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# Chapter 15. Iterators

It was the end of a very long day.

Phil

An iterator is a value that produces a sequence of values, typically for a loop to operate on. Rust’s standard library provides iterators that traverse vectors, strings, hash tables, and other collections, but also iterators to produce lines of text from an input stream, connections arriving at a network server, values received from other threads over a communications channel, and so on. And of course, you can implement iterators for your own purposes. Rust’s `for` loop provides a natural syntax for using iterators, but iterators themselves also provide a rich set of methods for mapping, filtering, joining, collecting, and so on.

Rust’s iterators are flexible, expressive, and efficient. Consider the following function, which returns the sum of the first `n` positive integers (often called the `n`th triangle number):

````fn` `triangle``(``n`: `i32``)`` `-> `i32` `{`
`    ``let`` ``mut`` ``sum`` ``=`` ``0``;`
`    ``for`` ``i`` ``in`` ``1``..``n``+``1`` ``{`
`        ``sum`` ``+=`` ``i``;`
`    ``}`
`    ``sum`
`}````

The expression `1..n+1` is a `Range<i32>` value. A `Range<i32>` is an iterator that produces the integers from its start value (inclusive) to its end value (exclusive), so you can use it as the operand of the `for` loop to sum the values from `1` to `n`.

But iterators also have a `fold` method, which you can use in the equivalent definition:

````fn` `triangle``(``n`: `i32``)`` `-> `i32` `{`
`    ``(``1``..``n``+``1``).``fold``(``0``,`` ``|``sum``,`` ``item``|`` ``sum`` ``+`` ``item``)`
`}````

Starting with `0` as the running total, `fold` takes each value that `1..n+1` produces and applies the closure `|sum, item| sum ...`

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