Adds vector sum example. (#461)

This is part of tracking issue #450.

src/science.md

@@ -7,14 +7,16 @@
 | [Mathematical functions on complex numbers][ex-mathematical-functions] | [![num-badge]][num] | [![cat-science-badge]][cat-science] |
 | [Calculating the side length of a triangle][ex-calculating-side-length-of-triangle] | [![std-badge]][std] | [![cat-science-badge]][cat-science] |
 | [Verifying tan is equal to sin divided by cos][ex-tan-equal-to-sin-divided-by-cos] | [![std-badge]][std] | [![cat-science-badge]][cat-science] |
-[ex-creating-complex-numbers]: science/mathematics/complex-numbers.html#creating-complex-numbers
+| [Vector Sum][ex-ndarray-vec-sum] | [![ndarray-badge]][ndarray] | [![cat-science-badge]][cat-science] |
 | [Distance between two points on the Earth][ex-latitude-longitude] | [![std-badge]][std] | [![cat-science-badge]][cat-science] |
 
+[ex-creating-complex-numbers]: science/mathematics/complex-numbers.html#creating-complex-numbers
 [ex-adding-complex-numbers]: science/mathematics/complex-numbers.html#adding-complex-numbers
 [ex-creating-complex-numbers]: science/mathematics/complex-numbers.html#creating-complex-numbers
 [ex-latitude-longitude]: science/mathematics.html#distance-between-two-points-on-earth
 [ex-mathematical-functions]: science/mathematics/complex-numbers.html#mathematical-functions
 [ex-calculating-side-length-of-triangle]: science/mathematics/trigonometry.html#calculating-the-side-length-of-a-triangle
 [ex-tan-equal-to-sin-divided-by-cos]: science/mathematics/trigonometry.html#verifying-tan-is-equal-to-sin-divided-by-cos
+[ex-ndarray-vec-sum]: science/mathematics/linear-algebra/vector_sum.html
 
 {{#include links.md}}

src/science/mathematics/linear-algebra/vector_sum.md

@@ -0,0 +1,51 @@
+## Vector Sum
+[![ndarray-badge]][ndarray]
+
+The [ndarray] crate supports a number of ways to create arrays -- this recipe
+focuses on creating [`ndarray::Array`]s from `std::Vec` via [`from_vec`]. Adding two
+arrays together is no different than adding two numbers together. Using the `&`
+operand on the arrays within an arithmetic operation prevents the operation from
+consuming the arrays. Without `&`, the arrays are consumed.
+
+In the first example, arrays `a` and `b` are moved in the let-statement `z = a +
+b`. In the second example, the arrays `c` and `d` are not moved and instead, a
+new array is created for `w`. Updating either of `c` or `d` after the vector sum
+has no effect the value of `w`. Additionally, while printing `c` works as
+expected, it would be an error to print `b` due to the move. See [Binary
+Operators With Two Arrays] for additional detail.
+
+```rust
+extern crate ndarray;
+use ndarray::Array;
+
+fn main() {
+  let a = Array::from_vec(vec![1., 2., 3., 4., 5.]);
+  let b = Array::from_vec(vec![5., 4., 3., 2., 1.]);
+  let mut c = Array::from_vec(vec![1., 2., 3., 4., 5.]);
+  let mut d = Array::from_vec(vec![5., 4., 3., 2., 1.]);
+
+  let z = a + b;
+  let w =  &c + &d;
+
+  let epsilon = 1e-8;
+  for elem in z.iter() {
+    let diff: f32 = *elem - 6.;
+    assert!(diff.abs() < epsilon);
+  }
+
+  println!("c = {}", c);
+  c[0] = 10.;
+  d[1] = 10.;
+
+  for elem in w.iter() {
+    let diff: f32 = *elem - 6.;
+    assert!(diff.abs() < epsilon);
+  }
+
+}
+```
+
+[Binary Operators With Two Arrays]: https://docs.rs/ndarray/*/ndarray/struct.ArrayBase.html#binary-operators-with-two-arrays
+[`from_vec`]: https://docs.rs/ndarray/*/ndarray/struct.ArrayBase.html#method.from_vec
+[ndarray]: https://docs.rs/crate/ndarray/*
+[`ndarray::Array`]: https://docs.rs/ndarray/*/ndarray/struct.ArrayBase.html