Why we need 1,2,4,8 bytes to store logical variable in fortran?

I don't understand that since logical type only has two cases: true and false, then why we need logical(1),logical(2),logical(4),logical(8) in Fortran?

We just need 1 bit.

Can somebody give an explanation?


  • First, Fortran doesn't say that we have logical types taking up 1, 2, 4 and 8 bytes each, and they certainly aren't logical(1), logical(2), logical(4), and logical(8). An implementation may choose to offer those, calling them those names.

    A logical variable can indeed be of only two values. From the (F90, although F2008 says the same in a different place) standard

    The logical type has two values which represent true and false.

    A processor must provide one or more representation methods for data of type logical. Each such method is characterized by a value for a type parameter called the kind type parameter.

    [Emphasis here and later verbatim.]

    For a logical type of default kind the rules of storage association ( say that:

    (1) A nonpointer scalar object of type default integer, default real, or default logical occupies a single numeric storage unit.
    (5) A nonpointer scalar object of type [..] nondefault logical [..] occupies a single unspecified storage unit that is different for each case.

    So, the compiler must offer a logical type which is of the same size as an integer and real type, but, equally, it can offer representations taking up 1 bit, 1 byte, or whatever. The kind number, and size, for any given representation (hence my first paragraph: the question isn't universally valid) is implementation-specific. That said, there is no SELECTED_LOGICAL_KIND (or such) intrinsic until Fortran 2023.

    As to why multiple representations can be useful, that comes down to offering a choice, perhaps for special cases such as for arrays and ideal memory management (some people like to play non-portable tricks). However, memory access/alignment requirements suggest that a scalar logical would be at least one byte (or padding make it the same). For C interoperability (F2003+) there is a kind C_BOOL corresponding to the companion C processor's _Bool, which needn't be the same size.