how to convert an int to a bitmask?

Question

Is there a way to convert an int to a bitmask?

example:

int i = 33;

should be converted to (not sure of the datatype)

bool[] bitmask = new[] {true, false, false, false, false, true};

Update
In reaction to most answers:

I need to do this:

BitArray bits = new BitArray(BitConverter.GetBytes(showGroup.Value));
List<String> showStrings = new List<string>();
for (int i = 0; i < bits.Length; i++)
{
    if(bits[i])
        showStrings.Add((i+1).ToString().PadLeft(2, '0'));
}

How would that go without converting it to a bitarray?

score 32 · Accepted Answer · answered Nov 02 '09 at 17:15

32

An int already is a bitmask. If you want to twiddle the bits, you can use bitwise operators freely on ints. If you want to convert the int to an enum that has the Flags attribute, a simple cast will suffice.

answered Nov 02 '09 at 17:15

JSBձոգչ

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3

And if you need to access the ith bit (with i being 0-indexed), use `1 << i`. – Brian Nov 02 '09 at 17:52

Boris Callens · Answer 2 · 2009-11-03T07:42:55.617

13

Found it

BitArray bits = new BitArray(System.BitConverter.GetBytes(showGroup.Value));

edited Nov 03 '09 at 07:42

answered Nov 02 '09 at 17:16

Boris Callens

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You'll probably find that the in method that @JS Bangs suggested will be much quicker. – Kieron Nov 02 '09 at 17:18
Well, it's such a tiny conversion, it will hardly matter. But for the sake of correctness I would love to learn :) – Boris Callens Nov 02 '09 at 17:22
2

There is an example on SO: http://stackoverflow.com/questions/8447/enum-flags-attribute – vgru Nov 02 '09 at 17:23
Thanks. [more characters to please the SO overlord] – Boris Callens Nov 03 '09 at 07:41

Michael Petrotta · Answer 3 · 2009-11-02T17:56:34.650

You could construct a bool[32] and loop through all bits in the int, masking it with 2^(loop counter) and setting the bools in the array appropriately.

Are you sure you need this, though? Most operations with bitmasks work with ints directly.

To answer the question in your edit:

int val = 35;
List<string> showStrings = new List<string>();
for (int i = 0; i < 32; i++)
{
    if (( (1 << i) & val) > 0)
    {
        showStrings.Add((i + 1).ToString().PadLeft(2, '0'));
    }
}

prints:

01  
02  
06

Not the most obvious solution if you're not used to bit arithmetic, true. Mask each bit in the integer value with 2^(bit-index), and if the resulting value is greater than zero (indicating that the bit at that index is set), do something. 1 << i (left-shifting) is equivalent to 2^i, and may have the same performance characteristics once JITted, but I'm used to this form.

Expressed as a macro-like method:

bool IsSet(int val, int index)
{
    return (( (1 << (index-1)) & val) > 0);
}

score 4 · Answer 4 · answered Nov 02 '09 at 17:18

4

int val = 33;
var bitarray = new BitArray(new[] { val });
var att = bitarray.Cast<bool>().ToArray();

answered Nov 02 '09 at 17:18

Matthew Whited

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I like this. For those interested, the Cast method is an Extension Method and requires "using System.Linq" declaration which under Mono requires a reference to System.Core – Wayne Phipps Feb 10 '13 at 15:51

score 2 · Answer 5 · answered Nov 02 '09 at 17:47

Since you asked, here is a solution without using BitArray:

// First define a bitmask enum for the bits you are interested in
[Flags]
public enum BitFlags
{
  Flag1 = 1,
  Flag2 = 2,
  Flag3 = 4,
  Flag4 = 8,
  Flag5 = 16
  // ...
}

int index = 0;
List<string> showStrings = new List<string>();
foreach(int flag in Enum.GetValues(typeof(BitFlags))cast<int>())
{
  index += 1;
  if ((input & flag) == flag)
    showStrings.Add(index.ToString().PadLeft(2, '0'));
}

It is about the same amount of code, with negligible performance difference. It does however let you strongly define your bit values and you can choose to omit bits in the BitFlags enum that you don't care about.

Thanks. I didn't know this was possible. Another tool in my belt :) — Boris Callens, Nov 03 '09 at 08:05

how to convert an int to a bitmask?

5 Answers5