Dictionaries¶
Lists and dictionaries¶
A list is a mapping from its indexes to its items: v:1040 59 27 maps
0 -> 1040
1 -> 59
2 -> 27A dictionary is a mapping from a list of keys to a list of values.
q)show d:`tom`dick`harry!1040 59 27
tom | 1040
dick | 59
harry| 27The indexes of v are 0 1 2. The indexes of d are `tom`dick`harry.
The values of v and d are the same.
q)value d
1040 59 27
q)value `v
1040 59 27Construction¶
Use Dict to make a dictionary from a list of keys and a list of values.
q)show d:`a`b`c!1 2 3
a| 1
b| 2
c| 3The lists must be the same length. The keys should be unique (no duplicates) but no error is signalled if duplicates are present.
Avoid duplicating keys in a dictionary or (column names in a) table.
Q does not reject duplicate keys, but operations on dictionaries and tables with duplicate keys are undefined.
If you know the keys are unique you can set the u attribute on them.
(`u#`a`b`c)!100 200 300
The dictionary will then function as a hash table – and indexing will be faster.
Items of the key and value lists can be of any datatype, including dictionaries or tables.
Keys and values¶
q)key d
`a`b`c
q)value d
1 2 3Keywords key and value return the key and value lists respectively.
Indexing¶
A dictionary is a mapping from its key items to its value items.
A list is a mapping from its indexes to its items. If the indexes of a list are its keys, it is unsurprising to find a dictionary is indexed by its keys.
q)k:`a`b`c`d`e
q)v:10 20 30 40 50
q)show dic:k!v
a| 10
b| 20
c| 30
d| 40
e| 50
q)dic[`d`b]
40 20
q)v[3 1]
40 20Nor that we can omit index brackets the same way.
q)dic `d`b
40 20
q)v 3 1
40 20Indexing out of the domain works as for lists, returning a null of the same type as the first value item.
q)v 5
0N
q)dic `x
0NBut unlike a list, indexed assignment to a dictionary has upsert semantics.
q)v[5 1]:42 100
'length
[0] v[5 1]:42 100
^
q)dic[`x`b]:42 100
q)dic
a| 10
b| 100
c| 30
d| 40
e| 50
x| 42Dictionary indexing uses Find to search the keys.
q)d:k!v
q)d[x] ~ v[k?x]
1bwhere and Find¶
Find and where both return indexes from lists. Also from dictionaries.
q)d:`a`b`c`d!10 20 30 10
q)where d=10
`a`d
q)d?30
`cReverse dictionary lookup: use Find for the key of the first matching value, or where for all of them.
q)dns:`netbox`google`apple!`$("104.130.139.23";"216.58.212.206";"17.172.224.47")
q)dns `apple
`17.172.224.47
q)dns?`$"17.172.224.47"
`apple
q)where dns=`$"17.172.224.47"
,`appleOrder¶
Dictionaries are ordered.
q)first dic
10
q)last dic
42
q)k:`a`b`c
q)v:1 2 3
q)(k!v) ~ reverse[k]!reverse v
0bTaking and dropping from a dictionary¶
Dictionaries are ordered, so you can take and drop items from either end of them.
q)d
a| 10
b| 20
c| 30
d| 10
q)-2#d
c| 30
d| 10
q)-1 _ d
a| 10
b| 20
c| 30You can also take and drop selected items.
q)`b`d#d
b| 20
d| 10
q)`b`x _ d
a| 10
c| 30
d| 10Joining dictionaries¶
Join on dictionaries has upsert semantics.
q)(`a`b`c!10 20 30),`c`d!400 500
a| 10
b| 20
c| 400
d| 500Empty and singleton dictionaries¶
Just like a list, a dictionary may be empty or have a single item. But its key and value must still be lists.
q)()!() / general empty dictionary
q)(`symbol$())!`float$() / typed empty dictionary
q)sd:(enlist `a)!enlist 1 / singleton dictionary
a| 1
q)key sd
,`a
q)value sd
,1Column dictionaries¶
When a dictionary’s value items are all same-length lists, it is a column dictionary.
q)show bd:`name`dob`sex!(`jack`jill`john;1982.09.15 1984.07.05 1990.11.16;`m`f`m)
name| jack jill john
dob | 1982.09.15 1984.07.05 1990.11.16
sex | m f mFlip it and we see a table.
q)flip bd
name dob sex
-------------------
jack 1982.09.15 m
jill 1984.07.05 f
john 1990.11.16 m
Step dictionaries
Tables
Q for Mortals
§5. Dictionaries,