Short answer:
A table is a reference to a (column) dictionary.
The internal representation of a table is nearly identical to that of a dictionary. We can use the flip command to create a table from a dictionary:
q)t: ([] a: 1 2 3; b: 4 5 6; c: 7 8 9) q)d: `a`b`c ! (1 2 3; 4 5 6; 7 8 9) q)t ~ flip d 1b q)
In fact, when a dictionary is flip‘ed, the underlying core data structure remains untouched. The table itself is a simple, small object that refers to the original dictionary. Using .Q.w, we can measure how much more memory a table takes than the corresponding dictionary:
q).Q.w[]`used // memory usage baseline 112992j q)x:`a`b`c!3 3#til 9 // create a small dictionary q).Q.w[]`used 113424j q)x:flip x q).Q.w[]`used // memory usage delta is 113456j // just 32 more bytes q)
No matter how large the underlying dictionary is, creating a table is fast and still takes only 32 bytes:
q)x:`a`b`c!3 100000#til 10 q).Q.w[]`used 1686192j q)x:flip x q).Q.w[]`used 1686224j // 32 = 1686224 - 1686192 q)
Now, Let’s examine how a keyed table is related to a dictionary. We start by creating a simple keyed table:
q)t: ([] a: 1 2 3; b: 4 5 6; c: 7 8 9) q)keyedTable: `a`b xkey t q)keyedTable a b| c ---| - 1 4| 7 2 5| 8 3 6| 9 q)keys keyedTable `a`b q)
Since keyedTable is a table, one might expect it to have the same type as t but instead, q presents the following surprise:
q)type t 98h // type table as expected q)type keyedTable 99h // *NOT* 98h q)
Type 99h is the type number for dictionaries. If keyedTable really is a dictionary, we should be able to extract its key and value:
q)key keyedTable a b --- 1 4 2 5 3 6 q)value keyedTable c - 7 8 9 q)
Indeed, keyedTable is a dictionary – one that holds unkeyed tables for both its key and its value:
q)type key keyedTable 98h q)type value keyedTable 98h q)
This suggests that we can create a keyed table by using the ! (dict) operator with two unkeyed tables:
q)(key keyedTable)!(value keyedTable) a b| c ---| - 1 4| 7 2 5| 8 3 6| 9 q)
Lastly, joining the two flipped tables brings us back to the original dict.
q)(flip key keyedTable),(flip value keyedTable) a| 1 2 3 b| 4 5 6 c| 7 8 9 q)
For more information, see Creating dictionaries and tables from C.
