Replace empty string with null in array efficiently

Question

I want to know the most efficient way of replacing empty strings in an array with null values.

I have the following array:

string[] _array = new string [10];
_array[0] = "A";
_array[1] = "B";
_array[2] = "";
_array[3] = "D";
_array[4] = "E";
_array[5] = "F";
_array[6] = "G";
_array[7] = "";
_array[8] = "";
_array[9] = "J";

and I am currently replacing empty strings by the following:

for (int i = 0; i < _array.Length; i++)
{
    if (_array[i].Trim() == "")
    {
        _array[i] = null;
    }
}

which works fine on small arrays but I'm chasing some code that is the most efficient at doing the task because the arrays I am working with could be much larger and I would be repeating this process over and over again.

Is there a linq query or something that is more efficient?

MarcinJuraszek · Accepted Answer · 2016-04-21 02:05:16Z

6

You might consider switching _array[i].Trim() == "" with string.IsNullOrWhitespace(_array[i]) to avoid new string allocation. But that's pretty much all you can do to make it faster and still keep sequential. LINQ will not be faster than a for loop.

You could try making your processing parallel, but that seems like a bigger change, so you should evaluate if that's ok in your scenario.

Parallel.For(0, _array.Length, i => {
    if (string.IsNullOrWhitespace(_array[i]))
    {
        _array[i] = null;
    }
});

edited Apr 21, 2016 at 2:05

answered Apr 21, 2016 at 2:01

MarcinJuraszek

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Igor · Accepted Answer · 2016-04-21 02:03:38Z

3

As far as efficiency it is fine but it also depends on how large the array is and the frequency that you would be iterating over such arrays. The main problem I see is that you could get a NullReferenceException with your trim method. A better approach is to use string.IsNullOrEmpty or string.IsNullOrWhiteSpace, the later is more along the lines of what you want but is not available in all versions of .net.

for (int i = 0; i < _array.Length; i++)
{
    if (string.IsNullOrWhiteSpace(_array[i]))
    {
        _array[i] = null;
    }
}

edited Apr 21, 2016 at 2:03

answered Apr 21, 2016 at 2:02

Igor

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4 Comments

JanR Over a year ago

you will no longer need the .Trim()

Igor Over a year ago

@JanR - thanks, I did not catch that in my alteration, that was the whole point of my answer :)

Paul Bartlett Over a year ago

I cannot imagine you can get much faster than this unless you resort to native code and do assembly! or you implement IsNullOrWhiteSpace yourself in the loop to save a Call instruction. The code for IsNulOrWhitespace is here referencesource.microsoft.com/#mscorlib/system/…

Igor Over a year ago

@PaulBartlett - the only thing that could also be done is to use the TPL library like MarcinJuraszek proposed. Other than that I agree, this is probably as good as it gets.

Ian · Accepted Answer · 2016-04-21 02:02:29Z

2

LINQ is mainly used for querying not for assignment. To do certain action on Collection, you could try to use List. If you use List instead of Array, you could do it with one line instead:

_list.ForEach(x => string.IsNullOrWhiteSpace(x) ? x = null; x = x);

answered Apr 21, 2016 at 2:02

Ian

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1 Comment

MarcinJuraszek Over a year ago

It's less code, but will actually be slower than an array+for loop.

ohiodoug · Accepted Answer · 2016-04-21 02:04:32Z

A linq query will do essentially the same thing behind the scenes so you aren't going to gain any real efficiency simply by using linq.

When determining something more efficient, look at a few things:

How big will your array grow?
How often will the data in your array change?
Does the order of your array matter?

You've already answered that your array might grow to large sizes and performance is a concern.

So looking at options 2 and 3 together, if the order of your data doesn't matter then you could keep your array sorted and break the loop after you detect non-empty strings.

Ideally, you would be able to check the data on the way in so you don't have to constantly loop over your entire array. Is that not a possibility?

Hope this at least gets some thoughts going.

Paul Bartlett · Accepted Answer · 2016-04-21 02:36:14Z

0

It's ugly, but you can eliminate the CALL instruction to the RTL, as I mentioned earlier, with this code:

if (_array[i] != null) {
  Boolean blank = true;
  for(int j = 0; j < value.Length; j++) {
    if(!Char.IsWhiteSpace(_array[i][j])) { 
        blank = false;
        break;
    }
  }

  if (blank) {
    _array[i] = null;
  }
}

But it does add an extra assignment and includes an extra condition and it is just too ugly for me. But if you want to shave off nanoseconds off a massive list then perhaps this could be used. I like the idea of parallel processing and you could wrap this with Parallel.

answered Apr 21, 2016 at 2:36

Paul Bartlett

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Comments

Maddy · Accepted Answer · 2018-05-17 18:25:47Z

-1

Use the below code

_array = _array.Select(str => { if (str.Length == 0) str = null; return str; }).ToArray();

edited May 17, 2018 at 18:25

answered Apr 21, 2016 at 2:18

Maddy

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1 Comment

Maddy Over a year ago

You can also use string.IsNullOrWhitespace(str) instead of str.length ==0 in the above code snippet

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