When I first started learning q, I had a difficult time understanding the differences between tables, keyed tables and dictionaries. The differences seemed very subtle at the time. Just recently, I was explaining a colleague (java developer with some exposure to q/kdb) how you can check meta of a table, look up the keys and types. All this seems trivial to those of us who are full time q developers but someone who only touches the surface of q/kdb in their daily jobs will have no idea about these features.
q is all about tables/dictionaries (and lists) and if you don’t know how to differentiate them properly, then you are going to have a tough time. In this post, I have highlighted some of the key similarities and differences between tables/keyed tables and dictionaries.
Type:
Each datatype in q has an identifier. You can see full list of datatypes here. Notice that tables have type of 98h and dictionaries have a type of 99h.
Types are a good way to identify what kind of datatype you are dealing with.
Dictionaries
A dictionary is simply a hash table which means you have a mapping between a domain list and a range list.
You can create a dictionary using ! operator.
q)d:`sym`price!(`aapl`ibm`msft;100 200 300) q)d sym | aapl ibm msft price| 100 200 300
Tables and keyed tables
Tables are quite self-explanatory. They are a collection of columns.
A sample table will look like:
q)tab:([]sym:`aapl`msft`ibm;price:100 200 300) q)tab sym price ---------- aapl 100 msft 200 ibm 300
We can key the sym column by using xkey.
q)kTab:`sym xkey tab
q)kTab
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----| -----
aapl| 100
msft| 200
ibm | 300
We can also directly create keyed tables by modifying the original syntax for tables:
q)([sym:`aapl`msft`ibm];price:100 200 300) sym | price ----| ----- aapl| 100 msft| 200 ibm | 300
By now, you would have noticed the similarity between a keyed table and a dictionary. They both look the same and indeed, they are the same. Furthermore, if you look deeper, you will notice that a table is a flip of a dictionary! If you were to take a table and switch it’s rows and columns (transpose), you will get a dictionary.
Keyed tables and dictionaries are the same
This is self-explanatory once you see the data itself and understand that both data structures contain a mapping of a domain to its range. Both have the same type as well:
q)type d 99h q)type kTab 99h
Table is a flip of a dictionary
Notice what happens when I flip the table:
q)d sym | aapl ibm msft price| 100 200 300 q)flip tab sym | aapl msft ibm price| 100 200 300
They are the same! If you still don’t believe me, you can do a type on both and you will notice that they are both of type 99h (dictionaries).
q)type(flip tab) 99h q)type d 99h
To summarize, dictionaries are mappings of list of domains to list of ranges. A table is a collection of columns and also, a flip of a dictionary. A keyed table is a dictionary.