Weird result of using recursion to concat array in javascript

Question

I want to use recursion to build list to array function but the expected result is reversed to real solution. How could I improve the function of listToArray(list)

function arrayToList(arr){
    if(arr.length==1){
        return {value:arr.pop(), rest:null};
    }else{
        return {value:arr.pop(), rest: arrayToList(arr)};
    }
}

//weired result can't find answer
function listToArray(list){
    if(list.rest == null){
        return [list.value];
    }else{
        return [list.value].concat(listToArray(list.rest));
    }
}

console.log(arrayToList([10, 20]));
// → {value: 10, rest: {value: 20, rest: null}}
console.log(listToArray(arrayToList([10, 20, 30])));
// → [10, 20, 30]

Well pop removes the last element.... so you are reading the values from the end.... — epascarello
– epascarello, Commented Feb 10, 2022 at 14:44
What is your question? What is weird about the result? What do you want to improve, specifically? — Sebastian Simon
– Sebastian Simon, Commented Feb 10, 2022 at 14:44
listToArray should output [10, 20, 30] but the result is [30, 20, 10] — The blank
– The blank, Commented Feb 10, 2022 at 14:48

epascarello · Accepted Answer · 2022-02-10 14:48:02Z

1

pop() removes the last item so you are reading from the end to the start. So read from the start to the end using shift()

function arrayToList(arr){
    if(arr.length==1){
        return {value:arr.shift(), rest:null};
    }else{
        return {value:arr.shift(), rest: arrayToList(arr)};
    }
}

//weired result can't find answer
function listToArray(list){
    if(list.rest == null){
        return [list.value];
    }else{
        return [list.value].concat(listToArray(list.rest));
    }
}

console.log(arrayToList([10, 20]));
// → {value: 10, rest: {value: 20, rest: null}}
console.log(listToArray(arrayToList([10, 20, 30])));
// → [10, 20, 30]

answered Feb 10, 2022 at 14:48

epascarello

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1 Comment

The blank Over a year ago

Thanks, make a stupid mistake

Lajos Arpad · Accepted Answer · 2022-02-10 15:45:38Z

The simplest solution is to concat the other way around, so replace

[list.value].concat(listToArray(list.rest));

with

(listToArray(list.rest)).concat([list.value]);

See the snippet below

function arrayToList(arr){
    if(arr.length==1){
        return {value:arr.pop(), rest:null};
    }else{
        return {value:arr.pop(), rest: arrayToList(arr)};
    }
}

//weired result can't find answer
function listToArray(list){
    if(list.rest == null){
        return [list.value];
    }else{
        return (listToArray(list.rest)).concat([list.value]);
    }
}

console.log(arrayToList([10, 20]));
// → {value: 10, rest: {value: 20, rest: null}}
console.log(listToArray(arrayToList([10, 20, 30])));
// → [10, 20, 30]

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