>>> ["foo", "bar", "baz"].index("bar")
1
Reference: Data Structures > More on Lists
Caveats follow
Note that while this is perhaps the cleanest way to answer the question as asked, index
is a rather weak component of the list
API, and I can't remember the last time I used it in anger. It's been pointed out to me in the comments that because this answer is heavily referenced, it should be made more complete. Some caveats about list.index
follow. It is probably worth initially taking a look at the documentation for it:
list.index(x[, start[, end]])
Return zero-based index in the list of the first item whose value is equal to x. Raises a ValueError
if there is no such item.
The optional arguments start and end are interpreted as in the slice notation and are used to limit the search to a particular subsequence of the list. The returned index is computed relative to the beginning of the full sequence rather than the start argument.
Linear time-complexity in list length
An index
call checks every element of the list in order, until it finds a match. If your list is long, and you don't know roughly where in the list it occurs, this search could become a bottleneck. In that case, you should consider a different data structure. Note that if you know roughly where to find the match, you can give index
a hint. For instance, in this snippet, l.index(999_999, 999_990, 1_000_000)
is roughly five orders of magnitude faster than straight l.index(999_999)
, because the former only has to search 10 entries, while the latter searches a million:
>>> import timeit
>>> timeit.timeit('l.index(999_999)', setup='l = list(range(0, 1_000_000))', number=1000)
9.356267921015387
>>> timeit.timeit('l.index(999_999, 999_990, 1_000_000)', setup='l = list(range(0, 1_000_000))', number=1000)
0.0004404920036904514
Only returns the index of the first match to its argument
A call to index
searches through the list in order until it finds a match, and stops there. If you expect to need indices of more matches, you should use a list comprehension, or generator expression.
>>> [1, 1].index(1)
0
>>> [i for i, e in enumerate([1, 2, 1]) if e == 1]
[0, 2]
>>> g = (i for i, e in enumerate([1, 2, 1]) if e == 1)
>>> next(g)
0
>>> next(g)
2
Most places where I once would have used index
, I now use a list comprehension or generator expression because they're more generalizable. So if you're considering reaching for index
, take a look at these excellent Python features.
Throws if element not present in list
A call to index
results in a ValueError
if the item's not present.
>>> [1, 1].index(2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: 2 is not in list
If the item might not be present in the list, you should either
- Check for it first with
item in my_list
(clean, readable approach), or
- Wrap the
index
call in a try/except
block which catches ValueError
(probably faster, at least when the list to search is long, and the item is usually present.)
Best Answer
First, not all copying of a list is bad. Sometimes a copy can take advantage of CPU cache and be extremely fast, it depends on the list, size, and other factors.
Second, for modifying a list "in-place" you need to use a type of list that is mutable. In your sample you use
listOf
which returns theList<T>
interface, and that is read-only. You need to directly reference the class of a mutable list (i.e.ArrayList
), or it is idiomatic Kotlin to use the helper functionsarrayListOf
orlinkedListOf
to create aMutableList<T>
reference. Once you have that, you can iterate the list using thelistIterator()
which has a mutation methodset()
.This will change the values in the list as iteration occurs and is efficient for all list types. To make this easier, create helpful extension functions that you can re-use (see below).
Mutating using a simple extension function:
You can write extension functions for Kotlin that do an in place mutable iteration for any
MutableList
implementation. These inline functions will perform as fast as any custom use of the iterator and is inlined for performance. Perfect for Android or anywhere.Here is a
mapInPlace
extension function (which keeps the naming typical for these type of functions such asmap
andmapTo
):Example calling any variation of this extension function:
This is not generalized for all
Collection<T>
, because most iterators only have aremove()
method, notset()
.Extension functions for Arrays
You can handle generic arrays with a similar method:
And for each of the primitive arrays, use a variation of:
About the Optimization using only Reference Equality
The extension functions above optimize a little by not setting the value if it has not changed to a different instance, checking that using
===
or!==
is Referential Equality. It isn't worth checkingequals()
orhashCode()
because calling those has an unknown cost, and really the referential equality catches any intent to change the value.Unit Tests for Extension Functions
Here are unit test cases showing the functions working, and also a small comparison to the stdlib function
map()
that makes a copy: