array vs hash key search

Question

So I'm a longtime perl scripter who's been getting used to python since I changed jobs a few months back. Often in perl, if I had a list of values that I needed to check a variable against (simply to see if there is a match in the list), I found it easier to generate hashes to check against, instead of putting the values into an array, like so:

$checklist{'val1'} = undef;
$checklist{'val2'} = undef;
...
if (exists $checklist{$value_to_check}) { ... }

Obviously this wastes some memory because of the need for a useless right-hand value, but IMO is more efficients and easier to code than to loop through an array.

Now in python, the code for this is exactly the same no matter if you're searching an list or a dictionary:

if value_to_check in checklist_which_can_be_list_or_dict:
   <code>

So my real question here is: in perl, the hash method was preferred for speed of processing vs. iterating through an array, but is this true in python? Given the code is the same, I'm wondering if python does list iteration better? Should I still use the dictionary method for larger lists?

If speed of processing is primary concern then also consider NumPy. NumPy's arrays are more compact than Python lists. Also, Access in reading and writing items is also faster with NumPy. — Gagandeep Singh, Nov 16 '11 at 05:42

score 6 · Answer 1 · answered Nov 16 '11 at 06:02

Dictionaries are hashes. An in test on a list has to walk through every element to check it against, while an in test on a dictionary uses hashing to see if the key exists. Python just doesn't make you explicitly loop through the list.

Python also has a set datatype. It's basically a hash/dictionary without the right-hand values. If what you want is to be able to build up a collection of things, then test whether something is already in that collection, and you don't care about the order of the things or whether a thing is in the collection multiple times, then a set is exactly what you want!

array vs hash key search

1 Answers1