Wolfram: Association. Get Value
Get Key's Value by Key
Lookup[asso, key]
-
return the value of key.
If the key is missing, return
Missing["KeyAbsent", key]
xx = Association[ "a" -> 3, "b" -> 2 ] (* <|a -> 3, b -> 2|> *) Lookup[xx, "b"] (* 2 *) Lookup[xx, "c"] (* Missing["KeyAbsent", "c"] *)
Lookup[asso, key, default]
-
use default if the key missing.
xx = Association[ "a" -> 3, "b" -> 2 ]; Lookup[xx, "c", 99] (* 99 *)
Lookup[asso, listOfKeys]
-
Return a list of the values. From list of keys.
Lookup[listOfAssoc, key]
-
Return a list of values. From the list of associations.
Alternative, Syntactical Form of Get Key's Value by Key
asso[key]
-
return the value of a key.
If the key is missing, return
Missing["KeyAbsent", key]
xx = Association[ "a" -> 3, "b" -> 2 ] xx["b"] (* 2 *) xx["c"] (* Missing["KeyAbsent", "c"] *)
Key Representation
Key[expr]
-
represent a key in a Wolfram: Association (Key Value List)
normally, when you lookup a key, e.g.
Lookup[asso, key]
or
asso[key]
the key is simple, such as a string, number, symbol.
but when asso has complicated expression as key e.g.
Association[ f[x] -> 3, {3,4} -> 2 ]
you need to quote the key by
Key[key]
, e.g.Lookup[asso, Key[{3,4}]]
xx = Association[ 1 -> 11, "b" -> 22, {3,4} -> 33 ]; xx["b"] === 22 (* True *) Lookup[xx, "b"] === 22 (* True *) (* HHHH------------------------------ *) xx[1] === 11 (* True *) Lookup[xx, 1] === 11 (* True *) (* HHHH------------------------------ *) Lookup[xx, {3,4}] (* {Missing[KeyAbsent, 3], Missing[KeyAbsent, 4]} *) Lookup[xx, Key[{3,4}]] (* 33 *)
Key[expr][asso]
-
return the value of a key in a asso.
xx = Association[ "a" -> 3, "b" -> 2 ]; Key["b"][xx] (* 2 *)
Get Key's Value by Index
to get the value by index, just use Part
Part[ Association[ "a" -> 3, "b" -> 2 ], 1 ] (* 3 *) (* same as *) Association[ "a" -> 3, "b" -> 2 ][[1]] (* 3 *)
WolframLang, Association (Key Value List)
- Wolfram: Association (Key Value List)
- Wolfram: Create Association
- Wolfram: Association. Get Value
- Wolfram: Association. Check Key Exist
- Wolfram: Association. Check Value Exist
- Wolfram: Association. Add Item
- Wolfram: Association. Delete Items by Key
- Wolfram: Association. Delete Items by Index
- Wolfram: Association. Delete Items by Filter
- Wolfram: Association. Sort
- Wolfram: Association. Union, Intersection, Complement, Etc
- Wolfram: Map f to Association
- Wolfram: Association to List, Get All Keys or Values