03 控制流 · Python 基础语法速通

01if / elif / else

Python

x = 2
if x > 2:
    print(">")
elif x == 2:
    print("=")
else:
    print("<")

Kotlin

val x = 2
when {
    x > 2 -> println(">")
    x == 2 -> println("=")
    else -> println("<")
}

ArkTS

const x = 2;
if (x > 2) {
  console.info(">");
} else if (x === 2) {
  console.info("=");
} else {
  console.info("<");
}

02for 遍历

Python 的 for 本质是「遍历可迭代对象」，没有 C 式三条件 for。

Python

for i in range(3):
    print(i)          # 0 1 2

for ch in "hi":
    print(ch)

Kotlin

for (i in 0 until 3) {
    println(i)
}
for (ch in "hi") {
    println(ch)
}

ArkTS

for (let i = 0; i < 3; i++) {
  console.info(i);
}
for (const ch of "hi") {
  console.info(ch);
}

range(n) ≈ Kotlin 0 until n；要含尾用 range(a, b+1) 或记「右开」。

03while / break / continue

Python

n = 3
while n > 0:
    print(n)
    n -= 1
    if n == 1:
        continue
    if n == 0:
        break

Kotlin

var n = 3
while (n > 0) {
    println(n)
    n--
}

ArkTS

let n = 3;
while (n > 0) {
  console.info(n);
  n--;
}

04列表推导式

把「map + filter」写成一行，读起来像集合生成式。

Python

nums = [1, 2, 3, 4]
evens = [x * 2 for x in nums if x % 2 == 0]
# [4, 8]

Kotlin

val nums = listOf(1, 2, 3, 4)
val evens = nums.filter { it % 2 == 0 }.map { it * 2 }

ArkTS

const nums = [1, 2, 3, 4];
const evens = nums.filter((x) => x % 2 === 0).map((x) => x * 2);

推导式别嵌太深；超过两层就改回普通 for，可读性优先。

05match（3.10+）≈ when

Python

cmd = "run"
match cmd:
    case "run":
        print("go")
    case "stop":
        print("halt")
    case _:
        print("other")

Kotlin

when (cmd) {
    "run" -> println("go")
    "stop" -> println("halt")
    else -> println("other")
}

ArkTS

switch (cmd) {
  case "run":
    console.info("go");
    break;
  case "stop":
    console.info("halt");
    break;
  default:
    console.info("other");
}