Adds parallel map-reduce recipe

See #329

* Adds example in `concurrency.md`
* Adds links to `intro.md`

src/concurrency.md

@@ -5,6 +5,7 @@
 | [Mutate the elements of an array in parallel][ex-rayon-iter-mut] | [![rayon-badge]][rayon] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Search items using given predicate in parallel][ex-rayon-parallel-search] | [![rayon-badge]][rayon] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Sort a vector in parallel][ex-rayon-parallel-sort] | [![rayon-badge]][rayon] [![rand-badge]][rand] | [![cat-concurrency-badge]][cat-concurrency] |
+| [Map-reduce in parallel][ex-rayon-map-reduce] | [![rayon-badge]][rayon] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Generate jpg thumbnails in parallel][ex-rayon-thumbnails] | [![rayon-badge]][rayon] [![glob-badge]][glob] [![image-badge]][image] | [![cat-concurrency-badge]][cat-concurrency][![cat-filesystem-badge]][cat-filesystem] |
 | [Spawn a short-lived thread][ex-crossbeam-spawn] | [![crossbeam-badge]][crossbeam] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Draw fractal dispatching work to a thread pool][ex-threadpool-fractal] | [![threadpool-badge]][threadpool] [![num-badge]][num] [![num_cpus-badge]][num_cpus] [![image-badge]][image] | [![cat-concurrency-badge]][cat-concurrency][![cat-science-badge]][cat-science][![cat-rendering-badge]][cat-rendering] |
@@ -73,6 +74,61 @@ fn main() {
 }
 ```
 
+[ex-rayon-map-reduce]: #ex-rayon-map-reduce
+<a name="ex-rayon-map-reduce"></a>
+## Map-reduce in parallel
+
+[![rayon-badge]][rayon] [![cat-concurrency-badge]][cat-concurrency]
+
+This example uses [`rayon::filter`], [`rayon::map`], and [`rayon::reduce`]
+to calculate the coniditional average age of a vector of `Person` objects.
+
+[`rayon::filter`] allows us to (in parallel) conditionally include elements from
+a collection that satisfy the given predicate.  Similarly, [`rayon::map`] and
+[`rayon::reduce`] allow us to transform the filtered elements via a unary
+operation and reduce them to a single value via a given binary operation,
+respectively. We also show use of [`rayon::sum`], which has the same result as
+the reduce operation in this example.
+
+```rust
+extern crate rayon;
+
+use rayon::prelude::*;
+
+struct Person {
+    age: u32,
+}
+
+fn main() {
+    let v: Vec<Person> = vec![
+        Person { age: 23 },
+        Person { age: 19 },
+        Person { age: 42 },
+        Person { age: 17 },
+        Person { age: 17 },
+        Person { age: 31 },
+        Person { age: 30 },
+    ];
+
+    let num_over_30 = v.par_iter().filter(|&x| x.age > 30).count() as f32;
+    let sum_over_30 = v.par_iter()
+        .map(|x| x.age)
+        .filter(|&x| x > 30)
+        .reduce(|| 0, |x, y| x + y);
+    
+    let alt_sum_30: u32 = v.par_iter()
+        .map(|x| x.age)
+        .filter(|&x| x > 30)
+        .sum();
+
+    let avg_over_30 = sum_over_30 as f32 / num_over_30;
+    let alt_avg_over_30 = alt_sum_30 as f32/ num_over_30;
+
+    assert!((avg_over_30 - alt_avg_over_30).abs() < std::f32::EPSILON);
+    println!("The average age of people older than 30 is {}", avg_over_30);
+}
+```
+
 [ex-rayon-parallel-search]: #ex-rayon-parallel-search
 <a name="ex-rayon-parallel-search"></a>
 ## Search items using given predicate in parallel
@@ -456,7 +512,11 @@ fn run() -> Result<()> {
 [`par_iter`]: https://docs.rs/rayon/*/rayon/iter/trait.IntoParallelRefIterator.html#tymethod.par_iter
 [`par_iter_mut`]: https://docs.rs/rayon/*/rayon/iter/trait.IntoParallelRefMutIterator.html#tymethod.par_iter_mut
 [`par_sort_unstable`]: https://docs.rs/rayon/*/rayon/slice/trait.ParallelSliceMut.html#method.par_sort_unstable
+[`rayon::filter`]: https://docs.rs/rayon/*/rayon/iter/trait.ParallelIterator.html#method.filter
 [`rayon::find_any`]: https://docs.rs/rayon/*/rayon/iter/trait.ParallelIterator.html#method.find_any
+[`rayon::map`]: https://docs.rs/rayon/*/rayon/iter/trait.ParallelIterator.html#method.map
+[`rayon::reduce`]: https://docs.rs/rayon/*/rayon/iter/trait.ParallelIterator.html#method.reduce
+[`rayon::sum`]: https://docs.rs/rayon/*/rayon/iter/trait.ParallelIterator.html#method.sum
 [`std::find`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.find
 [`Walkdir::new`]: https://docs.rs/walkdir/1.0.7/walkdir/struct.WalkDir.html#method.new
 

src/intro.md

@@ -69,6 +69,7 @@ community. It needs and welcomes help. For details see
 | [Mutate the elements of an array in parallel][ex-rayon-iter-mut] | [![rayon-badge]][rayon] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Search items using given predicate in parallel][ex-rayon-parallel-search] | [![rayon-badge]][rayon] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Sort a vector in parallel][ex-rayon-parallel-sort] | [![rayon-badge]][rayon] [![rand-badge]][rand] | [![cat-concurrency-badge]][cat-concurrency] |
+| [Map-reduce in parallel][ex-rayon-map-reduce] | [![rayon-badge]][rayon] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Generate jpg thumbnails in parallel][ex-rayon-thumbnails] | [![rayon-badge]][rayon] [![glob-badge]][glob] [![image-badge]][image] | [![cat-concurrency-badge]][cat-concurrency][![cat-filesystem-badge]][cat-filesystem] |
 | [Spawn a short-lived thread][ex-crossbeam-spawn] | [![crossbeam-badge]][crossbeam] | [![cat-concurrency-badge]][cat-concurrency] |
 | [Draw fractal dispatching work to a thread pool][ex-threadpool-fractal] | [![threadpool-badge]][threadpool] [![num-badge]][num] [![num_cpus-badge]][num_cpus] [![image-badge]][image] | [![cat-concurrency-badge]][cat-concurrency][![cat-science-badge]][cat-science][![cat-rendering-badge]][cat-rendering] |
@@ -201,6 +202,7 @@ community. It needs and welcomes help. For details see
 [ex-rand-range]: basics.html#ex-rand-range
 [ex-random-port-tcp]: net.html#ex-random-port-tcp
 [ex-rayon-iter-mut]: concurrency.html#ex-rayon-iter-mut
+[ex-rayon-map-reduce]: concurrency.html#ex-rayon-map-reduce
 [ex-rayon-parallel-search]: concurrency.html#ex-rayon-parallel-search
 [ex-rayon-parallel-sort]: concurrency.html#ex-rayon-parallel-sort
 [ex-rayon-thumbnails]: concurrency.html#ex-rayon-thumbnails