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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?

Vladimir F Героям слава
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user15964
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2 Answers2

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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 4.3.2.2:

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 (14.6.3.1) 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.

Jeff Hammond
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francescalus
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  • `SELECTED_LOGICAL_KIND` has been added in Fortran 2023. https://fortran.bcs.org/2022/AGM22_Reid.pdf or the specification have details. – Jeff Hammond Apr 04 '23 at 06:32
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LOGICAL

The FORTRAN standard requires logical variables to be the same size as INTEGER/REAL >variables (see the chapter on memory management) although only one bit is really needed to implement this type.

The values used to implement the logical constants .TRUE. and
.FALSE. differ:

          |    VMS     |    Sun    |   IRIX    |    -----------|------------|-----------|-----------|-----------
.TRUE.    |    -1      |     1     |     1     |    -----------|------------|-----------|-----------|-----------
.FALSE.   |     0      |     0     |     0     |    -----------|------------|-----------|-----------|-----------

Unix machines naturally adopted the C convention, VMS has a seemingly strange value for .TRUE., however on a closer look you will see that
if .FALSE. is "all bits 0", .TRUE. should be "all bits 1", in two's complement signed integers the number with all bits set to 1 is -1.

http://www.ibiblio.org/pub/languages/fortran/ch2-3.html

It looks like its for simpler memory management

http://www.ibiblio.org/pub/languages/fortran/ch2-19.html

Dexters
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  • Actually, this is not entirely true. As I understand it, the Fortran standard only defines that the first *bit* of the variable as the "meaningful" bit. So if you are testing for nonzero to mean true, you aren't guaranteed to be standards-compliant. If you assign "2" to a logical variable, strictly speaking, this will be interpreted as a .FALSE. since the first bit is zero. – ereisch Jan 12 '21 at 17:12