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Dictionaries are an extension of lists which provide the foundation for creating tables. In mathematical terms, dictionary creates the
“domain → Range”
or in general (short) creates
“key → value”
relationship between elements.
A dictionary is an ordered collection of key-value pairs that is roughly equivalent to a hash table. A dictionary is a mapping defined by an explicit I/O association between a domain list and a range list via positional correspondence. The creation of a dictionary uses the “xkey” primitive (!)
ListOfDomain ! ListOfRange
The most basic dictionary maps a simple list to a simple list.
| Input (I) | Output (O) |
|---|---|
| `Name | `John |
| `Age | 36 |
| `Sex | “M” |
| Weight | 60.3 |
q)d:`Name`Age`Sex`Weight!(`John;36;"M";60.3) / Create a dictionary d q)d Name | `John Age | 36 Sex | "M" Weight | 60.3 q)count d / To get the number of rows in a dictionary. 4 q)key d / The function key returns the domain `Name`Age`Sex`Weight q)value d / The function value returns the range. `John 36 "M" 60.3 q)cols d / The function cols also returns the domain. `Name`Age`Sex`Weight
Lookup
Finding the dictionary output value corresponding to an input value is called looking up the input.
q)d[`Name] / Accessing the value of domain `Name `John q)d[`Name`Sex] / extended item-wise to a simple list of keys `John "M"
Lookup with Verb @
q)d1:`one`two`three!9 18 27 q)d1[`two] 18 q)d1@`two 18
Operations on Dictionaries
Amend and Upsert
As with lists, the items of a dictionary can be modified via indexed assignment.
d:`Name`Age`Sex`Weight! (`John;36;"M";60.3)
/ A dictionary d
q)d[`Age]:35 / Assigning new value to key Age
q)d
/ New value assigned to key Age in d
Name | `John
Age | 35
Sex | "M"
Weight | 60.3
Dictionaries can be extended via index assignment.
q)d[`Height]:"182 Ft" q)d Name | `John Age | 35 Sex | "M" Weight | 60.3 Height | "182 Ft"
Reverse Lookup with Find (?)
The find (?) operator is used to perform reverse lookup by mapping a range of elements to its domain element.
q)d2:`x`y`z!99 88 77 q)d2?77 `z
In case the elements of a list is not unique, the find returns the first item mapping to it from the domain list.
Removing Entries
To remove an entry from a dictionary, the delete ( _ ) function is used. The left operand of ( _ ) is the dictionary and the right operand is a key value.
q)d2:`x`y`z!99 88 77 q)d2 _`z x| 99 y| 88
Whitespace is required to the left of _ if the first operand is a variable.
q)`x`y _ d2 / Deleting multiple entries z| 77
Column Dictionaries
Column dictionaries are the basics for creation of tables. Consider the following example −
q)scores: `name`id!(`John`Jenny`Jonathan;9 18 27)
/ Dictionary scores
q)scores[`name] / The values for the name column are
`John`Jenny`Jonathan
q)scores.name / Retrieving the values for a column in a
/ column dictionary using dot notation.
`John`Jenny`Jonathan
q)scores[`name][1] / Values in row 1 of the name column
`Jenny
q)scores[`id][2] / Values in row 2 of the id column is
27
Flipping a Dictionary
The net effect of flipping a column dictionary is simply reversing the order of the indices. This is logically equivalent to transposing the rows and columns.
Flip on a Column Dictionary
The transpose of a dictionary is obtained by applying the unary flip operator. Take a look at the following example −
q)scores name | John Jenny Jonathan id | 9 18 27 q)flip scores name id --------------- John 9 Jenny 18 Jonathan 27
Flip of a Flipped Column Dictionary
If you transpose a dictionary twice, you obtain the original dictionary,
q)scores ~ flip flip scores 1b
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