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第 13题(2019-08-7):为什么for循环嵌套顺序会影响性能? #7

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qappleh opened this issue Aug 7, 2019 · 1 comment

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@qappleh
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qappleh commented Aug 7, 2019

var t1 = new Date().getTime()
for (let i = 0; i < 100; i++) {
  for (let j = 0; j < 1000; j++) {
    for (let k = 0; k < 10000; k++) {
    }
  }
}
var t2 = new Date().getTime()
console.log('first time', t2 - t1)

for (let i = 0; i < 10000; i++) {
  for (let j = 0; j < 1000; j++) {
    for (let k = 0; k < 100; k++) {

    }
  }
}
var t3 = new Date().getTime()
console.log('two time', t3 - t2)  
@qappleh
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qappleh commented Aug 8, 2019

两个循环的次数的是一样的,但是 j 与 k 的初始化次数是不一样的

第一个循环的 j 的初始化次数是 100 次,k 的初始化次数是 10w 次
第二个循环的 j 的初始化次数是 1w 次, k 的初始化次数是 1000w 次
所以相同循环次数,外层越大,越影响性能

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