如何使用递归计算数组中数字的实例？

Question

I try to write a function (called: tally) using recursion (part of the exercise) to scan through an array of numbers and return an object with the numbers as key and the number of instances as value.

Example:

tally([2,3,4,5,5,5,5,5,5,5,6,7,,6,7,6,7,5,4,3,4,5,5,6])
//{2: 1, 3: 2, 4: 3, 5: 10, 6: 4, 7: 3}

I created the framework but i am not sure about the syntax to make it work:

function tally(arr) {
    var obj = {}
    if (/*check if object ('obj') has a key corresponding to the array element*/) {
        //increase key's value by onee
    } else {
        //add key with value of 1
    }
    return obj
};

Any hints to complete the recursion function above? Please try to stick to my structure in your answers as much as possible since this is part of an exercise.

Answer 1

Here you are:

function tally(arr) {
    if (arr.length == 0) {
        return {}
    }
    var value = arr.pop()
    var obj = tally(arr)
    if (value in obj) {
        obj[value] += 1
    } else {
        obj[value] = 1
    }
    return obj
};

EDIT: It can also be done using slice() instead of pop() :

function tally(arr) {
    if (arr.length == 0) {
        return {}
    }
    var value = arr[0]
    var obj = tally(arr.slice(1))
    if (value in obj) {
        obj[value] += 1
    } else {
        obj[value] = 1
    }
    return obj
};

Answer 2

ok, so you are asked to do a recursion just for the sake of it.

This could be done (albeit is hacky) passing an extra parameter to tally . When you declare a function in vanilla js you can actually feed it extra stuff. So, in each recursion, pass obj as a second parameter:

EDIT Thanks @Bergi, you're right. I'll edit the code

 function tally(arr) { let obj = arguments.length>1? arguments[1]: {}; if(arr.length===0) { return obj; } let next_number=arr.pop(); obj[next_number]=obj[next_number]||0; obj[next_number]++; return tally(arr,obj); }; let inputArr = [2,3,4,5,5,5,5,5,5,5,6,7,6,7,6,7,5,4,3,4,5,5,6], outputObj=tally(inputArr); console.log(outputObj); console.log({outputEmpty:tally([])});

Answer 3

I am not sure how to guide you to an answer without giving it away entirely, but this is what I would recommend. (There are some problems such as you destroy arr in the process that you may want to consider)

function tally(arr, obj) {

    // if the length is zero we've gone through every value
    if(arr.length === 0)
        return obj

    // create obj if we didn't provide it
    if(obj === undefined)
        obj = {}

    // pull the last value from arr
    let val = arr.pop()

    if (/*check if object ('obj') has a key corresponding to the array element*/) {
        //increase key's value by onee
    } else {
        //add key with value of 1
    }

    // move onto the next value
    return tally(arr,obj)

}

EDIT: took @Bergi's input

Answer 4

Using extra parameter for an index, i , the result, r -

 const plus1 = (k = "", r = {}) => ( k in r? r[k] += 1: r[k] = 1, r ) const tally = (a = [], i = 0, r = {}) => i >= a.length? r: tally ( a, i + 1, plus1(a[i], r) ) console.log(tally([2,3,4,5,5,5,5,5,5,5,6,7,,6,7,6,7,5,4,3,4,5,5,6]))

Output

{
  "2": 1,
  "3": 2,
  "4": 3,
  "5": 10,
  "6": 4,
  "7": 3,
  "undefined": 1
}