KDB select query and vs function

Question

Just curious what does this KX code do:

" " {`$first x vs y}/: column

it is part of the large KX query which is used after in the update statement.

What is x vs y and where they come from and what /: does together with " " on the left side?

the whole query looks like this:

select count i by date, somecolumn from update somecolumn:(" " {`$first x vs y}/: othercolumn) from select from some_table

What is x vs y and where they come from and what /: does together with " " on the left side?

Thomas Smyth · Accepted Answer · 2017-11-29 09:14:57Z

3

/: is an adverb which means 'each right'.

One simple example is using the , with /: to join the left item to each of the right items:

q)1 2 3,/:4 5 6
1 2 3 4    / 1 2 3 join with 4
1 2 3 5    / 1 2 3 join with 5
1 2 3 6    / 1 2 3 join with 6

vs is 'vector scalar' which in your case separate strings from a delimiter " ".

q)" " vs "1 2 3"
,"1"
,"2"
,"3"

{`$first x vs y} is a function that cut y with x, take the first of the result and cast it to a symbol:

q){`$first x vs y}[" ";"1 2 3"]
`1

Combine them all together, " " {`$first x vs y}/: column means cut each of the strings in 'column' with " ", take the first of each results and cast them to symbols:

q)update res:" "{`$first x vs y}/:test from ([]test:("1 2 3";"4 5 6"))
test    res
-----------
"1 2 3" 1
"4 5 6" 4

edited Nov 29, 2017 at 9:14

Thomas Smyth

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answered Aug 12, 2014 at 8:39

WooiKent Lee

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Community · Accepted Answer · 2020-06-20 09:12:55Z

In simple words: It takes first value before space from a string and cast it to symbol.

Ex: Input "Hello World" will give output `Hello

 q> {`$first  x vs y}  [" ";"Hello World"]

Output: `Hello

You can test it with simple list (column in your query will be replaced by actual list from table corresponding to that column )

  q>  " "{`$first  x vs y}/: ("hello world" ; "test program")

output: hellotest

Comparing this to ex1, " " is moved to start which is due to syntax of "each right(/:)"

Alternate simpler version of this is:

   q){`$first   " "  vs x } each ("hello world" ; "test program")

WooiKent has already explained the functions. But here are some references:

Each-Right(/:) : It is a loop which iterates over values of right list

http://code.kx.com/q/ref/adverbs/#each-right

vs(Vector From Scalar): Break a string on some delimiter

http://code.kx.com/q/ref/casting/#vs

In your query, it takes a column whose type is string as input. For each element(row) of that column, takes first value before space. Finally cast that value to symbol.

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