[q-big]

> someone who had done pretty well in our DS

> Him: “I can’t. I need to be able to get both the key and the value and hash tables don’t store the key”

How could he do well in the data structures class? This is the definition of a hash map or hashed dictionary, so this is basic knowledge of data structures that is taught in this class and central to know to even have a chance of passing the exam.

[jvanderbot]

No, technically he's right. On paper, a hash map creates an index in an array-like structure from the key, but does not necessarily store the key in a retrievable way. The "Hash" in hashmap comes from the fact that the key is somehow hashed (an often irreversible procedure) to determine the memory location to store the value.

In practice it's not the case, but very technically from a purely theoretical standpoint, I think he's right.

EDIT: untrue on any finite sized array, due to collisions. See below, and sorry for the brain fart!

[hawk_]

There's no theoretical way for a hash map to work without storing the key - the reason for that is hash collisions in presence of which you do need to run equality comparison with stored keys otherwise you would overwrite data just because of hash clashes.

[tonyarkles]

>> In practice it's not the case, but very technically from a purely theoretical standpoint, I think he's right.

> There's no theoretical way for a hash map to work without storing the key - the reason for that is hash collisions in presence of which you do need to run equality comparison with stored keys otherwise you would overwrite data just because of hash clashes.

There's a ton of material in a Data Structures course. I'm sure the point about storing the key was mentioned at some point, but it's not the focus when talking about different hashing strategies etc. For people with a bunch of experience (e.g. I'd been writing Perl for years before this, so being able to iterate across a hash and get (k,v) pairs was obvious) it's a detail that is already there in your brain, but if the DS course is your first exposure to a hash map, it's a detail that can easily get lost in the huge forest of other brand new things to learn.

[q-big]

> but if the DS course is your first exposure to a hash map, it's a detail that can easily get lost in the huge forest of other brand new things to learn.

My experience differs: It is the common situation that the theory is taught in the lecture, and then each week you have to do quite some exercises to implement all the newly learned data structures in code (lots of work to do each week :-) ). So, if you forgot such a "detail", the code to write for next week simply won't work - 0 points.

This way, it is rather unlikely that you will forget such a "detail" in the forest of new things to learn. This statement in particular holds for students who had done pretty well in DS as the one you are talking about.

[tonyarkles]

Not to date myself too much but the course was taught in Eiffel :). On top of the theory, and on top of the practical details, there was also the fun of learning an obscure quirky programming language you were unlikely to ever use again.

I loved it all and had a great time. Lots of people really did not.

> So, if you forgot such a “detail”…

Sure, very true in the short term. Next semester though? That code doesn’t matter and most people don’t have flawless long term memory.

[marcosdumay]

Collision-free hashes exist. They are just not used on practice to build hash maps, but if you use a 128 bits hash with homogeneous distribution, handling collisions is irrelevant.

Besides, just because you are storing the keys, it doesn't mean it is viable to retrieve them. On the collision aware maps you see on CS, iterating through the keys is extremely inefficient.

The OP's description is clearly somebody that has learned the theory, but don't have any practical knowledge.

[nuancebydefault]

I never understood why a 'dictionary' (so called conveniently in python) is called a 'map' in c++ and yet, a 'hash' in ruby. On a side note, a map in ruby is an operation that actively maps one thing to another. So confusing. Finally reading what you just wrote makes the pieces of this puzzle to fall in place! Never had a data structures class.

[q-big]

According to https://stackoverflow.com/a/2884200 map and dictionary are just terms for the same data structure concept. On the other hand: there exist other backings for this data structure than hashing the key, such as binary trees (for example red-black trees or AVL trees).