Can someone explain a real-world, plain-language use for inject in Ruby?

Question

I'm working on learning Ruby, and came across inject. I am on the cusp of understanding it, but when I'm the type of person who needs real world examples to learn something. The most common examples I come across are people using inject to add up the sum of a (1..10) range, which I could care less about. It's an arbitrary example.

What would I use it for in a real program? I'm learning so I can move on to Rails, but I don't have to have a web-centric example. I just need something that has a purpose I can wrap my head around.

Thanks all.

Answer 1

inject can sometimes be better understood by its "other" name, reduce. It's a function that operates on an Enumerable (iterating through it once) and returns a single value.

There are many interesting ways that it can be used, especially when chained with other Enumerable methods, such as map. Often times, it can be a more concise and expressive way of doing something, even if there is another way to do it.

An example like this may seem useless at first:

range.inject {|sum, x| sum += x}

The variable range, however, doesn't have to be a simple explicit range. It could be (for example) a list of values returned from your database. If you ran a database query that returned a list of prices in a shopping cart, you could use .inject to sum them all and get a total.

In the simple case, you can do this in the SQL query itself. In a more difficult case, such as where some items have tax added to them and some don't, something like inject can be more useful:

cart_total = prices.inject {|sum, x| sum += price_with_tax(x)}

This sort of thing is also particularly useful when the objects in the Enumerable are complex classes that require more detailed processing than a simple numerical value would need, or when the Enumerable contains objects of different types that need to be converted into a common type before processing. Since inject takes a block, you can make the functionality here as complex as you need it to be.