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Rollup merge of #31870 - ivan:filter-explain, r=steveklabnik

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As a Rust newbie, I found the book's explanation for why the `filter` closure gets a reference very confusing, and tried to figure out why `filter` is somehow less consumptive than `map` -- but it isn't; that's controlled by `iter`/`into_iter`.  I flailed around for a while until @habnabit explained it to me, and in retrospect it is quite obvious :-)
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Manish Goregaokar 2016-02-25 04:21:11 +05:30
commit b660ca59ff
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10
src/doc/book/iterators.md
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@ -311,10 +311,12 @@ for i in (1..100).filter(|&x| x % 2 == 0) {
```
This will print all of the even numbers between one and a hundred.
(Note that because `filter` doesn't consume the elements that are
being iterated over, it is passed a reference to each element, and
thus the filter predicate uses the `&x` pattern to extract the integer
itself.)
(Note that, unlike `map`, the closure passed to `filter` is passed a reference
to the element instead of the element itself. The filter predicate here uses
the `&x` pattern to extract the integer. The filter closure is passed a
reference because it returns `true` or `false` instead of the element,
so the `filter` implementation must retain ownership to put the elements
into the newly constructed iterator.)
You can chain all three things together: start with an iterator, adapt it
a few times, and then consume the result. Check it out: