Tetradic @ (amend)

Applies a dyadic verb to a certain value.

Syntax: .[x;y;z;w]

where z is a dyadic verb to be applied, x is a structure to be indexed with y and w is the second argument of the verb.

o)a:1 2 3; @[a;2;+;1]
1 2 4
o)d:`a`b`c!(1 2;3 4;5 6);
o)@[d;`b;*;2]
a| 1 2
b| 6 8
c| 5 6
o)t:flip `a`b!(!3;3+!3)
a b
---
0 3
1 4
2 5
o)@[t;`a;:;0]
a b
---
0 3
0 4
0 5
o)

For destructive updates use variable symbol:

o)a:1 2 3; @[`a;2;+;1]
`a
o)a
1 2 4
o)