add solutions for array.md

solutions/compound-types/array.md

@@ -0,0 +1,63 @@
+1.
+```rust
+fn main() {
+    let arr: [i32; 5] = [1, 2, 3, 4, 5];
+
+    assert!(arr.len() == 5);
+}
+```
+
+2.
+```rust
+fn main() {
+    // we can ignore parts of the array type or even the whole type, let the compiler infer it for us
+    let arr0 = [1, 2, 3];
+    let arr: [_; 3] = ['a', 'b', 'c'];
+    
+    // Arrays are stack allocated, `std::mem::size_of_val` return the bytes which array occupies
+    // A char takes 4 byte in Rust: Unicode char
+    assert!(std::mem::size_of_val(&arr) == 12);
+}
+```
+
+3.
+```rust
+fn main() {
+    let list: [i32; 100] = [1; 100];
+
+    assert!(list[0] == 1);
+    assert!(list.len() == 100);
+}
+```
+
+4.
+```rust
+fn main() {
+    // fix the error
+    let _arr = [1, 2, 3];
+}
+```
+
+5.
+```rust
+fn main() {
+    let arr = ['a', 'b', 'c'];
+    
+    let ele = arr[0];
+
+    assert!(ele == 'a');
+}
+```
+
+6.
+```rust
+fn main() {
+    let names = [String::from("Sunfei"), "Sunface".to_string()];
+    
+    // `get` returns an Option<T>, it's safe to use
+    let name0 = names.get(0).unwrap();
+
+    // but indexing is not safe
+    let _name1 = &names[1];
+}
+```