In Clojure, when should I use a vector over a list, and the other way around

clojurelistsequencesvector

I read that Vectors are not seqs, but Lists are. I'm not sure what the rationale is for using one over the other. It seems that vectors are used the most, but is there a reason for that?

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Once again, it seems I've answered my own question by getting impatient and asking it in #clojure on Freenode. Good thing answering your own questions is encouraged on Stackoverflow.com :D

I had a quick discussion with Rich Hickey, and here is the gist of it.

[12:21] <Raynes>    Vectors aren't seqs, right?
[12:21] <rhickey>   Raynes: no, but they are sequential
[12:21] <rhickey>   ,(sequential? [1 2 3])
[12:21] <clojurebot>    true
[12:22] <Raynes>    When would you want to use a list over a vector?
[12:22] <rhickey>   when generating code, when generating back-to-front
[12:23] <rhickey>   not too often in Clojure

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Python List vs. Array – when to use

Basically, Python lists are very flexible and can hold completely heterogeneous, arbitrary data, and they can be appended to very efficiently, in amortized constant time. If you need to shrink and grow your list time-efficiently and without hassle, they are the way to go. But they use a lot more space than C arrays, in part because each item in the list requires the construction of an individual Python object, even for data that could be represented with simple C types (e.g. float or uint64_t).

The array.array type, on the other hand, is just a thin wrapper on C arrays. It can hold only homogeneous data (that is to say, all of the same type) and so it uses only sizeof(one object) * length bytes of memory. Mostly, you should use it when you need to expose a C array to an extension or a system call (for example, ioctl or fctnl).

array.array is also a reasonable way to represent a mutable string in Python 2.x (array('B', bytes)). However, Python 2.6+ and 3.x offer a mutable byte string as bytearray.

However, if you want to do math on a homogeneous array of numeric data, then you're much better off using NumPy, which can automatically vectorize operations on complex multi-dimensional arrays.

To make a long story short: array.array is useful when you need a homogeneous C array of data for reasons other than doing math.

Java – How to join two lists in Java

In Java 8:

List<String> newList = Stream.concat(listOne.stream(), listTwo.stream())
                             .collect(Collectors.toList());

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