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this post was submitted on 12 May 2026
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arrays don't have indices. lists have indices. arrays have offsets.
The real reason she won't call back.
indeed. permanently off by 1.
§6.7.9 of the C11 standard says they have elements with indices:
it also states in section 6.7.7 ("type names") that
note also that your example is the only occurrence of the word "index" in the entire document that isn't just referring to the actual index at the end.
Says who?
By definition, an index is
Since the arrays offsets alao tell us about the items’s position in the array, is it not then an index?
People take these terms way too seriously. Hell, many languages have their “list” implemented as an array. What then do you call the index/offset?
if you want my opinion (<- see now you can't tell me i'm wrong, it's an opinion) then the difference is that an array is by definition a memory address that's designated as the beginning of an array, and it's got an offset because the first element is at that specific address and further items are offset from that address. so you add the offset to the address to get the nth item. a list, meanwhile, can be basically any implementation under the hood, but it's commonly a linked list. the way you get the nth index there is you count up from the first position. since the implementation is opaque and may be spread out in memory you can't arithmetic your way to an index, you need to follow the pointers.
java's arraylist is a list backed by an array. java's vector is a list backed by a linked list.
That doesn’t really address what you call it. Names only really just exist to get your point across. Inexperienced devs may not know what an offset means (or why we use that), so index does the job. An experience dev knows how it works anyway, so whether you say index or offset won’t matter. By virtue of the common denominator, I simply use index everywhere.
depends on the area you're working in. it's a pretty important distinction in embedded software.
Why? I’ve worked as an embedded dev for a few years and nobody in my team cared what it was called.
because making sure offsets are correct come up a lot when you're memory-mapping IO.
Right but whether they’re correct or not doesn’t depend on the name you use. Every programmer worth his name knows arrays start at offset zero even if you don’t call it that.
except if you use lua.
...which doesn't have arrays, but still.
Um actually they have strides and offsets.
Adding my fuel to this fight
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/indexOf
javascript doesn't have arrays. the backing data structure is a doubly-linked list.
and the backing for that is linear or page addressed MOS transistors, spinning rust or flippy-round magnets.
do you have a source that indicates mainstream JS engines internally uses a list structure for arrays? I can't find one.
skipped a few steps there i think.
anyway, good question. led me to some cursed code.
the ArrayObject in spidermonkey is an interface to either a TypedArrayObject or a SharedArrayObject. those both have an inner ArrayBuffer object, which is a view into ArrayBufferObjectMaybeShared, which contains a refcounted vector of uint8 pointers, regardless of the datatype. soooo all arrays in javascript are... strings?
thanks for the considered reply. didn't mean to jump all the way down to electrons and sound so flippant.
my claim is that JavaScript arrays are arrays because the spec defines their behavior as such. the implementation details are absolutely interesting from a performance perspective and I was genuinely curious how an internally linked list implementation would actually work, real-world. regardless... almost every interaction I have ever had with a JS programmer has ended in "its strings all the way down".. so... I mean... yes-ish?
loved your poking of the hornets nest in this thread :-)
i was thinking between the linked list and the transistors :)
also, i mean... what you might call an array i might call a vector. js arrays allow elements of different types, so they are by definition not arrays in the traditional sense. them being chars internally does make sense in a gross way.
If I'm understanding you correctly, they're basically doing the same thing as Python under the hood and using a heap-allocated array (vector) of pointers? If so, that should still be orders of magnitude faster than a linked list.
If their implementation is actually a linked list, colour me shocked. My impression was that JavaScript is "decently fast". I've never even considered writing high-performance code in it, but I've heard that the compiler can optimise extremely aggressively, and it's used so widely that I couldn't imagine that it had glaring performance issues like what I would expect to see if every array was actually a linked list under the hood.
can't a jit move things around enough that a linked list could be transformed into a memory-backed array if the access pattern requires it.
Sure it can, as long as it retains behaviour according to whatever standard it needs to comply to. My point was rather that I would be very surprised if the actual implementation (at memory level) was a linked list.
That's a different kind of array (Float32Array etc.), not the "normal" kind.
i couldn't actually find any of that in spidermonkey. i was looking in js/vm/arrayobject and its parents, didn't see any others.
I'm guessing it's this? https://searchfox.org/firefox-main/source/js/src/builtin/Array.cpp
i did look at that but i started out here: https://searchfox.org/firefox-main/source/js/src/vm/ArrayObject.h because it inherits from NativeObject.
Yes, and since that's the wrong place I kept looking, and found the link I just sent you
i edited my thing, i did start at array.cpp but only found references to other places so i went digging down the stack.
I’m aware, I added fuel, not peace and love
You can't take my 'i', we have been together.
you can't spell offset without off. as in fuck.
(affectionate)