Modern Map Map

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Map

Holds key and value pairs in a hash table, so you find a value by its key.

Syntax

Map( of TK, TV )

Notes

• READING vs WRITING is the thing to know. m[ k ] inserts a default-constructed value when k is absent; TryGet and Contains do not. m[ k ] += 1 just working is why, and if m[ k ] = 0 silently growing the map is the price.
• The hash contract: K extends containers.Hashable (or is a built-in type), k = k must compare, and equal keys must hash equal.
• Open addressing with linear probing, power-of-two capacity, tombstones on removal, load factor 0.75. Open addressing rather than chaining means one allocation instead of one per entry, which matters more in a language with no garbage collector.
• Iteration order is unspecified and changes across insertions. It is not randomised per process -- stability within one unmodified map is more useful when debugging.

Examples

#import "std/containers"
using containers

dim ages as Map( of ustring, integer )

ages.Put( "ada", 36 )
print ages.Count             '' 1
print ages[ "ada" ]          '' 36

'' READING an absent key INSERTS it -- use Contains or TryGet to ask
print ages.Contains( "alan" )                 '' false
print ages[ "alan" ]         '' 0
print ages.Count             '' 2 -- the read grew the map

'' OrderedMap is the sibling that iterates in key order

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Library: Map
Category: Map
Documented under: Members