爲什麼這不是遞歸定義的減少工作（JS）？

我試圖重新定義JavaScript的降低使用遞歸函數。這是我的嘗試，這是行不通的。如果任何人都可以稍微改變它來使它工作，那將是很棒的，因爲我會更好地理解它。（這是functional-javascript-workshop的練習）。爲什麼這不是遞歸定義的減少工作（JS）？

function reduce(arr, fn, initial) { 
    if (arr.length) { 
    var newArr = arr.slice(1, arr.length); 
    return reduce(newArr, fn, fn(arr[0])); 
    } else { 
    return initial; 
    } 
} 

module.exports = reduce

它給了我下面的神祕的錯誤消息，我不知道如何解釋：

/usr/local/lib/node_modules/functional-javascript-workshop/exercises/basic_recursion/exercise.js:13 
    prev[curr] = ++prev[curr] || 1 
          ^

TypeError: Cannot create property 'undefined' on string 'exercitation' 
    at /usr/local/lib/node_modules/functional-javascript-workshop/exercises/basic_recursion/exercise.js:13:29 
    at reduce (/home/david/node-school/functional-workshop/solution.js:7:28) 
    at /usr/local/lib/node_modules/functional-javascript-workshop/exercises/basic_recursion/exercise.js:12:10 
    at obtainResult (/usr/local/lib/node_modules/functional-javascript-workshop/exercises/runner.js:100:21) 
    at Exercise.<anonymous> (/usr/local/lib/node_modules/functional-javascript-workshop/exercises/runner.js:66:27) 
    at next (/usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:188:19) 
    at /usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:195:7 
    at Exercise.<anonymous> (/usr/local/lib/node_modules/functional-javascript-workshop/exercises/runner.js:34:5) 
    at next (/usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:188:19) 
    at /usr/local/lib/node_modules/functional-javascript-workshop/node_modules/workshopper-exercise/exercise.js:195:7

來源

2017-04-25 David J.

只是想知道：不'如果（arr.length）'工作？我總是用'如果（arr.length> 0）'... – Danmoreng

你是不是向我們展示了正確的代碼。錯誤發生在第7行在你的'solution.js'中調用'exercise.js'中。 –

@Danmoreng，它測試真值 –

您需要更新累加器：

function reduce(arr, fn, acc) { 
 
    if (arr.length) { 
 
    var newArr = arr.slice(1, arr.length); 
 
    acc = fn(arr[0], acc) 
 
    return reduce(newArr, fn, acc); 
 
    } else { 
 
    return acc; 
 
    } 
 
} 
 

 
console.log(reduce([1,2,3], (val, sum) => sum + val, 0))

來源

2017-04-25 14:49:22 thedude

這就像你的意見人。（謝謝！） –

夥計遵守 – thedude

我投票給你只是因爲你是El Duderino – Greeso

你忘了通過ACC的當前值umulator，即initial，進fn呼叫。

來源

2017-04-25 14:51:21 Bergi

正如其他人所指出的，你錯過傳遞累加器fn。如果你很好奇，reduce可以表示爲單個三元組（?:）表達式 - 同樣，arr.slice(1)將從1切片到數組的末尾;沒有必要指定在這種情況下

const reduce = (arr, fn, acc) => 
 
    arr.length === 0 
 
    ? acc 
 
    : reduce(arr.slice(1), fn, fn(acc, arr[0])) 
 

 
const add = (x,y) => x + y 
 

 
console.log(reduce([1,2,3], add, 0)) // 6

因爲arr.length === 0，arr.slice(1)和arr[1]在功能上的編碼處理陣列以普通切片的結束，這是經常可以看到這些抽象成函數降低複雜性和認知負荷

// isEmpty :: [a] -> Boolean 
 
const isEmpty = arr => arr.length === 0 
 

 
// head :: [a] -> a 
 
const head = arr => arr[0] 
 

 
// tail :: [a] -> [a] 
 
const tail = arr => arr.slice(1) 
 

 
// reduce :: ([a], ((b, a) -> b), b) -> b 
 
const reduce = (arr, fn, acc) => 
 
    isEmpty(arr) 
 
    ? acc 
 
    : reduce(tail(arr), fn, fn(acc, head(arr))) 
 

 
// add :: (Number, Number) -> Number 
 
const add = (x,y) => x + y 
 

 
console.log(reduce([1,2,3], add, 0)) // 6

來源

2017-04-25 21:53:25 naomik

爲什麼這不是遞歸定義的減少工作（JS）？

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