.
But since sum is not rank 2 the each-prior doesn't generate pairs in the same way, it's equivalent to doing
q){x}':[1 2 3 2 1]
1 2 3 2 1
q){sum x}':[1 2 3 2 1]
1 2 3 2 1
You could force it to be rank 2:
q){sum(x;y)}':[1 2 3 2 1]
1 3 5 5 3
but this gives a different result since sum ignores nulls while + doesn't.
q)sum(0N;1)
1
q)0N+1
0N
Finally, an alternative way to achieve this using sum (and without using each-prior) is to shift the vector using prev and then sum
q){sum(prev x;x)}[1 2 3 2 1]
0N 3 5 5 3