javascript - 三元条件求expmod？

Question

我正在阅读 SICP in JS关于三元条件的非终止示例:

function is_even(n) {
    return n % 2 === 0;
}

function expmod(base, exp, m) {
    const half_exp = expmod(base, exp / 2, m);
    return exp === 0 ? 1
           : is_even(exp) ? half_exp * half_exp % m
           : base * expmod(base, exp - 1, m) % m;
}

console.log(expmod(4, 3, 5))

它解释说:

This would make the function not just inefficient, but actually non-terminating! The problem is that the constant declaration appears outside the conditional expression, which means that it is executed even when the base case exp === 0 is met.

我只是不明白它的想法，当 exp === 0 时，它以 1 终止，但为什么要执行 half_exp？

Answer 1

您在执行的第一行进行递归调用，无论如何。这意味着该函数是非终止的，也就是无限循环。

function expmod(base, exp, m) {
    const half_exp = expmod(base, exp / 2, m); // <- recursive call
    // code ...
}

递归调用应始终与某种检查配对以确保存在退出点。