for

for 用于遍历可迭代对象。

---

遍历一个范围：

let mut sum = 0;
for i in 1..3 { // 或者 1..<4
  sum += i;
}
// sum == 6 

---

遍历一个数组：

let arr = [10, 20, 30];
let mut sum = 0;
for value in arr {
  sum += value;
}
// sum == 60 

---

遍历一个记录：

let record = (a: 1, b: 2, c: 3);
let mut sum = 0;
for key in record {
  sum += record[key];
}
// sum == 6