Class pv.Nest
Represents a Nest operator for the specified array. Nesting allows elements in an array to be grouped into a hierarchical tree structure. The levels in the tree are specified by key functions. The leaf nodes of the tree can be sorted by value, while the internal nodes can be sorted by key. Finally, the tree can be returned either has a multidimensional array via #entries, or as a hierarchical map via #map. The #rollup routine similarly returns a map, collapsing the elements in each leaf node using a summary function.
For example, consider the following tabular data structure of Barley yields, from various sites in Minnesota during 1931-2:
{ yield: 27.00, variety: "Manchuria", year: 1931, site: "University Farm" },
{ yield: 48.87, variety: "Manchuria", year: 1931, site: "Waseca" },
{ yield: 27.43, variety: "Manchuria", year: 1931, site: "Morris" }, ...
To facilitate visualization, it may be useful to nest the elements first by
year, and then by variety, as follows:
var nest = pv.nest(yields)
.key(function(d) d.year)
.key(function(d) d.variety)
.entries();
This returns a nested array. Each element of the outer array is a key-values
pair, listing the values for each distinct key:
{ key: 1931, values: [
{ key: "Manchuria", values: [
{ yield: 27.00, variety: "Manchuria", year: 1931, site: "University Farm" },
{ yield: 48.87, variety: "Manchuria", year: 1931, site: "Waseca" },
{ yield: 27.43, variety: "Manchuria", year: 1931, site: "Morris" },
...
] },
{ key: "Glabron", values: [
{ yield: 43.07, variety: "Glabron", year: 1931, site: "University Farm" },
{ yield: 55.20, variety: "Glabron", year: 1931, site: "Waseca" },
...
] },
] },
{ key: 1932, values: ... }
Further details, including sorting and rollup, is provided below on the
corresponding methods.
Defined in: Nest.js.
| Constructor Attributes | Constructor Name and Description |
|---|---|
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pv.Nest(array)
Constructs a nest operator for the specified array.
|
| Method Attributes | Method Name and Description |
|---|---|
|
entries()
Returns a hierarchical nested array.
|
|
|
key(key)
Nests using the specified key function.
|
|
|
map()
Returns a hierarchical map of values.
|
|
|
rollup(f)
Returns a rollup map.
|
|
|
sortKeys(order)
Sorts the previously-added keys.
|
|
|
sortValues(order)
Sorts the leaf values.
|
Class Detail
pv.Nest(array)
Constructs a nest operator for the specified array. This constructor should
not be invoked directly; use pv.nest instead.
- Parameters:
- {array} array
- an array of elements to nest.
Method Detail
entries()
Returns a hierarchical nested array. This method is similar to
pv.entries, but works recursively on the entire hierarchy. Rather
than returning a map like #map, this method returns a nested
array. Each element of the array has a key and values
field. For leaf nodes, the values array will be a subset of the
underlying elements array; for non-leaf nodes, the values array will
contain more key-values pairs.
For an example usage, see the Nest constructor.
- Returns:
- a hierarchical nested array.
{pv.Nest}
key(key)
Nests using the specified key function. Multiple keys may be added to the
nest; the array elements will be nested in the order keys are specified.
- Parameters:
- {function} key
- a key function; must return a string or suitable map key.
- Returns:
- {pv.Nest} this.
map()
Returns a hierarchical map of values. Each key adds one level to the
hierarchy. With only a single key, the returned map will have a key for each
distinct value of the key function; the correspond value with be an array of
elements with that key value. If a second key is added, this will be a nested
map. For example:
pv.nest(yields)
.key(function(d) d.variety)
.key(function(d) d.site)
.map()
returns a map m such that m[variety][site] is an array, a subset of
yields, with each element having the given variety and site.
- Returns:
- a hierarchical map of values.
rollup(f)
Returns a rollup map. The behavior of this method is the same as
#map, except that the leaf values are replaced with the return value
of the specified rollup function f. For example,
pv.nest(yields)
.key(function(d) d.site)
.rollup(function(v) pv.median(v, function(d) d.yield))
first groups yield data by site, and then returns a map from site to median
yield for the given site.
- Parameters:
- {function} f
- a rollup function.
- Returns:
- a hierarchical map, with the leaf values computed by f.
- See:
- #map
{pv.Nest}
sortKeys(order)
Sorts the previously-added keys. The natural sort order is used by default
(see pv.naturalOrder); if an alternative order is desired,
order should be a comparator function. If this method is not called
(i.e., keys are unsorted), keys will appear in the order they appear
in the underlying elements array. For example,
pv.nest(yields)
.key(function(d) d.year)
.key(function(d) d.variety)
.sortKeys()
.entries()
groups yield data by year, then variety, and sorts the variety groups
lexicographically (since the variety attribute is a string).
Key sort order is only used in conjunction with #entries, which returns an array of key-values pairs. If the nest is used to construct a #map instead, keys are unsorted.
- Parameters:
- {function} order Optional
- an optional comparator function.
- Returns:
- {pv.Nest} this.
{pv.Nest}
sortValues(order)
Sorts the leaf values. The natural sort order is used by default (see
pv.naturalOrder); if an alternative order is desired, order
should be a comparator function. If this method is not called (i.e., values
are unsorted), values will appear in the order they appear in the
underlying elements array. For example,
pv.nest(yields)
.key(function(d) d.year)
.key(function(d) d.variety)
.sortValues(function(a, b) a.yield - b.yield)
.entries()
groups yield data by year, then variety, and sorts the values for each
variety group by yield.
Value sort order, unlike keys, applies to both #entries and #map. It has no effect on #rollup.
- Parameters:
- {function} order Optional
- an optional comparator function.
- Returns:
- {pv.Nest} this.