add [Iterator]

solutions/functional-programing/iterator.md

@@ -0,0 +1,238 @@
+1、
+```rust
+fn main() {
+    let arr = [0; 10];
+    for i in arr {
+        println!("{}", i)
+    }
+}
+```
+
+2、
+```rust
+fn main() {
+    let mut v = Vec::new();
+    for n in 1..101 {
+       v.push(n);
+    }
+
+    assert_eq!(v.len(), 100);
+}
+```
+
+3、
+```rust
+fn main() {
+    let v1 = vec![1, 2];
+
+    // moving ownership
+    let mut v1_iter = v1.into_iter();
+
+    assert_eq!(v1_iter.next(), Some(1));
+    assert_eq!(v1_iter.next(), Some(2));
+    assert_eq!(v1_iter.next(), None);
+}
+```
+
+```rust
+fn main() {
+    let v1 = vec![1, 2];
+
+    // borrowing
+    let mut v1_iter = v1.iter();
+
+    assert_eq!(v1_iter.next(), Some(&1));
+    assert_eq!(v1_iter.next(), Some(&2));
+    assert_eq!(v1_iter.next(), None);
+}
+```
+
+4、
+```rust
+fn main() {
+    let arr = vec![0; 10];
+    for i in arr.iter() {
+        println!("{}", i)
+    }
+
+    println!("{:?}",arr);
+}
+```
+
+5、
+```rust
+fn main() {
+    let mut names = vec!["Bob", "Frank", "Ferris"];
+
+    for name in names.iter_mut() {
+        *name = match name {
+            &mut "Ferris" => "There is a rustacean among us!",
+            _ => "Hello",
+        }
+    }
+
+    println!("names: {:?}", names);
+}
+```
+
+6、
+```rust
+fn main() {
+    let mut values = vec![1, 2, 3];
+    let mut values_iter = values.iter_mut();
+
+    if let Some(v) = values_iter.next() {
+        *v = 0;
+    }
+
+    assert_eq!(values, vec![0, 2, 3]);
+}
+```
+
+7、
+```rust
+struct Fibonacci {
+    curr: u32,
+    next: u32,
+}
+
+// Implement `Iterator` for `Fibonacci`.
+// The `Iterator` trait only requires a method to be defined for the `next` element.
+impl Iterator for Fibonacci {
+    // We can refer to this type using Self::Item
+    type Item = u32;
+    
+    // Here, we define the sequence using `.curr` and `.next`.
+    // The return type is `Option<T>`:
+    //     * When the `Iterator` is finished, `None` is returned.
+    //     * Otherwise, the next value is wrapped in `Some` and returned.
+    // We use Self::Item in the return type, so we can change
+    // the type without having to update the function signatures.
+    fn next(&mut self) -> Option<Self::Item> {
+        let new_next = self.curr + self.next;
+
+        self.curr = self.next;
+        self.next = new_next;
+
+        // Since there's no endpoint to a Fibonacci sequence, the `Iterator` 
+        // will never return `None`, and `Some` is always returned.
+        Some(self.curr)
+    }
+}
+
+// Returns a Fibonacci sequence generator
+fn fibonacci() -> Fibonacci {
+    Fibonacci { curr: 0, next: 1 }
+}
+
+fn main() {
+    let mut fib = fibonacci();
+    assert_eq!(fib.next(), Some(1));
+    assert_eq!(fib.next(), Some(1));
+    assert_eq!(fib.next(), Some(2));
+    assert_eq!(fib.next(), Some(3));
+    assert_eq!(fib.next(), Some(5));
+}
+```
+
+8、
+```rust
+fn main() {
+    let v1 = vec![1, 2, 3];
+
+    let v1_iter = v1.iter();
+
+    // The sum method will take the ownership of the iterator and iterates through the items by repeatedly calling next method
+    let total: i32 = v1_iter.sum();
+
+    assert_eq!(total, 6);
+
+    println!("{:?}",v1);
+}
+```
+
+9、
+```rust
+use std::collections::HashMap;
+fn main() {
+    let names = [("sunface",18), ("sunfei",18)];
+    let folks: HashMap<_, _> = names.into_iter().collect();
+
+    println!("{:?}",folks);
+
+    let v1: Vec<i32> = vec![1, 2, 3];
+
+    let v2: Vec<_> = v1.into_iter().collect();
+
+    assert_eq!(v2, vec![1, 2, 3]);
+}
+```
+
+10、
+```rust
+fn main() {
+    let v1: Vec<i32> = vec![1, 2, 3];
+
+    let v2: Vec<_> = v1.iter().map(|x| x + 1).collect();
+
+    assert_eq!(v2, vec![2, 3, 4]);
+}
+```
+
+11、
+```rust
+use std::collections::HashMap;
+fn main() {
+    let names = ["sunface", "sunfei"];
+    let ages = [18, 18];
+    let folks: HashMap<_, _> = names.into_iter().zip(ages.into_iter()).collect();
+
+    println!("{:?}",folks);
+}
+```
+
+12、
+```rust
+#[derive(PartialEq, Debug)]
+struct Shoe {
+    size: u32,
+    style: String,
+}
+
+fn shoes_in_size(shoes: Vec<Shoe>, shoe_size: u32) -> Vec<Shoe> {
+    shoes.into_iter().filter(|s| s.size == shoe_size).collect()
+}
+
+fn main() {
+    let shoes = vec![
+        Shoe {
+            size: 10,
+            style: String::from("sneaker"),
+        },
+        Shoe {
+            size: 13,
+            style: String::from("sandal"),
+        },
+        Shoe {
+            size: 10,
+            style: String::from("boot"),
+        },
+    ];
+
+    let in_my_size = shoes_in_size(shoes, 10);
+
+    assert_eq!(
+        in_my_size,
+        vec![
+            Shoe {
+                size: 10,
+                style: String::from("sneaker")
+            },
+            Shoe {
+                size: 10,
+                style: String::from("boot")
+            },
+        ]
+    );
+}
+```