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| function arrayToList(array) { | |
| let result = {}; | |
| if (Array.isArray(array)) { | |
| let currListItem = result; | |
| for (let item of array) { | |
| let newListItem = { | |
| value: item, | |
| rest: null | |
| }; | |
| if (typeof currListItem.rest === 'undefined') { | |
| result = newListItem; | |
| } else { | |
| currListItem.rest = newListItem; | |
| } | |
| currListItem = newListItem; | |
| } | |
| } | |
| return result; | |
| } | |
| function listToArray(list) { | |
| let result = []; | |
| if (typeof list === 'undefined' || list.value === undefined || list.rest === undefined) { | |
| return result; | |
| } else { | |
| result.push(list.value); | |
| while (list.hasOwnProperty('rest') && list.rest !== null) { | |
| list = list.rest; | |
| if (list.hasOwnProperty('value')) { | |
| result.push(list.value); | |
| } | |
| } | |
| } | |
| return result; | |
| } | |
| function prepend(element, list) { | |
| return { | |
| value: element, | |
| rest: list | |
| }; | |
| } | |
| function nth(list, number) { | |
| return listToArray(list)[number]; | |
| } | |
| function nthRecursive(list, number) { | |
| if (number === 0) { | |
| return list.value; | |
| } else if (list.rest === null) { | |
| return undefined; | |
| } else { | |
| return nthRecursive(list.rest, number-1); | |
| } | |
| } | |
| console.log(arrayToList()); | |
| console.log(arrayToList([10, 20])); | |
| // → {value: 10, rest: {value: 20, rest: null}} | |
| console.log(arrayToList([4, 3, 2, 1])); | |
| // → {value: 4, rest: {value: 3, rest: {value: 2, rest: {value: 1, rest: null}}}} | |
| console.log(listToArray()); | |
| // → [] | |
| console.log(listToArray(arrayToList([10, 20, 30]))); | |
| // → [10, 20, 30] | |
| console.log(listToArray({value: 10, rest: {xxx: 20, yyy: null}})); | |
| // → [10] | |
| console.log(listToArray({value: 10, rest: {value: 20, yyy: null}})); | |
| // → [10, 20] | |
| console.log(listToArray({value: 10, rest: {xxx: 20, rest: null}})); | |
| // → [10] | |
| console.log(prepend(10, prepend(20, null))); | |
| // → {value: 10, rest: {value: 20, rest: null}} | |
| console.log(nth(arrayToList([10, 20, 30]), 1)); | |
| // → 20 | |
| console.log(nth(arrayToList([10, 20, 30]), 3)); | |
| // → undefined | |
| console.log(nth(arrayToList([10, 20, 30]), -2)); | |
| // → undefined | |
| console.log(nthRecursive(arrayToList([10, 20, 30]), 1)); | |
| // → 20 | |
| console.log(nthRecursive(arrayToList([10, 20, 30]), 3)); | |
| // → undefined | |
| console.log(nthRecursive(arrayToList([10, 20, 30]), -2)); | |
| // → undefined | |
| // now with the help of the hints... | |
| function arrayToListWithHints(array) { | |
| let result = {}; | |
| if (Array.isArray(array)) { | |
| let list = null; | |
| array = array.reverse(); | |
| for (let item of array) { | |
| list = { | |
| value: item, | |
| rest: list | |
| }; | |
| } | |
| result = list; | |
| } | |
| return result; | |
| } | |
| function listToArrayWithHints(list) { | |
| let result = []; | |
| if (typeof list === 'undefined' || list.value === undefined || list.rest === undefined) { | |
| return result; | |
| } else { | |
| for (let node = list; node; node = node.rest) { | |
| if (node.hasOwnProperty('value')) { | |
| result.push(node.value); | |
| } | |
| } | |
| } | |
| return result; | |
| } | |
| console.log('\nWith Hints:'); | |
| console.log(arrayToListWithHints()); | |
| console.log(arrayToListWithHints([10, 20])); | |
| // → {value: 10, rest: {value: 20, rest: null}} | |
| console.log(arrayToListWithHints([4, 3, 2, 1])); | |
| // → {value: 4, rest: {value: 3, rest: {value: 2, rest: {value: 1, rest: null}}}} | |
| console.log(listToArrayWithHints()); | |
| // → [] | |
| console.log(listToArrayWithHints(arrayToListWithHints([10, 20, 30]))); | |
| // → [10, 20, 30] | |
| console.log(listToArrayWithHints({value: 10, rest: {xxx: 20, yyy: null}})); | |
| // → [10] | |
| console.log(listToArrayWithHints({value: 10, rest: {value: 20, yyy: null}})); | |
| // → [10, 20] | |
| console.log(listToArrayWithHints({value: 10, rest: {xxx: 20, rest: null}})); | |
| // → [10] | |
| console.log(nth(arrayToListWithHints([10, 20, 30]), 1)); | |
| // → 20 | |
| console.log(nth(arrayToListWithHints([10, 20, 30]), 3)); | |
| // → undefined | |
| console.log(nth(arrayToListWithHints([10, 20, 30]), -2)); | |
| // → undefined |
Hey Jonurry, thanks for those solutions! I was scratching my head around to try to solve these ones.
I was wondering why did you put the first '=== undefined' (on lines 23 and 109), between semicolons, while the other ones not. Is there any specific reason for it? Or it is just your personal preference?
Hey @PiotrNap. Glad it helped you.
The reason that the first undefined is in single quotes is because typeof returns a string value so the comparison has to be a string as well.
Alright, that makes sense ! Ty for answering :)
Hello Jonurry, I'm a newbie in js so I have some questions about your solutions. I hope you can help me.
In arrayToList function, I don't know how the value of "result" changes in the loop. In your loop, the "result" only appears in the if (typeof currListItem.rest === 'undefined') but when I try testing it like this :
if (typeof currListItem.rest === 'undefined') {
result = newListItem;
console.log(true)
} else {
currListItem.rest = newListItem;
console.log(false)
}
The if (typeof currListItem.rest === 'undefined') only works in the first time, so I don't know how the "result" can change its value.
Hints
Building up a list is easier when done back to front. So
arrayToListcould iterate over the array backward (see the previous exercise) and, for each element, add an object to the list. You can use a local binding to hold the part of the list that was built so far and use an assignment likelist = {value: X, rest: list}to add an element.To run over a list (in
listToArrayandnth), a for loop specification like this can be used:for (let node = list; node; node = node.rest) {}Can you see how that works? Every iteration of the loop,
nodepoints to the current sublist, and the body can read itsvalueproperty to get the current element. At the end of an iteration,nodemoves to the next sublist. When that isnull, we have reached the end of the list and the loop is finished.The recursive version of
nthwill, similarly, look at an ever smaller part of the “tail” of the list and at the same time count down the index until it reaches zero, at which point it can return thevalueproperty of the node it is looking at. To get the zeroeth element of a list, you simply take thevalueproperty of its head node. To get element N + 1, you take the _N_th element of the list that’s in this list’s rest property.