Find last string in nested arrays recursively?

Question

These are the arrays:

[ 'markdown',
  [ 'para', 'this is a paragraph' ],
  [ 'para', {class: 'custom-class'}, 'another paragraph' ],
  [ 'hr' ],
  [ 'bulletlist',
    [ 'listitem', 'This is a list' ],
    [ 'listitem', 'This is another list' ] ] ]

What I want to do is to find the last string in each of them. In this case, 'this is a paragraph', 'another paragraph', 'This is a list', 'This is another list'. (I'm not interested in arrays with only one string, in this case, hr. But I guess that's another SO question.)

The best I could achieve was this:

for (i = 1; i < tree.length; i++) {
  var node = tree[i]
  var lastItem = node[node.length - 1] 

  console.log(lastItem)
}

\\ this is a paragraph
\\ {class: 'custom-class'}
\\ hr
\\ [ 'listitem', 'This is a list' ]
\\ [ 'listitem', 'This is another list' ]

I think you can see the problem, though 1) the loop sometimes find a nested array/object 2) it's not recursive.

How should I modify the loop so it always find the last string of these arrays?

hr is the last string in [ 'hr' ]. Why that is not included? — thefourtheye
– thefourtheye, Commented Mar 8, 2015 at 10:12

elclanrs · Accepted Answer · 2015-03-08 11:30:09Z

2

Using a reducer with an accumulator, and recursion:

var findLast = function(xs) {
  return xs.reduce(function(acc, x) {
    if (Array.isArray(x)) {
      var last = x[x.length-1]
      if (Array.isArray(last)) {
        return acc.concat(findLast(x))
      }
      return acc.concat(last)
    }
    return acc
  },[])
}

console.log(findLast(yourArray))
/*^
[ 'this is a paragraph',
  'another paragraph',
  'hr',
  'This is a list',
  'This is another list' ]
*/

Note that recursion isn't a necessity, since a recursive problem can always be solved using a loop, and a stack if necessary. Recursion is more elegant, but in JavaScript it would blow up the stack with many nested arrays (until browsers ship TCO) but a loop won't:

var findLast = function(xs) {
  var out = []
  for (var i = 0; i < xs.length; i++) {
    var x = xs[i]
    if (Array.isArray(x)) {
      var last = x[x.length-1]
      if (Array.isArray(last)) {
        xs = x
        i = 0
      } else {
        out.push(last)
      }
    }
  }
  return out
}

This solves the general problem you described, but if you want to exclude arrays with a single element you need to add one more condition:

if (Array.isArray(x) && x.length > 1) {

Demo: http://jsfiddle.net/e46wnbv3/

edited Mar 8, 2015 at 11:30

answered Mar 8, 2015 at 10:31

elclanrs

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1 Comment

wyc Over a year ago

Thanks, it worked. Just one thing: how can I do it so that I can actually modify those last items and not just return a copy of them? (That was something that I could do with my original code above.)

Elad Stern · Accepted Answer · 2015-03-08 11:17:29Z

1

I'm having a bit of a hard time understanding your object structure, so I'll start by giving a worded answer. If your object structure becomes clearer I could write an actual coded example.

It's ~~impossible~~ very difficult not to use a recursion here because you don't know how deeply an array might be nested. What you need to do is make a function that receives an object as a parameter. This function then loops through all the elements of the object, calling itself if it sees any of them is an array. When it reaches the last element, it is printed.

Basically along the lines of:

function findLastString(obj)
    loop obj.length
        if obj[i] == array
        then findLastString(obj[i])
    end loop
    if obj[obj.length-1] != array
    then print(obj[obj.length-1])
end function

edited Mar 8, 2015 at 11:17

answered Mar 8, 2015 at 10:29

Elad Stern

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2 Comments

elclanrs Over a year ago

"It's impossible not to use a recursion" -- It is not impossible, you can always use a loop and a stack if necessary.

Elad Stern Over a year ago

Ok, the word "impossible" is a bit harsh, but it really doesn't make sense to handle this type of structure without a recursion.

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