【Use Recursion to Create a Countdown】フリーコードキャンプを日本語で解説！

おいーす、もりたけです。

フリーコードキャンプを日本語で解説！の第108弾です。

JavaScript Algorithms and Data Structures Certification (300 hours)の中の、

Basic JavaScriptの中の、

Use Recursion to Create a Countdown

です。

さっそく見ていきましょう。

まずは本文から。

In a previous challenge, you learned how to use recursion to replace a for loop. Now, let’s look at a more complex function that returns an array of consecutive integers starting with 1 through the number passed to the function.

As mentioned in the previous challenge, there will be a base case. The base case tells the recursive function when it no longer needs to call itself. It is a simple case where the return value is already known. There will also be a recursive call which executes the original function with different arguments. If the function is written correctly, eventually the base case will be reached.

For example, say you want to write a recursive function that returns an array containing the numbers 1 through n. This function will need to accept an argument, n, representing the final number. Then it will need to call itself with progressively smaller values of n until it reaches 1. You could write the function as follows:

function countup(n) {

  if (n < 1) {

    return [];

  } else {

    const countArray = countup(n – 1);

    countArray.push(n);

    return countArray;

  }

}

console.log(countup(5)); // [ 1, 2, 3, 4, 5 ]

At first, this seems counterintuitive since the value of n decreases, but the values in the final array are increasing. This happens because the push happens last, after the recursive call has returned. At the point where n is pushed into the array, count(n - 1) has already been evaluated and returned [1, 2, ..., n - 1].

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We have defined a function called countdown with one parameter (n). The function should use recursion to return an array containing the integers n through 1 based on the n parameter. If the function is called with a number less than 1, the function should return an empty array. For example, calling this function with n = 5 should return the array [5, 4, 3, 2, 1]. Your function must use recursion by calling itself and must not use loops of any kind.

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解説していきます。

今回は再帰関数の複雑な処理についてです。

１からnの数値を含む配列を返す再帰関数について考えてみましょう。

この関数は、最終的な数値を表す引数nをとります。そして、1に達するまでnの値を徐々に小さくして自分自身を呼び出し、その都度配列を吐き出せば行けそうですね。

次のように書くことができます。