JS 加法乘法错误解决 number-precision¶

问题描述¶

一个项目中，需要计算 0.56 * 100 然后转换成字符串，结果计算错误，所以调研使用这个库实现。

我具体使用 number-precision 这个库，可以实现 JS 精确四则运算。

为什么使用这个库¶

0.1 + 0.2 = 0.30000000000000004
1.0 - 0.9 = 0.09999999999999998
0.105.toFixed(2) = 0.1 // not 0.11

安装¶

npm install number-precision --save

主要方法¶

可以计算加减乘除（plus，minus，times, divides）以及求相近值（strip, round）

NP.strip(num)         // strip a number to nearest right number
NP.plus(num1, num2, num3, ...)   // addition, num + num2 + num3, two numbers is required at least.
NP.minus(num1, num2, num3, ...)  // subtraction, num1 - num2 - num3
NP.times(num1, num2, num3, ...)  // multiplication, num1 * num2 * num3
NP.divide(num1, num2, num3, ...) // division, num1 / num2 / num3
NP.round(num, ratio)  // round a number based on ratio

具体使用¶

import NP from 'number-precision';

NP.strip(0.09999999999999998); // = 0.1
NP.plus(0.1, 0.2);             // = 0.3, not 0.30000000000000004
NP.plus(2.3, 2.4);             // = 4.7, not 4.699999999999999
NP.minus(1.0, 0.9);            // = 0.1, not 0.09999999999999998
NP.times(3, 0.3);              // = 0.9, not 0.8999999999999999
NP.times(0.362, 100);          // = 36.2, not 36.199999999999996
NP.divide(1.21, 1.1);          // = 1.1, not 1.0999999999999999
NP.round(0.105, 2);            // = 0.11, not 0.1

如果你想避免下面的警告（报错：数值转换整数越界，结果不精确），可以在文档开头增加下面的提示

PS: If you want to get rid of XXX is beyond boundary when transfer to integer, the results may not be accurate, use this at the beginning of your app to turn off boundary checking.

NP.enableBoundaryChecking(false); // default param is true

正文已经结束，后面是简单的源码分析，看一下作者如何实现精确计算。

源码分析¶

这里使用 TS 实现精确计算。

type numType = number | string;
/**
 * @desc 解决浮动运算问题，避免小数点后产生多位数和计算精度损失。
 * 问题示例：2.3 + 2.4 = 4.699999999999999，1.0 - 0.9 = 0.09999999999999998
 */

/**
 * 把错误的数据转正
 * strip(0.09999999999999998)=0.1
 */
function strip(num: numType, precision = 15): number {
  return +parseFloat(Number(num).toPrecision(precision));
}

/**
 * Return digits length of a number
 * @param {*number} num Input number
 */
function digitLength(num: numType): number {
  // Get digit length of e
  const eSplit = num.toString().split(/[eE]/);
  const len = (eSplit[0].split('.')[1] || '').length - +(eSplit[1] || 0);
  return len > 0 ? len : 0;
}

/**
 * 把小数转成整数，支持科学计数法。如果是小数则放大成整数
 * @param {*number} num 输入数
 */
function float2Fixed(num: numType): number {
  if (num.toString().indexOf('e') === -1) {
    return Number(num.toString().replace('.', ''));
  }
  const dLen = digitLength(num);
  return dLen > 0 ? strip(Number(num) * Math.pow(10, dLen)) : Number(num);
}

/**
 * 检测数字是否越界，如果越界给出提示
 * @param {*number} num 输入数
 */
function checkBoundary(num: number) {
  if (_boundaryCheckingState) {
    if (num > Number.MAX_SAFE_INTEGER || num < Number.MIN_SAFE_INTEGER) {
      console.warn(`${num} is beyond boundary when transfer to integer, the results may not be accurate`);
    }
  }
}

/**
 * 精确乘法
 */
function times(num1: numType, num2: numType, ...others: numType[]): number {
  if (others.length > 0) {
    return times(times(num1, num2), others[0], ...others.slice(1));
  }
  const num1Changed = float2Fixed(num1);
  const num2Changed = float2Fixed(num2);
  const baseNum = digitLength(num1) + digitLength(num2);
  const leftValue = num1Changed * num2Changed;

  checkBoundary(leftValue);

  return leftValue / Math.pow(10, baseNum);
}

/**
 * 精确加法
 */
function plus(num1: numType, num2: numType, ...others: numType[]): number {
  if (others.length > 0) {
    return plus(plus(num1, num2), others[0], ...others.slice(1));
  }
  const baseNum = Math.pow(10, Math.max(digitLength(num1), digitLength(num2)));
  return (times(num1, baseNum) + times(num2, baseNum)) / baseNum;
}

/**
 * 精确减法
 */
function minus(num1: numType, num2: numType, ...others: numType[]): number {
  if (others.length > 0) {
    return minus(minus(num1, num2), others[0], ...others.slice(1));
  }
  const baseNum = Math.pow(10, Math.max(digitLength(num1), digitLength(num2)));
  return (times(num1, baseNum) - times(num2, baseNum)) / baseNum;
}

/**
 * 精确除法
 */
function divide(num1: numType, num2: numType, ...others: numType[]): number {
  if (others.length > 0) {
    return divide(divide(num1, num2), others[0], ...others.slice(1));
  }
  const num1Changed = float2Fixed(num1);
  const num2Changed = float2Fixed(num2);
  checkBoundary(num1Changed);
  checkBoundary(num2Changed);
  // fix: 类似 10 ** -4 为 0.00009999999999999999，strip 修正
  return times(num1Changed / num2Changed, strip(Math.pow(10, digitLength(num2) - digitLength(num1))));
}

/**
 * 四舍五入
 */
function round(num: numType, ratio: number): number {
  const base = Math.pow(10, ratio);
  return divide(Math.round(times(num, base)), base);
}

let _boundaryCheckingState = true;
/**
 * 是否进行边界检查，默认开启
 * @param flag 标记开关，true 为开启，false 为关闭，默认为 true
 */

// 这里可以设置边界检查（默认是true）
function enableBoundaryChecking(flag = true) {
  _boundaryCheckingState = flag;
}

// 输出上面的方法
export { strip, plus, minus, times, divide, round, digitLength, float2Fixed, enableBoundaryChecking };
export default {
  strip,
  plus,
  minus,
  times,
  divide,
  round,
  digitLength,
  float2Fixed,
  enableBoundaryChecking,
};

参考链接¶

https://developer.aliyun.com/mirror/npm/package/number-precision

Last update: September 12, 2022