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我搞不懂浮點數：CS 101 - yllan

所有議程請見議程表。

歡迎來到 https://hackmd.io/@iPlayground/2019 共筆

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預備知識

  • 指數運算
2^3 = 2 * 2 * 2 = 8
2^0 = 1
2^(-3) = 1 / 8

議程目標

希望新手聽完，能知道浮點數運作原理。
老手能夠學到一些edge case。
大家都能避開常見的坑。

0.1+0.2

定點數

浮點數

19.9 ~ 10011.1110011001100…

func testFloatEqual() {
    let e:Double = 1.9 * 10 * 10
    let r:Double = 1.9 * 100
    XCTAssertEqual(e, r) // V
    XCTAssertNotEqual(e.description, r.description) // X
}
    求講者怎麼測的？

浮點數的加法容易出現問題，盡量用乘的。誤差會比較小

與錢有關的，最好都不要扯上關係

DON'T: Use floating point type to store money

var m: Float = 0
for _ in 0..<1000000 {
    m += 0.01
}
// m = 9865.22363

DO: Use Decimal type to store money

var money: Decimal = 0
for _ in 0..<1000000 {
    money += Decimal(1) / Deciaml(100)
}
// money = 10000.000000

能用 Double 就不要用 Float

浮點數運算要避免大數字與小數字加減

range取 max(0.01, utl)

tags:iPlayground2019
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