重构这个确定数字是十、百、千等的基本代码 [英] refactor this basic code that determines if number is in tens, hundreds, thousands etc

问题描述

 if (n<100) {
     x = 10;
 } else if (n<1000) {
     x = 100;
 } else if (n<10000) {
     x = 1000;
 } else if (n....

等等.等

对于此类问题，是否有一种简洁、可扩展的方法?我的大脑决定停止工作.

Is there a nice concise, scalable method for this type of problem? My brain has decided to stop working.

推荐答案

var x = Math.pow( 10, Math.floor( Math.log(n) / Math.log(10) ) );

这个可能更可取，因为它可以作为常量使用，因此不需要每次都计算:

This one might be preferable, because it's available as a constant so it doesn't need to be calculated each time:

var x = Math.pow( 10, Math.floor( Math.log(n) / Math.LN10 ) );

基本上归结为对数四舍五入.

It basically comes down to rounding down logarithmically.

编辑:由于 log 和 / 的组合通常会导致错误，因此将商四舍五入可能是个好主意以及(目前，Math.log(1000)/Math.LN10 === 2.9999999999999996).

Because a combination of log and / often tends to result in errors, it might be a good idea to round the quotient as well (currently, Math.log(1000) / Math.LN10 === 2.9999999999999996).

var x = Math.pow( 10, Math.floor( Math.round( Math.log(1000) / Math.LN10 * 1e10 ) / 1e10 ) );

这种四舍五入可能会导致新的错误，但对于正常"输入，现在的答案通常是正确的.

This rounding might result in new errors, but for 'normal' inputs the answer is now more often correct.

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