Product

prd x    prd[x]

Where x is a numeric list, returns its product.

Nulls are treated as 1s.

q)prd 7                    / product of atom (returned unchanged)
7
q)prd 2 3 5 7              / product of list
210
q)prd 2 3 0N 7             / 0N is treated as 1
42
q)prd (1 2 3 4;2 3 5 7)    / product of list of lists
2 6 15 28
q)prd 101b
0b
q)prd "abc"
'type

prd is an aggregate function, equivalent to */.