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I am trying to implement a LHS pattern match with a RHS action code in Python How can I get a fast hashtable match. Is this possible in Python? I need to fast match features in terms of x,y,c where x and y are coordinates and c is the color at index(x,y) of a 2d array.

George Chrtistoforou
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    Hashtable is called [`dict`](https://docs.python.org/3/tutorial/datastructures.html#dictionaries) in Python. – Amadan Apr 07 '19 at 23:22
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    And check this answer to see which types can be hashed: https://stackoverflow.com/a/44880799/937153 – Unni Apr 07 '19 at 23:23

2 Answers2

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An hashmap is a dictionary in python.

There are several ways to create dictionaries, here's 2:

d = dict(k=v)

or 

d = {k:v}

To get the value of a key:

k = d.get("k")

or 

k = d[k]

To set the value of a key:

d[k] = "ok"

Notes:

In Python, dictionaries (or “dicts”, for short) are a central data structure:

Dicts store an arbitrary number of objects, each identified by a unique dictionary key. Dictionaries are often also called maps, hashmaps, lookup tables, or associative arrays. They allow the efficient lookup, insertion, and deletion of any object associated with a given key.

Resources:

  1. Dictionaries in Python

  2. Dictionaries, Maps, and Hash Tables in Python

Pedro Lobito
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  • A dictionary is a hashmap, not a hashtable. You can map values to hashable keys, but the keys are still just a set, not a table. – sleblanc Apr 07 '19 at 23:55
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The native type dict is a hashmap, not a hashtable, therefore you can only values to a key.

You can however simulate a hashtable by using (x, y) tuples as keys:

d = {}
d[(1,0)] = True
d[(1,1)] = False

This works because the tuple type in Python is hashable, meaning that as long as the values that it wraps are hashable, it can convert the value to a key.

Otherwise, you could extend the dict type to provide additional methods, letting you access values in a Java- or C-style 2D array:

d[1][0] = True
d[1][1] = False
sleblanc
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