Mapping bits to an integer

[ferrad01]

I have a large number (say 1000) of on/off bits of data which I want to represent as a set of integers (in this case 250 4 byte integers). I'm not sure how to do this as I don't think there is a bit datatype (1-bit). Logical*1 is 1 byte (8 bits). I'd like to do something like:

logical*1 :: lbig(1000)
integer*4 :: ibig(250)

equivalence(ibig(1), lbig(1))

but that doesn't work as the logical datatype is too big.

[jimdempseyatthecove]

Look at using BIT, BIC and BIS

integer*4 :: ibig(250)

...
ibig = 0

logical function yourTest(yourArray, yourBit)
integer*4 :: yourArray(250)
integer*4 :: yourBit
yourTest = (BIT(yourArray(yourBit / 32), MOD(yourBit, 32)) .ne. 0)
end function yourTest

The yourSet and yourClear will be variations on the above

You can also use C/C++ bit array funcitons

Jim Dempsey

[Robert_van_Amerongen]

If I understand it correctly, you want 1000 1/0 values, obtained from 1000 1 byte logicals, use to set the 4 bytes of 250 integers? If so, why don't use the TRANSFER statement

ibig = TRANSFER(lbig, ibig)

This works also fine in case you prefer to have the integer array being one with 1000 elements each 8 bits long!

Robert

[ferrad01]

Thanks Jim,
This is what I need- (not sure how the TRANSFER function is supposed to work)

[rwg]

This example shows how to store and test 8000 bits. To store and test 1000 bits you need only 32 Integers so declaring ibig as integer*4::ibig(32) is sufficent.

[jimdempseyatthecove]

Quoting rwg

This example shows how to store and test 8000 bits. To store and test 1000 bits you need only 32 Integers so declaring ibig as integer*4::ibig(32) is sufficent.

This is correct, the code example provided was more or less a sketch of how to use a bit table.

The programmer can take this as a starting point. There are additional issues to consider for any complete implementation.

1) does the design require static allocation or dynamic allocation
2) should the bit field chunk be INTEGER*4 or INTEGER (KIND=INT_PTR_KIND())
3) Is the bit field used by a single thread or multiple threads

Issue 3, when used by multiple threads, will require atomic operations. However, by reconsidering to use 1 bit in single byte, you can then avoid an atomic operation and simply write the 0/1 into the byte.

Jim Dempsey