added string retain example

Author: BartMassey

string-retain/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "string-retain"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

string-retain/src/main.rs

@@ -0,0 +1,27 @@
+fn retain<F>(s: &mut String, mut pred: F)
+    where F: FnMut(char) -> bool
+{
+    let mut nretained = 0;
+    for c in s.chars() {
+        if pred(c) {
+            nretained += 1;
+        }
+    }
+    let mut result = String::with_capacity(nretained);
+    for c in s.chars() {
+        if pred(c) {
+            result.push(c);
+        }
+    }
+    *s = result;
+}
+
+fn main() {
+    let mut s = "Hello, world!".to_string();
+    let mut copy_of_s = s.clone();
+    let is_vowel = |c| "aieou".contains(c);
+    copy_of_s.retain(is_vowel);
+    println!("{}", copy_of_s);
+    retain(&mut s, is_vowel);
+    println!("{}", s);
+}