feat: add fold

src/SUMMARY.md

@@ -37,6 +37,7 @@
     - [AsRef](rust/r3/as-ref.md)
     - [Callback](rust/r3/callback.md)
     - [Async](rust/r3/async.md)
+    - [Iterators](rust/r3/iterators.md)
   - [R2 - Expert](rust/r2/mod.md)
     - [Hello Github Action](rust/r2/hello-github-action.md)
     - [Hello Actix CloudRun](rust/r2/hello-actix-cloudrun.md)

src/rust/r3/iterators.md

@@ -0,0 +1,25 @@
+# Iterators
+
+## How to sum value by key in object list?
+
+```rust,editable
+use std::collections::HashMap;
+
+fn main() {
+    println!(
+        "{:?}",
+        vec![
+            ("key1", 10),
+            ("key2", 20),
+            ("key1", 5),
+            ("key3", 15),
+            ("key2", 25),
+        ]
+        .into_iter()
+        .fold(HashMap::new(), |mut acc, (key, value)| {
+            *acc.entry(key).or_insert(0) += value;
+            acc
+        })
+    );
+}
+```