Weird Inline Array Initialization

Question

I found this code snippet on a different post that I don't quite understand and would appriciate someone explaining.

private bool[] GetPageNumbersToLink(IPagedResult result)
{
   if (result.TotalPages <= 9)
      return new bool[result.TotalPages + 1].Select(b => true).ToArray();

   ...

C# Array initialization - with non-default value

My take on this:

new bool[result.TotalPages + 1]

this creates an array of bools with length equal to that of the totalpages, the default value being false on all of them.

.Select(b => true)

this uses a linq query to return only the items that are true, i.e. none of them

.ToArray();

linq returns IEnumerable or something, so it needs to go back to a new array, of length 0 as none were selected in the first place.

I think that's what it's doing, but it doesn't make any sense.

One sometimes get confused by the word Select since it sounds like: "filter" the source (however that is called Where); in fact it means: "project" the source. — Jeppe Stig Nielsen
– Jeppe Stig Nielsen, Commented Nov 13, 2014 at 19:59

Jon Skeet · Accepted Answer · 2014-11-13 19:55:59Z

8

(EDIT: Oops, misread the post (unless it was edited without me noticing).)

No, that's not what it's doing. The result is effectively:

Enumerable.Repeat(true, result.TotalPages + 1).ToArray();

The Select call doesn't depend on the value that it's selecting at all - it's just returning true whatever the input is. Select doesn't perform filtering, just projection.

In other words:

// Array filled with (result.TotalPages + 1) x false
new bool[result.TotalPages + 1]

// Sequence of length (result.TotalPages + 1) x true
.Select(b => true)

// Array filled with (result.TotalPages + 1) x true
.ToArray();

Anyway it's inefficient. It's occasionally unfortunate that .NET doesn't provide something like:

public static T[] CreateFilledArray<T>(T value, int size)
{
    T[] ret = new T[size];
    for (int i=0; i < size; i++)
    {
        ret[i] = value;
    }
    return ret;
}

edited Nov 13, 2014 at 19:55

answered Aug 16, 2009 at 20:11

Jon Skeet

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2 Comments

Eric Schoonover Over a year ago

The first line of this answer is a bit misleading... it's not actually doing what the OP describes. Select isn't performing any type of filter, it's returning true for each item in the array.

Jeppe Stig Nielsen Over a year ago

It looks like you meant to make that last method a generic one.

Thomas Levesque · Accepted Answer · 2009-08-16 20:12:57Z

5

.Select(b => true)

this uses a linq query to return only the items that are true, i.e. none of them

No, it means that for each item, it returns true. It doesn't do any test on the value of the array item

This code just creates an array with all items set to true

I prefer this form (no intermediate array) :

return new Enumerable.Range(0,result.TotalPages).Select(b => true).ToArray();

answered Aug 16, 2009 at 20:12

Thomas Levesque

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1 Comment

Jon Skeet Over a year ago

Enumerable.Repeat is a more descriptive way of doing this, IMO.

Filip Navara · Accepted Answer · 2009-08-16 20:13:49Z

2

I believe the .Select(b => true) part returns "true" for every element in the array, so you actually get IEnumerable with result.TotalPages + 1 items and then it's converted to an array.

answered Aug 16, 2009 at 20:13

Filip Navara

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