When to Use a Hash Table

Jun 24, 2025

When we write code in a language like C#, Java, Python or Rust we often have to choose between arrays or lists and hash tables. An array or list stores elements one after another. A hash or dictionary stores elements in locations that are based on the values of the elements themselves (hash codes).

Often we can realize a performance improvement in programs by using hashed collections correctly. If our program loops over an array to find an element by value, it is likely faster to use a hash table instead.

Here are some signs it is worth trying a dictionary:

We often need to test if an element is contained in a collection of elements—has the element already been added?

We often need to insert or remove elements (not just at the end).

Nested loops can lead to significant slowdowns—could an inner loop be replaced with a hash table lookup?

We want to represent sparse data, such as an array where only a small number of elements have existing values.

If a program needs the packed elements in a linear collection, arrays or lists are a better choice. They are more memory-efficient and faster to loop over. But if the program has excessive searching by value, a hash collection is probably an improvement.

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