In one of my previous posts, I talked about functional forms of select, exec, update and delete. I also briefly touched on the importance of functional queries and why you should get used to them as much as possible if you are serious about q.
In this post, I will talk about functional forms of amend (@ and .) which I think are even more important than select/update. You can use amend to modify lists/dictionaries/tables via a custom or built-in function. A major advantage of amend is its ability to modify both in-memory and on-disk tables.
There are two versions of amend: @ and .@ is for higher-level modification where as . modifies nested elements.
Syntax
Syntax for both takes four arguments where the fourth argument is optional
@[L;I;f;y] and .[L;I;f;y] L - Original list I - indices/sublist f - function y - argument for the function (optional)
Examples
Multiplying some elements of a list by 2
q)L:10?100 q)L 12 10 1 90 73 90 43 90 84 63 q)I:2 5 7 q)@[L;I;*;2] 12 10 2 90 73 180 43 180 84 63
As you can see, our original list L has 10 elements. We want to modify elements with indices 2 5 and 7 (i.e. L[I]) which happen to be:
q)L[I] 1 90 90
The function we want to apply to this sublist is * and its argument is 2. After the operation, we get a list where the indices 2 5 and 7 have been multiplied by 2. Similarly, we can add 2 to those indices by changing our function to +.
q)@[L;I;+;2] 12 10 3 90 73 92 43 92 84 63
Most of the times, you will be using amend for custom functions. Let’s define a function that multiplies our sublist by 4 and then adds 2.
q)f:{2+4*x}
q)@[L;I;f]
12 10 6 90 73 362 43 362 84 63
Here our function is monadic so @ only needs 3 arguments instead of 4. We can redefine f to make it dyadic:
q)@[L;I;f;2]
12 10 4 90 73 182 43 182 84 63
q)f:{y+4*x}
q)@[L;I;f;2]
12 10 6 90 73 362 43 362 84 63
As you can see the result is same as above.
If you want to update the values of L after applying append, change L to `L.
q)L
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q)@[`L;I;f;2]
`L
q)L
12 10 4 90 73 182 43 182 84 63
Dot (.)
@ is used for higher level amend. For in depth amend, you need to use .
Let’s consider this list:
q)L:(1 3 9;2 20; 5 4 18 19) q)L 1 3 9 2 20 5 4 18 19
Let’s trying adding 2 to 0th element of the list using @
q)@[L;0;+;2] 3 5 11 2 20 5 4 18 19
Wait, @ modified the entire first row. I only wanted to add 2 to the very first value (1). We need to use . to do that.
q).[L;0 0;+;2] 3 3 9 2 20 5 4 18 19
Here we told amend to only modify the first element in the first row (0;0). To change the second element in the second row, we will do:
q).[L;1 1;+;2] 1 3 9 2 22 5 4 18 19
In short, the only difference @ and . is that . can append in depth.
Some more examples:
q)@[1 5 9 2 5;0 1;neg]
-1 -5 9 2 5
q)L
1 3 9 2 20 5 4 18 19
q)@[L;where L=20;-;5] / where L=20 returns 4
1 3 9 2 15 5 4 18 19
q)@[L;0 4 6;:;10]
10 3 9 2 10 5 10 18 19
q)@[L;0 4 6;{5*9-5*x}]
20 3 9 2 -455 5 -55 18 19
q)L:(1 2 9;5 7 10 20 2;5 2 8 0 1 7 3) q)L 1 2 9 5 7 10 20 2 5 2 8 0 1 7 3 q).[L;1 4;div;5] 1 2 9 5 7 10 20 0 5 2 8 0 1 7 3 q).[L;(2;2);*;9] 1 2 9 5 7 10 20 2 5 2 72 0 1 7 3
Play around on your own a bit to get a better understanding of how @ and . work. In this post, I have only used lists but you can use dictionaries and tables as well.