c# array select

Question

I have a bool array

a[1] = true
a[2] = false
a[3] = true
a[4] = true

how do i select only true values to a new array?

Thanks!

What use do you have for an array only containing several copies of the value true? — Anon.
– Anon., Commented Jan 28, 2011 at 3:58
@Artur: It's declared like this: bool[] arrayName; How do I know? Because that's how you declare arrays in C#. I don't see how this is possibly relevant. — Cody Gray
– Cody Gray ♦, Commented Jan 28, 2011 at 4:13
@Artur: Then it wouldn't be a "bool array", as explained in the top line. It would be "some other class, with an overridden [] operator and type conversion to bool". — Cody Gray
– Cody Gray ♦, Commented Jan 28, 2011 at 4:22

Community · Accepted Answer · 2020-07-23 06:58:13Z

13

I don't really know why you would want to do this but...

bool[] a = {true, false, true, true};
bool[] b = a.Where(x => x).ToArray();

If you just want to count how many "true"s there are:

int c = a.Count(x => x);

edited Jul 23, 2020 at 6:58

CommunityBot

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answered Jan 28, 2011 at 4:01

Mike

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5 Comments

Alastair Pitts Over a year ago

You really don't want to get into the habit of performing a boolVar == true check. It's not very good code.

Mike Over a year ago

Its explicit so that the questioner can get a better idea of what is happening and hopefully learn. And why is it bad to perform this explicit check. I bet it gets compiled to the exact same thing.

Cody Gray Over a year ago

Indeed, the generated IL is the same for both constructs in C#. But that's not necessarily true for all languages. In general, I agree that being explicit is good in your answer, but you might want to add a clarifier that the == true part is not necessary. Sometimes, people just copy and paste code they get in answers here into their IDE.

Jonathan Wood Over a year ago

@AlastairPitts: I don't see any reason to not use the explicit "boolVar == true check," other than to save a few keystrokes. If you think that's bad code, perhaps you should indicate why.

SzymonPajzert Over a year ago

For example, in JavaScript values that would be evaluated a truthy would not pass this equality check. On the other hand, in functional languages where concept of LINQ comes from this check would be a smelly code as well, since it would be more idiomatic to write something like a.Where(Identity), but since the C# hasn't established such function, (x => x) would be in a good proximity.

Chris Shain · Accepted Answer · 2011-01-28 03:59:05Z

8

If you mean a new array containing the indices of 'a' that had a value of true...

// Assuming here that a begins at 0, unlike your example...
Enumerable.Range(0, a.Length).Where(i=>a[i]).ToArray();

answered Jan 28, 2011 at 3:59

Chris Shain

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Comments

Cheng Chen · Accepted Answer · 2011-01-28 04:00:20Z

4

bool[] result = a.Where(x => x).ToArray();

answered Jan 28, 2011 at 4:00

Cheng Chen

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2 Comments

richard Over a year ago

I think this only works in C# 3.0, where lambda expressions were introduced.

richard Over a year ago

@Timwi: Thanks, I should have been more specific, it works for version of .NET 3.0 and later. ;-)

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