將數組連接到自身的速度比循環數組創建更多索引的速度快嗎？

Question

我只是考慮看看通過源的three.js所GitHub的頁面上的例子，我碰到這個ImprovedNoise類，這基本上是一個柏林噪聲腳本來了：

https://github.com/mrdoob/three.js/blob/master/examples/js/ImprovedNoise.js

在很該ImprovedNoise功能的頂部是這樣的：

var p = [151,160,137,91,90,15,131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10, 
     23,190,6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,88,237,149,56,87, 
     174,20,125,136,171,168,68,175,74,165,71,134,139,48,27,166,77,146,158,231,83,111,229,122,60,211, 
     133,230,220,105,92,41,55,46,245,40,244,102,143,54,65,25,63,161,1,216,80,73,209,76,132,187,208, 
     89,18,169,200,196,135,130,116,188,159,86,164,100,109,198,173,186,3,64,52,217,226,250,124,123,5, 
     202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,223,183,170,213,119, 
     248,152,2,44,154,163,70,221,153,101,155,167,43,172,9,129,22,39,253,19,98,108,110,79,113,224,232, 
     178,185,112,104,218,246,97,228,251,34,242,193,238,210,144,12,191,179,162,241,81,51,145,235,249, 
     14,239,107,49,192,214,31,181,199,106,157,184,84,204,176,115,121,50,45,127,4,150,254,138,236,205, 
     93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180]; 

for (var i=0; i < 256 ; i++) { 

    p[256+i] = p[i]; 

} 

一旦p陣列建立你會發現，p被填充數字0的隨機排序陣列255，腳本在做了for循環中的每一個位置該陣列並有效地鎖定自己的位置從256到511的順序。順序是相同的，但索引被移動了256.

所以我的問題是這樣的：它在JavaScript中更快地遍歷數組這或簡單地做..

p = p.concat(p); 

Answer 1

它是依賴於瀏覽器：

• Firefox 4的concat約爲1/2。
• Chrome 11提供的concat方法速度提高了10倍。
• IE9給出的concat方法約快3/4
• Safari 5給出的concat方法約快1/3。
• Opera 11給出的concat方法約快1/4。

自己嘗試一下：

http://jsperf.com/concat-vs-forloop

Answer 2

好吧我會爲你的時間。將編輯爲包含更多瀏覽器（執行任一代碼1000000次）。好的瀏覽器測試！機規格是I5 430M和Win7上64位

測試代碼4GB RAM：

<script> 

function do1(){ 
p = [151,160,137,91,90,15,131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10, 
     23,190,6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,88,237,149,56,87, 
     174,20,125,136,171,168,68,175,74,165,71,134,139,48,27,166,77,146,158,231,83,111,229,122,60,211, 
     133,230,220,105,92,41,55,46,245,40,244,102,143,54,65,25,63,161,1,216,80,73,209,76,132,187,208, 
     89,18,169,200,196,135,130,116,188,159,86,164,100,109,198,173,186,3,64,52,217,226,250,124,123,5, 
     202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,223,183,170,213,119, 
     248,152,2,44,154,163,70,221,153,101,155,167,43,172,9,129,22,39,253,19,98,108,110,79,113,224,232, 
     178,185,112,104,218,246,97,228,251,34,242,193,238,210,144,12,191,179,162,241,81,51,145,235,249, 
     14,239,107,49,192,214,31,181,199,106,157,184,84,204,176,115,121,50,45,127,4,150,254,138,236,205, 
     93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180]; 
for (var i=0; i < 256 ; i++) { 

    p[256+i] = p[i]; 

} 
} 

function do2(){ 
p = [151,160,137,91,90,15,131,13,201,95,96,53,194,233,7,225,140,36,103,30,69,142,8,99,37,240,21,10, 
     23,190,6,148,247,120,234,75,0,26,197,62,94,252,219,203,117,35,11,32,57,177,33,88,237,149,56,87, 
     174,20,125,136,171,168,68,175,74,165,71,134,139,48,27,166,77,146,158,231,83,111,229,122,60,211, 
     133,230,220,105,92,41,55,46,245,40,244,102,143,54,65,25,63,161,1,216,80,73,209,76,132,187,208, 
     89,18,169,200,196,135,130,116,188,159,86,164,100,109,198,173,186,3,64,52,217,226,250,124,123,5, 
     202,38,147,118,126,255,82,85,212,207,206,59,227,47,16,58,17,182,189,28,42,223,183,170,213,119, 
     248,152,2,44,154,163,70,221,153,101,155,167,43,172,9,129,22,39,253,19,98,108,110,79,113,224,232, 
     178,185,112,104,218,246,97,228,251,34,242,193,238,210,144,12,191,179,162,241,81,51,145,235,249, 
     14,239,107,49,192,214,31,181,199,106,157,184,84,204,176,115,121,50,45,127,4,150,254,138,236,205, 
     93,222,114,67,29,24,72,243,141,128,195,78,66,215,61,156,180]; 
p = p.concat(p); 
} 

function timeit(func){ 
var date1 = new Date(); 
var start = date1.getTime(); 
for (i=0;i<1000000;i++) func(); 
var date2 = new Date(); 
var end = date2.getTime(); 
alert(end-start); 
} 

timeit(do1); 
//timeit(do2); // uncomment to activate 

</script> 

鉻11

方法1（循環）：

• 4669ms
• 4809ms
• 5103ms
• 個5025ms
• 4786ms

方法2（CONCAT）：

• 387ms
• 370ms
• 494ms
• 640毫秒
• 394ms

歌劇11.1（3次測試..需要長期）

方法1（循環）

• 7884毫秒
• 7621毫秒
• 7546毫秒

注：對於一個我跑了> 98000ms .. IDK發生了什麼事。

方法2（的concat）

• 27684毫秒
• 28479毫秒
• 23539毫秒

IE 9

方法1（循環）

• 個6065ms
• 6026ms
• 6214ms

方法2（的concat）

• 8064毫秒
• 8105毫秒
• 7954毫秒

Answer 3

你並不需要在所有重複的陣列，只需使用P [I％256]來訪問其數字成員。