Timeout/inefficiency error with "for" loop and array.reduce in Javascript

Question

Working through a codewars challenge (Simple Fun #237: Suffix Sums), and while I'm passing all the tests it's giving me a time-out error. The challenge is to make a new array from a given one, where each index of the new array is the summation of that same index in the original to the end of the original array.

For an array 1, 2, 3, -6, the output should be 0, -1, -3, -6.

b[0]= 1 + 2 + 3 - 6 = 0  
b[1]=     2 + 3 - 6 = -1  
b[2]=         3 - 6 = -3  
b[3]=           - 6 = -6

My code is this

function suffixSums(a) {
 var res=[]
 for(i=0;i<a.length;i++){
  var newarray=a.slice([i])
   res.push(newarray.reduce(function(acc, val){ return acc + val },0))
  }
  return res
}

Any clue what's up? I'm just learning still obviously, optimization is a whole new world for me

Hint: start at the end.

BPS
– BPS

2017-05-04 16:32:20 +00:00
Commented May 4, 2017 at 16:32 — BPS
– BPS, Commented May 4, 2017 at 16:32

Nina Scholz · Accepted Answer · 2017-05-04 20:28:36Z

1

You could use the successor of an item and add the actual value while iterating from the end.

function suffixSums(a) {
    var i = a.length - 1;
    while (i--) {
        a[i] += a[i + 1];
    }
    return a;
}

console.log(suffixSums([1, 2, 3, -6]));

edited May 4, 2017 at 20:28

answered May 4, 2017 at 16:42

Nina Scholz

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Comments

ibrahim mahrir · Accepted Answer · 2017-05-04 16:30:40Z

0

You are wasting time because you calculate the sum of each items from scratch, you can start from the end of the array and accumulate the sum as you go:

function suffixSums(a) {
    var res = [],
        sum = 0;                              // sum of items from end to the current index (initialized with 0)
    for(var i = a.length - 1; i >= 0; i--) {  // loop from the end
       sum += a[i];                           // add the current number to sum
       res[i] = sum;                          // add the sum to the result array at index i (this line could be changed to: res.unshift(sum);)
    }
    return res;
}

console.log(suffixSums([1, 2, 3, -6]));

edited May 4, 2017 at 16:30

answered May 4, 2017 at 16:24

ibrahim mahrir

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4 Comments

skdfsdfa Over a year ago

Awesome! Thanks for the commentary, that helps a lot. Default thinking is definitely left to right, didn't occur to me to switch it

Bergi Over a year ago

@skdfsdfa You can do it left-to-right as well, you just have to substract the whole sum from every element

ibrahim mahrir Over a year ago

@Bergi left-to-right is undoable unless you already got the sum, in which case you'll have to loop the array twice.

Bergi Over a year ago

@ibrahimmahrir single-pass or double-pass doesn't make a difference for algorithmic complexity :-)

James · Accepted Answer · 2017-05-04 16:29:04Z

0

If you don't mind changing the original array (Codewars doesn't mind):

function suffixSums(a) {
 for(var i = a.length - 2; i > -1; i--)
   a[i] += a[i+1];
 return a;
}

answered May 4, 2017 at 16:29

James

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Comments

Redu · Accepted Answer · 2017-05-04 18:02:58Z

0

I would do this job like;

var arr = [1, 2, 3, -6],
    res = arr.reduceRight((r,n) => r[0] === void 0 ? [n] : [n+r[0]].concat(r),[]);
console.log(res);

answered May 4, 2017 at 18:02

Redu

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