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【メモ】JavaScriptで簡単なデータ処理をしたいときの備忘録

Last updated at Posted at 2025-03-19

NumPy使えよってのはさておき。about:blankでどうにかしちゃう系エンジニア向けの記事です。こんな感じのデータからいろいろ出していきます。あんまり大きいデータをいじる気はないのでパフォーマンスは考えていません。


const arr = [ 3, 1, 4, 1, 5, 9, 2, 6, 5, 3 ]

平均値


const average = data => data.reduce((s,e)=>s + e, 0)/data.length;

console.log(average(arr)); // 3.9

中央値


const median = data => {
    const sorted = data.sort((a,b) => {
      return a - b;
    })
    return (sorted[Math.floor(sorted.length/2)] + sorted[Math.floor((sorted.length - 1)/2)])/2
};

console.log(median(arr)); // 3.5

最頻値


const mode = data => [...new Set(data)].reduce((prev, curr) => {
    const prevCount = data.reduce((s, e) => s + (e === prev[0] ? 1 : 0), 0);
    const currCount = data.reduce((s, e) => s + (e === curr ? 1 : 0), 0);
    return (
        prevCount < currCount ? 
            [curr] :
            (prevCount === currCount ? [...prev, curr] : prev)
    )
}, []);

console.log(mode(arr)); // [ 3, 1, 5 ]

標準偏差

上記average関数を使います。$\sigma^2 = \bar{x^2}-(\bar{x})^2$という有名等式を使うだけです。


const stDev = data => Math.sqrt(average(data.map(e => e**2)) - average(data)**2);

console.log(stDev(arr)); // 2.3430749027719964

偏差値

average,stDevから愚直に求めるだけです。


const devValue = (data, value) => 50 + 10 * (value - average(data))/stDev(data) 

console.log(devValue(arr, 3)) // 46.15889360201312

度数表


const frequency = data => data.reduce((prev, curr) => {
    prev[curr] = (prev[curr] ?? 0) + 1;
    return prev;
}, {})

console.log(JSON.stringify(frequency(arr)));
// {"1":2,"2":1,"3":2,"4":1,"5":2,"6":1,"9":1}

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