{"id":6120,"date":"2025-03-04T17:01:35","date_gmt":"2025-03-05T01:01:35","guid":{"rendered":"https:\/\/wonghoi.humgar.com\/blog\/?p=6120"},"modified":"2025-12-01T02:10:45","modified_gmt":"2025-12-01T10:10:45","slug":"how-missing-keys-are-handled-in-dictionary-hashtables-in-c-python-and-matlab","status":"publish","type":"post","link":"https:\/\/wonghoi.humgar.com\/blog\/2025\/03\/04\/how-missing-keys-are-handled-in-dictionary-hashtables-in-c-python-and-matlab\/","title":{"rendered":"How missing keys are handled in Dictionary (Hashtables) in C++, Python and MATLAB"},"content":{"rendered":"\n

C++<\/h2>\n\n\n\n

In C++ (STL), the default behavior to touching (especially reading) a missing key (whether it’s map or unordered_map, aka hashtable) is that the new key will be automatically inserted with the default value<\/a> for the type (say 0 for integers). <\/p>\n\n\n\n

I have an example MapWithDefault<\/a> template wrapper implementation that allows you to endow the map (during construction) with a default value to be returned when the key you’re trying to read does not exist<\/p>\n\n\n\n

C++ has the at()<\/code> method but it involves throwing an exception when the key is not found. However enabling exceptions is a material performance overhead burden in C++.<\/p>\n\n\n\n

MATLAB<\/h2>\n\n\n\n

MATLAB’s hashtables are done with containers.Map()<\/code> and with more modern MATLAB, dictionary<\/code> objects (R2020b and on), unless you want to stick to valid MATLAB variable names as keys and exploit dynamic fieldnames.<\/p>\n\n\n\n

Neither containers.Map()<\/code> or dictionary<\/code> have a default value mechanism when a key is not found. It will just throw an error if the key you are trying to read does not exist. Use iskey()<\/code>\/isKey()<\/code> method to check if the key exist first and decide to read it or spit out a default value.<\/p>\n\n\n\n

Python<\/h2>\n\n\n\n

Natively Python dictionaries will throw a Key error exception if the requested key in []<\/code> operator (aka __getitem__()<\/code>) does not already exist.<\/p>\n\n\n\n

Use .get(key, default)<\/code> method if you want to assume a default value if the key is not found. The .get()<\/code> method does not throw an exception: the default<\/code> is None<\/code> if not specified.<\/p>\n\n\n\n

If you want C++’s default behavior of reading a new key means inserting the said key with a default, you have to explicitly import collections<\/code> package and use defaultdict<\/code>. I wouldn’t recommend this as the behavior is not intuitive and likely confusing in insidious ways.<\/p>\n\n\n\n

There’s a simiar approach to my MapWithDefault in Python dictionaries: subclass from dict<\/code> and define your own __missing__<\/code> dunder\/magic method<\/a> that returns a default when a key is missing, then use the parent (aka dict<\/code>)’s constructor to do an explicit (type\/class) conversion for existing dict<\/code> object into your child class object that has __missing__<\/code> implemented.<\/p>\n\n\n\n

Despite this approach is a little like my MapWithDefault, the __missing__<\/code> approach has more flexibility like allowing the default value to be dependent on the query key string, but it comes at the expense of making up a different class<\/strong> (namely a subclass of dict), not making a different instance<\/strong> per different default values.<\/p>\n\n\n\n

Monkey-patching instance methods is frowned upon in Python. So if you want the default value to tie to instances<\/strong>, the mechanism needs to be redesigned.<\/p>\n

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C++ In C++ (STL), the default behavior to touching (especially reading) a missing key (whether it’s map or unordered_map, aka hashtable) is that the new key will be automatically inserted with the default value for the type (say 0 for … Continue reading →<\/span><\/a><\/p>\n

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