Perplexed - Difference between Arrays and Hashes?

Both are collections used to store and retrieve (lookup) data.

You can think of an array as a list. Just a collection of data in some kind of order. You can look up a piece of data by using the number it comes in on the list, like, it’s first in the list, or second… like you have a shopping list and you ask, “OK, what’s the first thing on the list? Milk. What’s the second? Lettuce.” shopping_array[0] would give you "Milk", and shopping_array[1] would give "Lettuce".

You can think of a hash as a collection of pairs of information, and each pair is made of a name and a value. The name is called a key, and it must be unique. You use the key to return the value paired to it. So for a shopping list, it’d be more like a list where your finicky girlfriend wanted to make sure you got the right kind of stuff when you went shopping, and next to “milk” she wrote “low fat”, and next to “fish” it said “salmon”. So milk is the key for “low fat”. You can imagine it would look like a table when she wrote it, and hashes are sometimes called hash tables. You’d look at the list thinking, “What do we want for fish?” and the list would answer “salmon”. So the important thing with a hash isn’t the order of the information, the important thing is what is paired to what. shopping_hash["fish"] gives you "salmon".

Examples

So, a simple array might just contain some days of the week:

days = ["Monday", "Tuesday", "Wednesday"]

A hash could contain numbers followed by their names in Spanish:

dias = { "Monday" => "lunes", "Tuesday" => "martes", "Wednesday" => "mercoles" }

Here the English word would be the key, and the Spanish equivalent would be the value.

Hashes are called dictionaries in Python, and it might be easy to think of them this way, because a dictionary essentially has a key (the word) and a value (the definition). In terms of computer performance, hashes in Ruby are said to be very fast for looking up information compared to arrays.

Lookups:

Arrays: To retrieve data from our array, we can use the index (order) number, starting from 0. So, days[0] will return "Monday", days[1] will return "Tuesday".

Hashes: With a hash you use the key value instead of the index number. So dias["Monday"] returns "lunes", because in the dias hash, "Monday" is the key for the value lunes.

Now, you can store any kind of value in both arrays and hashes, as I understand. And as we’ve seen in the exercises, you can have arrays inside of arrays, and also hashes in arrays, hashes in hashes, and all sorts of other things. The basics, however, are pretty straightforward.

Hopefully I haven’t made any mistakes explaining this. If you try to play with a few of your own in labs.codecademy it should all come together for you.