Elqouent Javascript Exercise "nth"

Question

I have the following code for the nth function in the Chapter 4 exercises in Eloquent JavaScript:

function nth(list, num){
   var node = list;
   if(num === 0) {
     return node.value;
   } else {
      if (node.rest) {
         node = node.rest;
         nth(node, num - 1);
      } else {
         return undefined;
      }
   }
}

console.log(nth(arrayToList([10,20,30]),1));
// -> 20

I have an iterative version of the function that works as expected; however, this recursive version of the function returns undefined even though a call to console.log right before the return statement correctly prints node.value. Why?

Here is the arrayToList function as well:

function arrayToList(arr){
   var list = {value: arr[arr.length - 1], rest: null};
   for (var i = arr.length - 2; i >=0; i--){
      list = {value: arr[i], rest: list};
   }
   return list;
}

You're missing `return` on the recursive call. If you don't return what the recursive call returns, the result will be `undefined`. — Pointy, Aug 25 '16 at 22:05
Because you never do `return nth(node, num - 1);` so the function executes `nth(...)` and then never returns the result of that function. Always define explicit return values everywhere. — Akshat Mahajan, Aug 25 '16 at 22:05
an alternative: `var arrayToList = arr => arr.reduceRight((rest, value) => ({value, rest}), null);` and `var nth = (list, num) => list /* && num >= 0*/? num? nth(list.rest, num-1): list.value: void 0;` — Thomas, Aug 25 '16 at 22:17

score 1 · Accepted Answer · answered Aug 25 '16 at 22:05

1

You need to return the recursively obtained result:

return nth(node, num - 1);

answered Aug 25 '16 at 22:05

trincot

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Thanks! That was it – Chris Capps Aug 25 '16 at 22:21

Elqouent Javascript Exercise "nth"

1 Answers1