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@kobalab(小林 聡)

配列を「回転」させる

Last updated at Posted at 2024-10-18

[ 0, 1, 2, 3, 4 ] → [ 3, 4, 0, 1, 2 ] みたいなことをやりたいが、すぐ忘れるのでメモ。

    let array = [ 0, 1, 2, 3, 4 ];
    array = array.splice(3).concat(array);

これで array が [ 3, 4, 0, 1, 2 ] に変更される。再代入を忘れずに。

array.splice(3) は array から添字 3 以降の要素を取り除いて返す。つまり、[ 3, 4 ].concat([ 0, 1, 2]) → [ 3, 4, 0, 1, 2 ] となる寸法。

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Comments

diywmk9
@diywmk9

splice()だけで行なえば再代入不要です。

  const array = [0, 1, 2, 3, 4];
  array.splice(0, 0, ...array.splice(-2));
  console.log(array); // [3, 4, 0, 1, 2]

また、push()、pop()、shift()、unshift()を組み合わせれば、1要素ずつ左右にローテートする処理も作れます。

  const rotateLeft  = arr => arr.push(arr.shift());
  const rotateRight = arr => arr.unshift(arr.pop());

  const array = [0, 1, 2, 3, 4];

  for (let i = array.length; i--;) {
    rotateLeft(array);
    console.log(array);
  }
  // [1, 2, 3, 4, 0]
  // [2, 3, 4, 0, 1]
  // [3, 4, 0, 1, 2]
  // [4, 0, 1, 2, 3]
  // [0, 1, 2, 3, 4]

  for (let i = array.length; i--;) {
    rotateRight(array);
    console.log(array);
  }
  // [4, 0, 1, 2, 3]
  // [3, 4, 0, 1, 2]
  // [2, 3, 4, 0, 1]
  // [1, 2, 3, 4, 0]
  // [0, 1, 2, 3, 4]
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