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Comment by chadash | Hacker News Reader

> why do you care if an applicant understand how a hash map works; and what's particular about a hash map?

Because a hash map is:

1) a pretty basic concept in data structures

2) Variations on hash maps are used all the time in the real world. If you use objects in javascript, dictionaries in python, or maps in C++, then you are using things that essentially implement hash maps.

Point number 1 is like if I went to an orthopedic surgeon and they couldn't tell me what the liver does. You can say "well the liver has nothing to do with my finger that got smashed in a car crash, so what do I care." Or you can say, "that seems like a red flag. Maybe I'd be safer choosing a different doctor."

* Note: I have no idea how often the liver comes up in orthopedic finger surgery and for all I know it's a lot. But I think you get the point.

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>Variations on hash maps are used all the time

Yes, you use them. You don't build them. To torture your analogy it's like asking the surgeon to explain how their bone saw works. Why should they know? All they care is that it cuts bone.

Knowing how to build a bone saw is a completely different skill set than surgery. Building a hash table is a job for a software engineer even if not every engineer does it frequently. Still my analogy wasn’t great.

Analogies aside, hashmaps (in one implementation or another), arrays, vectors/lists, strings and arguably sets are very very common data structures in most modern languages. I don’t expect someone to be able to build a hashmap from scratch but I do expect an experienced engineer to have some basic idea of the pieces that go into building it as well as it’s properties (not necessarily ordered, O(1)ish sets/gets, collisions, etc). Knowing this kind of thing helps you understand when to use an object in JavaScript vs a map. Or how dictionaries differ between python 2 and 3. If you understand the underlying data structure, then you know what questions to ask.

* in python 2, a dictionary’s keys are not guaranteed to have consistent order. In more recent versions of python 3, they ARE guaranteed to have consistent ordering. This has ramifications for the code you write, and it’s especially confusing if you don’t understand hashmaps because 99% of the time, the order will be maintained in python 2 even though it isn’t guaranteed. But relying on things that are true most of the time is a very bad way to write production code :)

>I don’t expect someone to be able to build a hashmap from scratch but I do expect an experienced engineer to have some basic idea of the pieces that go into building it as well as it’s properties (not necessarily ordered, O(1)ish sets/gets, collisions, etc). Knowing this kind of thing helps you understand when to use an object in JavaScript vs a map. Or how dictionaries differ between python 2 and 3. If you understand the underlying data structure, then you know what questions to ask.

Yeah, I'd say this is all in knowing how to use a hash. How to build one would go into the underlying data structure.

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