add functional_programming

data/1.rust初阶/2.rust基本概念/12.函数式编程/config.json

@@ -2,7 +2,9 @@
   "node_id": "rust-4844e545058847c8b3198330fcedd12c",
   "keywords": [],
   "children": [],
-  "export": [],
+  "export": [
+    "functional_programming.json"
+  ],
   "keywords_must": [],
   "keywords_forbid": []
 }
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data/1.rust初阶/2.rust基本概念/12.函数式编程/functional_programming.json

+{
+    "type": "code_options",
+    "author": "jackymao_com",
+    "source": "functional_programming.md",
+    "notebook_enable": false,
+    "exercise_id": ""
+}
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data/1.rust初阶/2.rust基本概念/12.函数式编程/functional_programming.md

+# 函数式编程
+
+虽然 rust 语言主要是指令式编程，但也借鉴了很多函数式编程的精华，譬如 rust 中的迭代器、闭包、高阶函数等。
+
+Rust 迭代器函数式编程主要用到的有：
+- map
+- for_each
+- filter
+- filter_map
+- fold
+- reduce
+- zip
+- collect
+
+用法如下:
+
+```rust
+
+#![allow(unused_variables)]
+#![allow(dead_code)]
+
+use std::collections::HashMap;
+
+fn main() {
+        
+    let v: Vec<_> = (0..20).collect();
+    // println!("{:?}", v);
+    
+    let values = vec![1, 2, 3, 4, 5].into_iter();
+    let _sum = values.clone().reduce(|acc, x| acc + x);
+    // println!("{:?} {:?}", &values, _sum);
+    
+    let m = (1..5).fold(2, |mul, x| mul * x);
+    let n = (1..101).fold(0, |a, b| a + b);
+    // println!("{:?} {:?}", m, n);
+    
+    // fold is like reduce but can return an accumulator of a different type than the items of the iterator
+    let values = vec!["Hello", "World", "!"].into_iter();
+    let _sentence = values.clone().fold(String::new(), |acc, x| acc + x);
+    // println!("{:?} {:?}", values, _sentence);
+    
+    let v = vec!["Hello", "World", "!"].into_iter();
+    let w: Vec<String> = v.clone().map(String::from).collect();
+    // println!("{:?} {:?}", v, w);
+    
+    let even: Vec<_> = (0..10).filter(|x| x % 2 == 0).collect();
+    // println!("{:?}", even);
+    
+    let v = vec![-1, 2, -3, 4, 5].into_iter();
+    let _positive_numbers: Vec<i32> = v.clone()
+            .inspect(|x| println!("Before filter: {}", x))
+            .filter(|x: &i32| x.is_positive())
+            .inspect(|x| println!("After filter: {}", x))
+            .collect();
+    println!("{:?} {:?}", v, _positive_numbers);
+    
+    let v: Vec<i32> = [1, 2, 3].into_iter().map(|x| x + 1).rev().collect();
+    assert_eq!(v, [4, 3, 2]);
+    
+    let names = vec!["Alice", "Bob", "Charlie"];
+    let scores = vec!["90", "80", "95"];
+    let score_map: HashMap<_, _> = names.iter()
+    	.zip(scores.iter())
+    	.collect();
+    println!("{:?} {:?} {:?}", names, scores, score_map);
+    
+    fn filter_map_test() {
+        let v = vec!["Hello", "World", "!"].into_iter();
+    
+        let w: Vec<String> = v.clone()
+            .filter_map(|x| {
+                if x.len() > 2 {
+                    Some(String::from(x))
+                } else {
+                    None
+                }
+            })
+            .collect();
+        println!("{:?} {:?}", v, w);
+    
+        assert_eq!(w, vec!["Hello".to_string(), "World".to_string()]);
+    }
+    
+    filter_map_test();
+    
+}
+
+```
+
+使用的时候如果编译出现错误，就要注意到有的函数是会消耗迭代器的元素，有的会返回一个迭代器，有的是惰性的。
+
+下面的例子能正确编译的是：
+
+## 答案
+
+```rust
+
+fn main() {
+    let arr = vec![0; 10];
+    for i in arr.iter() {
+        println!("{:?}", i)
+    }
+
+    println!("{:?}",arr);
+}
+
+```
+
+## 选项
+
+###
+
+```rust
+
+fn main() {
+    let arr = vec![0; 10];
+    for i in &arr.iter() {
+        println!("{:?}", i)
+    }
+
+    println!("{:?}",arr);
+}
+
+```
+
+###
+
+```rust
+
+fn main() {
+    let arr = vec![0; 10];
+    for i in arr {
+        println!("{:?}", i)
+    }
+
+    println!("{:?}",arr);
+}
+
+```
+
+###
+
+```rust
+
+fn main() {
+    let arr = vec![0; 10];
+    for i in arr.into_iter() {
+        println!("{:?}", i)
+    }
+
+    println!("{:?}",arr);
+}
+
+```
+
+
+<!-- 
+ref:
+https://kerkour.com/rust-functional-programming 
+https://doc.rust-lang.org/std/iter/struct.Map.html
+https://www.cnblogs.com/chenguifeng/p/12243999.html
+https://course.rs/advance/functional-programing/iterator.html#消费者与适配器
+-->
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