In high school and college math courses, we learn about ways to express groups of items as subsets and supersets. This branch of mathematics is called set theory.
X ∈ A
means “Item X exists in set A”, or “X is an element of set A”. The symbol looks like an E, which is a helpful mnemonic, but it doesn’t visually convey its meaning very well. I thought it should be redesigned, so I stepped up to the task in another blog post: “Set Membership” Symbol Redesign.
Set theory is a powerful tool, and it is becoming more important for every-day computer science. It is used to describe things that need to be processed via list-comprehensions in functional languages as well as other languages.