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Struggling to understand two statements
I am studying the official example code from here: https://github.com/kxcontrib/websocket/blob/master/AppendixB/pubsub.q and after thinking for a while I still don’t understand what exactly a few lines do:
trade:flip `time`sym`price`size!"nsfi"$:(); subs:2!flip `handle`func`params!"is*"$:();
The following is the things I already know:
- Q generally read from right to left;
- We assign the results to trade/subs;
- flip is used to transpose an input
- `time `sym are “symbols”
- “nsfi” is used to specify the type of columns
Then here comes the things I don’t know…
- What does $:() do?
- How about the bang between symbols and “nsfi”?
- What does 2!flip mean?..
Thanks!
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3 Replies
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Yes, it is creating a table schema of specific type for the cols. So when you are manipulating the table, it throws a type error if the type is not of that defined in the schema.
https://code.kx.com/q/ref/insert/#type
More information here: https://code.kx.com/q4m3/8_Tables/#82-empty-tables-and-schema
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1. : represents each left and $ is casting.
https://code.kx.com/q/ref/maps/#each-left-and-each-right
https://code.kx.com/q4m3/7_Transforming_Data/#72-cast
So for the line “nsfi”$:() it is actually casting an empty list with every type on the left (hence each left) so that it creates a list of that specific type. You can use .Q.s1 to view the string representation of the object.
https://code.kx.com/q/ref/dotq/#qs1-string-representation
q)"n"$() `timespan$() q).Q.s1 "nsfi"$:() "(`timespan$();`symbol$();`float$();`int$())"
2. bang (!) is used to create a dictionay with the keys on the left and values on the right with the correct length of list. Then flipping a dict will turn it into a tbl.
https://code.kx.com/q/ref/dict/
https://code.kx.com/q/kb/faq/#flip-a-column-dictionary
3. 2! enkeys the table by the first 2 cols.
https://code.kx.com/q/ref/enkey/
Hope this helps!
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Hi mauricelim, thanks for the quick help! Can I say that the big picture is to define two empty tables called trade and subs here?
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