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Hash Map (Dictionaries)

Hash maps (associative arrays, dictionaries) store key-value pairs and provide fast lookup by key. They are ideal for mapping names to values, counting, and grouping data.

# Creation
d = {'a': 1, 'b': 2}
empty = {}

# Access and set
d['c'] = 3
v = d.get('x', 0)  # default if missing

# Iteration
for k, val in d.items():
    print(k, val)

# Counting
from collections import Counter
cnt = Counter(['a','b','a'])

% containers.Map as a hash map
keys = {'a','b'};
vals = {1,2};
m = containers.Map(keys, vals);

% Access
m('c') = 3; % add
if isKey(m, 'x')
  v = m('x');
else
  v = 0;
end

Gotchas

  • Keys must be hashable (immutable) in Python; lists cannot be keys but tuples can.
  • Order: Python 3.7+ preserves insertion order for dicts, but don't rely on it for logic unless intended.
  • Matlab containers.Map requires matching key/value cell arrays for construction and may be slower than native arrays for numeric-heavy code.