What does the q @ symbol mean in the context of a function call?
What's going on in the example here?
Source: http://code.kx.com/q/ref/select/#index-at
@[d;1 1 1;+;3]
((1 2 3;4 5 6 7);(17 18;19;20 21);(13 14;15 16 17 18;19 20))
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http://code.kx.com/wiki/JB:QforMortals2/functions#Functional_Forms_of_Amend
@ [L, I, F, Y]
L = your list
I = index of items to change f = function to apply y = second parameter to f
You can see what it does by posting +
in functions and adding some log output. In this case, it indexes into the second element d@1
, extracts (8 9;10;11 12)
, adds 3, leads to (11 12;13;14 15)
, uses this as next input, adds 3, leads to (14 15;16;17 18)
.
q)@[d;1 1 1;{0N!("x is:",.Q.s1 x;"y is:",.Q.s1 y);x+y};3]
("x is:(8 9;10;11 12)";"y is:3")
("x is:(11 12;13;14 15)";"y is:3")
("x is:(14 15;16;17 18)";"y is:3")
(1 2 3;4 5 6 7)
(17 18;19;20 21)
(13 14;15 16 17 18;19 20)
Or it can be seen using over ( /
):
q)@/[d;;{0N!(x;y);x+y};3]1 1 1 ((8 9;10;11 12);3) ((11 12;13;14 15);3) ((14 15;16;17 18);3) (1 2 3;4 5 6 7) (17 18;19;20 21) (13 14;15 16 17 18;19 20)
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