Maps (hashtables) in the real world

Question

I'm trying to explain Map (aka hash table, dict) to someone who's new to programming. While the concepts of Array (=list of things) and Set (=bag of things) are familiar to everyone, I'm having a hard time finding a real-world metaphor for Maps (I'm specifically interested in python dicts and Javascript Objects). The often used dictionary/phone book analogy is incorrect, because dictionaries are sorted, while Maps are not - and this point is important to me. So the question is: what would be a real world phenomena or device that behaves like Map in computing?

Answer 1

I agree with delnan in that the human example is probably too close to that of an object. This works well if you are trying to transition into explaining how objects are implemented in loosely typed languages, however a map is a concept that exists in Java and C# as well. This could potentially be very confusing if they begin to use those languages.

Essentially you need to understand that maps are instant look-ups that rely on a unique set of values as keys. These two things really need to be stressed, so here's a decent yet highly contrived example:

Lets say you're having a party and everyone is supposed to bring one thing. To help the organizer, everyone says what their first name is and what they're bringing. Now lets pretend there are two ways to store this information. The first is by putting it down on a list and the second is by telling someone with a didactic memory. The contrived part is that they can only identify you through you're first name (so he's blind and has a cochlear implant so everyone sounds like a robot, best I can come up with).

List: To add, you just append to the bottom of the list. To back out you just remove yourself from the list. If you want to see who is bringing something and what they're bringing, then you have to scan the entire list until you find them. If you don't find them after scanning, then they're clearly they're not on the list and not bringing anything. The list would clearly allow duplicates of people with the same first name.

Dictionary (contrived person): You don't append to the end of the list, you just tell him someone's first name and what they're bringing. If you want to know what someone is bringing you just ask by name and he immediately tells you. Likewise if two people of the same name tell him they're bringing something, he'll think its the same person just changing what they're bringing. If someone hasn't signed up you would ask by name, but he'd be confused and ask you what you're talking about. Also you would have to say when you tell the guy that someone is no longer bringing something he would lose all memory of them, so yeah highly contrived.

You might also want to show why the list is sufficient if you don't care who brings what, but just need to know what all is being brought. Maybe even leave the names off the list, to stress key/value pairs with the dictionary.

Answer 2

Perhaps it would be the analogy of a human being that your meeting for the first time:

Each person has an unordered amount of attributes, each of these attributes can only have 1 value, which is unique (like hair=long, eye_color=blue). And you would discover these attributes in no particular order.

So for a person she can have a shoesize=38, hair_color=brown and eye_color=blue and when reciting (human_dict.get('shoe_size')) this to someone else you would mention the attributes in no particular order except by attribute name.