jsreduce的用法,函数式编程的七件武器之reduce与map
jsreduce的用法,函数式编程的七件武器之reduce与map有了reduce,我们得以用“描述”的方式,以decalratively的方式求得average:# begin_src js :results output const myArray = [22 9 60 12 4 56]; const sumAndLog = (x y) => { console.log(`${x} ${y}=${x y}`); return x y; }; myArray.reduce(sumAndLog 0); # end_src # RESULTS: : 0 22=22 : 22 9=31 : 31 60=91 : 91 12=103 : 103 4=107 : 107 56=163求均值求数列的和首先从耳熟能详的求数列之和起步。const myArray = [22 9 60 12 4 56]; const su
JavaScript是当今流行语言中对函数式编程支持最好的编程语言。函数式编程的七个函数分别为:
- reduce() and reduceRight() to apply an operation to a whole array reducing it to a single result
- map() to transform one array into another by applying a function to each of its elements
- flat() to make a single array out of an array of arrays
- flatMap() to mix together mapping and flattening
- forEach() to simplify writing loops by abstracting the necessary looping code
以及 search 与 selection 的函数:
- filter() to pick some elements from an array
- find() and findIndex() to search for elements that satisfy a condition
- A pair of predicates every() and some() to check an array for a Boolean test
一、array.reduce() 将数列降维至一个值
当我们处理array的时候,总是陷入到无穷尽的loop循环之中,掉入进琐碎的陷阱,戕害我们的思维和大脑。
reduce的基本工作原理如下:
求数列的和
首先从耳熟能详的求数列之和起步。
const myArray = [22 9 60 12 4 56];
const sum = (x y) => x y;
const mySum = myArray.reduce(sum 0); // 163
观察其运行轨迹:
# begin_src js :results output
const myArray = [22 9 60 12 4 56];
const sumAndLog = (x y) => {
console.log(`${x} ${y}=${x y}`);
return x y;
};
myArray.reduce(sumAndLog 0);
# end_src
# RESULTS:
: 0 22=22
: 22 9=31
: 31 60=91
: 91 12=103
: 103 4=107
: 107 56=163
求均值
有了reduce,我们得以用“描述”的方式,以decalratively的方式求得average:
const average = arr => arr.reduce(sum 0) / arr.length;
console.log(average(myArray)); // 27.166667
求均值的第二种方法,将length写到里面:
const average2 = (sum val ind arr) => {
sum = val;
return ind === arr.length - 1 ? sum / arr.length
: sum; //将这作为思考的原材料
};
console.log(myArray.reduce(average2 0)); // 27.166667s
更近一步,将average作为固有属性:
Array.prototype.average = function() {
return this.reduce((x y) => x y 0) / this.length;
};
let myAvg = [22 9 60 12 4 56].average(); // 27.166667
单词计算多个值
虽然 reduce 只能返回单个结果,但是此返回结果却可以包含多个元素,比如是object。
const average3 = arr => {
const sumCount = arr.reduce(
(accum value) => ({sum: value accum.sum count: accum.count 1})
{sum: 0 count: 0}
);
return sumCount.sum / sumCount.count;
};
console.log(average3([7 11 19 23]));
以array的方式改写:
const average4 = arr => {
const sumCount = arr.reduce(
(accum value) => [accum[0] value xaccum[1] 1]
[0 0]
);
return sumCount[0] / sumCount[1];
};
console.log(average4(myArray)); // 27.166667
从右往左的折叠
工作原理如下图:
比如 reverse 字符串的常规解决方案为:
const reverseString = str => {
let arr = str.split("");
arr.reverse();
return arr.join("");
};
console.log(reverseString("MONTEVIDEO")); // OEDIVETNOM
而reduceRight的解题方案呢,
const reverseString2 = str =>
str.split("").reduceRight((x y) => x y "");
console.log(reverseString2("OEDIVETNOM")); // MONTEVID
二、array.map 从数学到编程
map首先是数学上的概念。
从object中提取数据
const markers = [
{name: "AR" lat: -34.6 lon: -58.4}
{name: "BO" lat: -16.5 lon: -68.1}
{name: "BR" lat: -15.8 lon: -47.9}
{name: "CL" lat: -33.4 lon: -70.7}
{name: "CO" lat: 4.6 lon: -74.0}
{name: "EC" lat: -0.3 lon: -78.6}
{name: "PE" lat: -12.0 lon: -77.0}
{name: "PY" lat: -25.2 lon: -57.5}
{name: "UY" lat: -34.9 lon: -56.2}
{name: "VE" lat: 10.5 lon: -66.9}
];
let averageLat = average(markers.map(x => x.lat)); // -15.76
let averageLon = average(markers.map(x => x.lon)); // -65.53
// extended array.prototype
let averageLat2 = markers.map(x => x.lat).average();
let averageLon2 = markers.map(x => x.lon).average();
悄无声息的处理数据
看一个我们想当然的应用:
["123.45" "67.8" "90"].map(parseFloat);
// [123.45 67.8 90]
["123.45" "-67.8" "90"].map(parseInt);
// [123 NaN NaN]
这是因为 parseInt 有一个 optional 的参数 radix。
数列的表示方法
现在我们来创建一个 range.
const range = (start stop) =>
new Array(stop - start).fill(0).map((v i) => start i);
// 必须写一个v,也必须写 new
let from2To6 = range(2 7); // [2 3 4 5 6]
尝试求乘方:
const range = (start stop) =>
new Array(stop - start).fill(0).map((v i) => start i);
const factorialByRange = n => range(1 n 1).reduce((x y) => x * y 1);
factorialByRange(5); // 120
factorialByRange(3);
尝试字母表:
const ALPHABET = range("A".charCodeAt() "Z".charCodeAt() 1).map(x =>
String.fromCharCode(x)
);
// ["A" "B" "C" ... "X" "Y" "Z"]
用 reduce 构造 map
reduce是所有其他函数的起点,
const myMap = (arr fn) => arr.reduce((x y) => x.concat(fn(y)) []);
尝试两种不同的解决方案:
const myArray = [22 9 60 12 4 56];
const dup = x => 2 * x;
console.log(myArray.map(dup)); // [44 18 120 24 8 112]
console.log(myMap(myArray dup)); // [44 18 120 24 8 112]
console.log(myArray); // [22 9 60 12 4 56]