Types and kinds#

These intrinsics allow for explicitly casting one type of variable to another or can be used to conditionally execute code blocks based on variable types when working with polymorphic variables.

Fortran Data Types#

Fortran provides five basic intrinsic data types:

Integer type
The integer types can hold only whole number values.
Real type
Stores floating point numbers, such as 2.0, 3.1415, -100.876, etc.
Complex type
A complex number has two parts, the real part and the imaginary part. Two consecutive floating point storage units store the two parts.
Logical type
There are only two logical values: .true. and .false.
Character type
The character type stores strings. The length of the string can be specified by the len specifier. If no length is specified, it is 1.

These “types” can be of many “kinds”. Often different numeric kinds take up different storage sizes and therefore can represent different ranges; but a different kind can have other meanings. A character variable might represent ASCII characters or UTF-8 or Unicode characters, for example.

You can derive your own data types from these fundamental types as well.

Implicit Typing#

Fortran allows a feature called implicit typing, i.e., you do not have to declare some variables before use. By default if a variable is not declared, then the first letter of its name will determine its type:

Variable names starting with i-n (the first two letters of “integer”) specify integer variables.
All other variable names default to real.

However, in most circles it is considered good programming practice to declare all the variables. For that to be enforced, you start your variable declaration section with a statement that turns off implicit typing: the statement

implicit none

For more information refer to the implicit statement.

aimag#

Name#

aimag(3) - [TYPE:NUMERIC] Imaginary part of complex number

Synopsis#

    result = aimag(z)

     elemental complex(kind=KIND) function aimag(z)

      complex(kind=KIND),intent(in) :: z

Characteristics#

The type of the argument z shall be complex and any supported complex kind
The return value is of type real with the kind type parameter of the argument.

Description#

aimag(3) yields the imaginary part of the complex argument z.

This is similar to the modern complex-part-designator %IM which also designates the imaginary part of a value, accept a designator can appear on the left-hand side of an assignment as well, as in val%im=10.0.

Options#

z
The complex value to extract the imaginary component of.

Result#

The return value is a real value with the magnitude and sign of the imaginary component of the argument z.

That is, If z has the value (x,y), the result has the value y.

Examples#

Sample program:

program demo_aimag
use, intrinsic :: iso_fortran_env, only : real_kinds, &
 & real32, real64, real128
implicit none
character(len=*),parameter :: g='(*(1x,g0))'
complex              :: z4
complex(kind=real64) :: z8
   ! basics
    z4 = cmplx(1.e0, 2.e0)
    print *, 'value=',z4
    print g, 'imaginary part=',aimag(z4),'or', z4%im

    ! other kinds other than the default may be supported
    z8 = cmplx(3.e0_real64, 4.e0_real64,kind=real64)
    print *, 'value=',z8
    print g, 'imaginary part=',aimag(z8),'or', z8%im

    ! an elemental function can be passed an array
    print *
    print *, [z4,z4/2.0,z4+z4,z4**3]
    print *
    print *, aimag([z4,z4/2.0,z4+z4,z4**3])

end program demo_aimag

Results:

 value= (1.00000000,2.00000000)
 imaginary part= 2.00000000 or 2.00000000
 value= (3.0000000000000000,4.0000000000000000)
 imaginary part= 4.0000000000000000 or 4.0000000000000000

 (1.00000000,2.00000000) (0.500000000,1.00000000) (2.00000000,4.00000000)
 (-11.0000000,-2.00000000)

   2.00000000       1.00000000       4.00000000      -2.00000000

Standard#

FORTRAN 77

cmplx#

Name#

cmplx(3) - [TYPE:NUMERIC] Conversion to a complex type

Synopsis#

    result = cmplx(x [,kind]) | cmplx(x [,y] [,kind])

     elemental complex(kind=KIND) function cmplx( x, y, kind )

      type(TYPE(kind=**)),intent(in)          :: x
      type(TYPE(kind=**)),intent(in),optional :: y
      integer(kind=**),intent(in),optional    :: KIND

Characteristics#

x may be integer, real, or complex.
y may be integer or real. y is allowed only if x is not complex.
KIND is a constant integer initialization expression indicating the kind parameter of the result.

The type of the arguments does not affect the kind of the result except for a complex x value.

if kind is not present and x is complex the result is of the kind of x.
if kind is not present and x is not complex the result if of default complex kind.

NOTE: a kind designated as ** may be any supported kind for the type

Description#

The cmplx(3) function converts numeric values to a complex value.

Even though constants can be used to define a complex variable using syntax like

      z = (1.23456789, 9.87654321)

this will not work for variables. So you cannot enter

      z = (a, b)  ! NO ! (unless a and b are constants, not variables)

so to construct a complex value using non-complex values you must use the cmplx(3) function:

      z = cmplx(a, b)

or assign values separately to the imaginary and real components using the %IM and %RE designators:

      z%re = a
      z%im = b

If x is complex y is not allowed and cmplx essentially returns the input value except for an optional change of kind, which can be useful when passing a value to a procedure that requires the arguments to have a different kind (and does not return an altered value):

      call something(cmplx(z,kind=real64))

would pass a copy of a value with kind=real64 even if z had a different kind

but otherwise is equivalent to a simple assign. So if z1 and z2 were complex:

      z2 = z1        ! equivalent statements
      z2 = cmplx(z1)

If x is not complex x is only used to define the real component of the result but y is still optional – the imaginary part of the result will just be assigned a value of zero.

If y is present it is converted to the imaginary component.

cmplx(3) and double precision#

Primarily in order to maintain upward compatibility you need to be careful when working with complex values of higher precision that the default.

It was necessary for Fortran to continue to specify that cmplx(3) always return a result of the default kind if the kind option is absent, since that is the behavior mandated by FORTRAN 77.

It might have been preferable to use the highest precision of the arguments for determining the return kind, but that is not the case. So with arguments with greater precision than default values you are required to use the kind argument or the greater precision values will be reduced to default precision.

This means cmplx(d1,d2), where d1 and d2 are doubleprecision, is treated as:

      cmplx(sngl(d1), sngl(d2))

which looses precision.

So Fortran 90 extends the cmplx(3) intrinsic by adding an extra argument used to specify the desired kind of the complex result.

      integer,parameter :: dp=kind(0.0d0)
      complex(kind=dp) :: z8
     ! wrong ways to specify constant values
      ! note this was stored with default real precision !
      z8 = cmplx(1.2345678901234567d0, 1.2345678901234567d0)
      print *, 'NO, Z8=',z8,real(z8),aimag(z8)

      z8 = cmplx(1.2345678901234567e0_dp, 1.2345678901234567e0_dp)
      ! again, note output components are just real
      print *, 'NO, Z8=',z8,real(z8),aimag(z8)
      !
      ! YES
      !
      ! kind= makes it work
      z8 = cmplx(1.2345678901234567d0, 1.2345678901234567d0,kind=dp)
      print *, 'YES, Z8=',z8,real(z8),aimag(z8)

A more recent alternative to using cmplx(3) is “F2018 component syntax” where real and imaginary parts of a complex entity can be accessed independently:

value%RE     ! %RE specifies the real part
or
value%IM     ! %IM specifies the imaginary part

Where the designator value is of course of complex type.

The type of a complex-part-designator is real, and its kind and shape are those of the designator. That is, you retain the precision of the complex value by default, unlike with cmplx.

The following are examples of complex part designators:

       impedance%re           !-- Same value as real(impedance)
       fft%im                 !-- Same value as AIMAG(fft)
       x%im = 0.0             !-- Sets the imaginary part of x to zero
       x(1:2)%re=[10,20]      !-- even if x is an array

NOTE for I/O#

Note that if format statements are specified a complex value is treated as two real values.

For list-directed I/O (ie. using an asterisk for a format) and NAMELIST output the values are expected to be delimited by “(” and “)” and of the form “(realpart,imaginary_part)”. For NAMELIST input parenthesized values or lists of multiple _real values are acceptable.

Options#

x
The value assigned to the real component of the result when x is not complex.

If x is complex, the result is the same as if the real part of the input was passed as x and the imaginary part as y.

     result = CMPLX (REAL (X), AIMAG (X), KIND).

That is, a complex x value is copied to the result value with a possible change of kind.

y
y is only allowed if x is not complex. Its value is assigned to the imaginary component of the result and defaults to a value of zero if absent.
kind
An integer initialization expression indicating the kind parameter of the result.

Result#

The return value is of complex type, with magnitudes determined by the values x and y.

The common case when x is not complex is that the real component of the result is assigned the value of x and the imaginary part is zero or the value of y if y is present.

When x is complex y is not allowed and the result is the same value as x with a possible change of kind. That is, the real part is real(x, kind) and the imaginary part is real(y, kind).

Examples#

Sample program:

program demo_aimag
implicit none
integer,parameter :: dp=kind(0.0d0)
real(kind=dp)     :: precise
complex(kind=dp)  :: z8
complex           :: z4, zthree(3)
   precise=1.2345678901234567d0

  ! basic
   z4 = cmplx(-3)
   print *, 'Z4=',z4
   z4 = cmplx(1.23456789, 1.23456789)
   print *, 'Z4=',z4
   ! with a format treat a complex as two real values
   print '(1x,g0,1x,g0,1x,g0)','Z4=',z4

  ! working with higher precision values
   ! using kind=dp makes it keep DOUBLEPRECISION precision
   ! otherwise the result would be of default kind
   z8 = cmplx(precise, -precise )
   print *, 'lost precision Z8=',z8
   z8 = cmplx(precise, -precise ,kind=dp)
   print *, 'kept precision Z8=',z8

  ! assignment of constant values does not require cmplx(3)00
   ! The following is intuitive and works without calling cmplx(3)
   ! but does not work for variables just constants
   z8 = (1.1111111111111111d0, 2.2222222222222222d0 )
   print *, 'Z8 defined with constants=',z8

  ! what happens when you assign a complex to a real?
   precise=z8
   print *, 'LHS=',precise,'RHS=',z8

  ! elemental
   zthree=cmplx([10,20,30],-1)
   print *, 'zthree=',zthree

  ! descriptors are an alternative
   zthree(1:2)%re=[100,200]
   print *, 'zthree=',zthree

end program demo_aimag

Results:

    Z4= (-3.000000,0.0000000E+00)
    Z4= (1.234568,1.234568)
    Z4= 1.234568 1.234568
    lost precision Z8= (1.23456788063049,-1.23456788063049)
    kept precision Z8= (1.23456789012346,-1.23456789012346)
    Z8 defined with constants= (1.11111111111111,2.22222222222222)
    LHS=   1.11111111111111      RHS= (1.11111111111111,2.22222222222222)
    zthree= (10.00000,-1.000000) (20.00000,-1.000000) (30.00000,-1.000000)
    zthree= (100.0000,-1.000000) (200.0000,-1.000000) (30.00000,-1.000000)

Standard#

FORTRAN 77, KIND added in Fortran 90.

int#

Name#

int(3) - [TYPE:NUMERIC] Truncate towards zero and convert to integer

Synopsis#

    result = int(a [,kind])

     elemental integer(kind=KIND) function int(a, KIND )

      TYPE(kind=**),intent(in) :: a
      integer,optional :: KIND

Characteristics#

a kind designated as ** may be any supported kind for the type
a shall be of type integer, real, or complex, or a boz-literal-constant.
KIND shall be a scalar integer constant expression.

Description#

int(3) truncates towards zero and return an integer.

Options#

a
is the value to truncate towards zero
kind
indicates the kind parameter of the result. If not present the returned type is that of default integer type.

Result#

returns an integer variable applying the following rules:

Case:

If a is of type integer, int(a) = a
If a is of type real and |a| < 1, int(a) equals 0. If |a| >= 1, then int(a) equals the integer whose magnitude does not exceed a and whose sign is the same as the sign of a.
If a is of type complex, rule 2 is applied to the real part of a.
If a is a boz-literal constant, it is treated as an integer with the kind specified.

The interpretation of a bit sequence whose most significant bit is 1 is processor dependent.

The result is undefined if it cannot be represented in the specified integer type.

Examples#

Sample program:

program demo_int
use,intrinsic :: iso_fortran_env, only : int8, int16, int32, int64
implicit none
integer :: i = 42
complex :: z = (-3.7, 1.0)
real :: x=-10.5, y=10.5

   print *, int(x), int(y)

   print *, int(i)

   print *, int(z), int(z,8)
   ! elemental
   print *, int([-10.9,-10.5,-10.3,10.3,10.5,10.9])
   ! note int(3) truncates towards zero

   ! CAUTION:
   ! a number bigger than a default integer can represent
   ! produces an incorrect result and is not required to
   ! be detected by the program.
   x=real(huge(0))+1000.0
   print *, int(x),x
   ! using a larger kind
   print *, int(x,kind=int64),x

   print *, int(&
   & B"111111111111111111111111111111111111111111111111111111111111111",&
   & kind=int64)
   print *, int(O"777777777777777777777",kind=int64)
   print *, int(Z"7FFFFFFFFFFFFFFF",kind=int64)

   ! elemental
   print *
   print *,int([ &
   &  -2.7,  -2.5, -2.2, -2.0, -1.5, -1.0, -0.5, &
   &  0.0,   &
   &  +0.5,  +1.0, +1.5, +2.0, +2.2, +2.5, +2.7  ])

end program demo_int

Results:

 >          -10   10
 >           42
 >           -3  -3
 >          -10  -10  -10   10   10  10
 >  -2147483648   2.14748467E+09
 >   2147484672   2.14748467E+09
 >   9223372036854775807
 >   9223372036854775807
 >   9223372036854775807
 >
 >  -2          -2          -2          -2          -1
 >  -1           0           0           0           1
 >   1           2           2           2           2

Standard#

FORTRAN 77

nint#

Name#

nint(3) - [TYPE:NUMERIC] Nearest whole number

Synopsis#

    result = nint( a [,kind] )

     elemental integer(kind=KIND) function nint(a, kind )

      real(kind=**),intent(in) :: a
      integer(kind=**),intent(in),optional :: KIND

Characteristics#

a kind designated as ** may be any supported kind for the type
a is type real of any kind
KIND is a scalar integer constant expression
The result is default integer kind or the value of kind if kind is present.

Description#

nint(3) rounds its argument to the nearest whole number with its sign preserved.

The user must ensure the value is a valid value for the range of the kind returned. If the processor cannot represent the result in the kind specified, the result is undefined.

If a is greater than zero, nint(a) has the value int(a+0.5).

If a is less than or equal to zero, nint(a) has the value int(a-0.5).

Options#

a
The value to round to the nearest whole number
kind
can specify the kind of the output value. If not present, the output is the default type of integer.

Result#

The result is the integer nearest a, or if there are two integers equally near a, the result is whichever such integer has the greater magnitude.

The result is undefined if it cannot be represented in the specified integer type.

Examples#

Sample program:

program demo_nint
implicit none
integer,parameter   :: dp=kind(0.0d0)
real,allocatable    :: in(:)
integer,allocatable :: out(:)
integer             :: i
real                :: x4
real(kind=dp)       :: x8

  ! basic use
   x4 = 1.234E0
   x8 = 4.721_dp
   print *, nint(x4), nint(-x4)
   print *, nint(x8), nint(-x8)

  ! elemental
   in = [ -2.7,  -2.5, -2.2, -2.0, -1.5, -1.0, -0.5, -0.4, &
        &  0.0,   &
        & +0.04, +0.5, +1.0, +1.5, +2.0, +2.2, +2.5, +2.7  ]
   out = nint(in)
   do i=1,size(in)
      write(*,*)in(i),out(i)
   enddo

  ! dusty corners
   ISSUES: block
   use,intrinsic :: iso_fortran_env, only : int8, int16, int32, int64
   integer :: icheck
      ! make sure input is in range for the type returned
      write(*,*)'Range limits for typical KINDS:'
      write(*,'(1x,g0,1x,g0)')  &
      & int8,huge(0_int8),   &
      & int16,huge(0_int16), &
      & int32,huge(0_int32), &
      & int64,huge(0_int64)

      ! the standard does not require this to be an error ...
      x8=12345.67e15 ! too big of a number
      icheck=selected_int_kind(ceiling(log10(x8)))
      write(*,*)'Any KIND big enough? ICHECK=',icheck
      print *, 'These are all wrong answers for ',x8
      print *, nint(x8,kind=int8)
      print *, nint(x8,kind=int16)
      print *, nint(x8,kind=int32)
      print *, nint(x8,kind=int64)
   endblock ISSUES

end program demo_nint

Results:

 >               1          -1
 >               5          -5
 >      -2.700000              -3
 >      -2.500000              -3
 >      -2.200000              -2
 >      -2.000000              -2
 >      -1.500000              -2
 >      -1.000000              -1
 >     -0.5000000              -1
 >     -0.4000000               0
 >      0.0000000E+00           0
 >      3.9999999E-02           0
 >      0.5000000               1
 >       1.000000               1
 >       1.500000               2
 >       2.000000               2
 >       2.200000               2
 >       2.500000               3
 >       2.700000               3
 >     Range limits for typical KINDS:
 >     1 127
 >     2 32767
 >     4 2147483647
 >     8 9223372036854775807
 >     Any KIND big enough? ICHECK=          -1
 >     These are all wrong answers for   1.234566949990144E+019
 >        0
 >          0
 >     -2147483648
 >      -9223372036854775808

Standard#

FORTRAN 77 , with KIND argument - Fortran 90

real#

Name#

real(3) - [TYPE:NUMERIC] Convert to real type

Synopsis#

  result = real(x [,kind])

   elemental real(kind=KIND) function real(x,KIND)

    TYPE(kind=**),intent(in) :: x
    integer(kind=**),intent(in),optional :: KIND

Characteristics#

the type of x may be integer, real, or complex; or a BOZ-literal-constant.
kind is a integer initialization expression (a constant expression)
- If kind is present it defines the kind of the real result
- if kind is not present
  - when x is complex the result is a real of the same kind as x.
  - when x is real or integer the result is a real of default kind
a kind designated as ** may be any supported kind for the type

Description#

real(3) converts its argument x to a real type.

The real part of a complex value is returned. For complex values this is similar to the modern complex-part-designator %RE which also designates the real part of a complex value.

      z=(3.0,4.0)     ! if z is a complex value
      print *, z%re == real(z) ! these expressions are equivalent

Options#

x
An integer, real, or complex value to convert to real.
kind
When present the value of kind defines the kind of the result.

Result#

real(x) converts x to a default real type if x is an integer or real variable.
real(x) converts a complex value to a real type with the magnitude of the real component of the input with kind type parameter the same as x.
real(x, kind) is converted to a real type with kind type parameter kind if x is a complex, integer, or real variable.

Examples#

Sample program:

program demo_real
use,intrinsic :: iso_fortran_env, only : dp=>real64
implicit none
complex              :: zr = (1.0, 2.0)
doubleprecision      :: xd=huge(3.0d0)
complex(kind=dp) :: zd=cmplx(4.0e0_dp,5.0e0_dp,kind=dp)

   print *, real(zr), aimag(zr)
   print *, dble(zd), aimag(zd)

   write(*,*)xd,real(xd,kind=kind(0.0d0)),dble(xd)
end program demo_real

Results:

00000000       2.00000000
0000000000000000       5.0000000000000000
7976931348623157E+308  1.7976931348623157E+308  1.7976931348623157E+308

Standard#

FORTRAN 77

dble#

Name#

dble(3) - [TYPE:NUMERIC] Converstion to double precision real

Synopsis#

    result = dble(a)

     elemental doubleprecision function dble(a)

      doubleprecision :: dble
      TYPE(kind=KIND),intent(in) :: a

Characteristics#

a my be integer, real, complex, or a BOZ-literal-constant
the result is a doubleprecision real.

Description#

dble(3) Converts a to double precision real type.

Options#

a
a value to convert to a doubleprecision real.

Result#

The return value is of type doubleprecision. For complex input, the returned value has the magnitude and sign of the real component of the input value.

Examples#

Sample program:

program demo_dble
implicit none
real:: x = 2.18
integer :: i = 5
complex :: z = (2.3,1.14)
   print *, dble(x), dble(i), dble(z)
end program demo_dble

Results:

  2.1800000667572021  5.0000000000000000   2.2999999523162842

Standard#

FORTRAN 77

transfer#

Name#

transfer(3) - [TYPE:MOLD] Transfer bit patterns

Synopsis#

    result = transfer(source, mold [,size] )

     type(TYPE(kind=KIND)) function transfer(source,mold,size)

      type(TYPE(kind=KIND)),intent(in) :: source(..)
      type(TYPE(kind=KIND)),intent(in) :: mold(..)
      integer(kind=**),intent(in),optional :: size

Characteristics#

source shall be a scalar or an array of any type.
mold shall be a scalar or an array of any type.
size shall be a scalar of type integer.
result has the same type as mold

Description#

transfer(3) copies the bitwise representation of source in memory into a variable or array of the same type and type parameters as mold.

This is approximately equivalent to the C concept of “casting” one type to another.

Options#

source
Holds the bit pattern to be copied
mold
the type of mold is used to define the type of the returned value. In addition, if it is an array the returned value is a one-dimensional array. If it is a scalar the returned value is a scalar.
size
If size is present, the result is a one-dimensional array of length size.

If size is absent but mold is an array (of any size or shape), the result is a one-dimensional array of the minimum length needed to contain the entirety of the bitwise representation of source.

If size is absent and mold is a scalar, the result is a scalar.

Result#

The result has the bit level representation of source.

If the bitwise representation of the result is longer than that of source, then the leading bits of the result correspond to those of source but any trailing bits are filled arbitrarily.

When the resulting bit representation does not correspond to a valid representation of a variable of the same type as mold, the results are undefined, and subsequent operations on the result cannot be guaranteed to produce sensible behavior. For example, it is possible to create logical variables for which var and .not. var both appear to be true.

Examples#

Sample program:

program demo_transfer
use,intrinsic :: iso_fortran_env, only : int32, real32
integer(kind=int32) :: i = 2143289344
real(kind=real32)   :: x
character(len=10)   :: string
character(len=1)    :: chars(10)
   x=transfer(i, 1.0)    ! prints "nan" on i686
   ! the bit patterns are the same
   write(*,'(b0,1x,g0)')x,x ! create a NaN
   write(*,'(b0,1x,g0)')i,i

   ! a string to an array of characters
   string='abcdefghij'
   chars=transfer(string,chars)
   write(*,'(*("[",a,"]":,1x))')string
   write(*,'(*("[",a,"]":,1x))')chars
end program demo_transfer

Results:

   1111111110000000000000000000000 NaN
   1111111110000000000000000000000 2143289344
   [abcdefghij]
   [a] [b] [c] [d] [e] [f] [g] [h] [i] [j]

Comments#

Joe Krahn: Fortran uses molding rather than casting.

Casting, as in C, is an in-place reinterpretation. A cast is a device that is built around an object to change its shape.

Fortran transfer(3) reinterprets data out-of-place. It can be considered molding rather than casting. A mold is a device that confers a shape onto an object placed into it.

The advantage of molding is that data is always valid in the context of the variable that holds it. For many cases, a decent compiler should optimize transfer(3) into a simple assignment.

There are disadvantages of this approach. It is problematic to define a union of data types because you must know the largest data object, which can vary by compiler or compile options. In many cases, an EQUIVALENCE would be far more effective, but Fortran Standards committees seem oblivious to the benefits of EQUIVALENCE when used sparingly.

Standard#

Fortran 90

logical#

Name#

logical(3) - [TYPE:LOGICAL] Conversion between kinds of logical values

Synopsis#

    result = logical(l [,kind])

     elemental logical(kind=KIND) function logical(l,KIND)

      logical(kind=**),intent(in) :: l
      integer(kind=**),intent(in),optional :: KIND

Characteristics#

a kind designated as ** may be any supported kind for the type
l is of type logical
KIND shall be a scalar integer constant expression. If KIND is present, the kind type parameter of the result is that specified by the value of KIND; otherwise, the kind type parameter is that of default logical.

Description#

logical(3) converts one kind of logical variable to another.

Options#

l
The logical value to produce a copy of with kind kind
kind
indicates the kind parameter of the result. If not present, the default kind is returned.

Result#

The return value is a logical value equal to l, with a kind corresponding to kind, or of the default logical kind if kind is not given.

Examples#

Sample program:

Linux
program demo_logical
! Access array containing the kind type parameter values supported by this
! compiler for entities of logical type
use iso_fortran_env, only : logical_kinds
implicit none
integer :: i

   ! list kind values supported on this platform, which generally vary
   ! in storage size as alias declarations
   do i =1, size(logical_kinds)
      write(*,'(*(g0))')'integer,parameter :: boolean', &
      & logical_kinds(i),'=', logical_kinds(i)
   enddo

end program demo_logical

Results:

 > integer,parameter :: boolean1=1
 > integer,parameter :: boolean2=2
 > integer,parameter :: boolean4=4
 > integer,parameter :: boolean8=8
 > integer,parameter :: boolean16=16

Standard#

Fortran 95 , related ISO_FORTRAN_ENV module - fortran 2009

kind#

Name#

kind(3) - [KIND:INQUIRY] Query kind of an entity

Synopsis#

    result = kind(x)

     integer function kind(x)

      type(TYPE,kind=**),intent(in) :: x(..)

Characteristics#

x may be of any intrinsic type. It may be a scalar or an array.
the result is a default integer scalar

Description#

kind(x)(3) returns the kind value of the entity x.

Options#

x
Value to query the kind of.

Result#

The return value indicates the kind of the argument x.

Note that kinds are processor-dependent.

Examples#

Sample program:

program demo_kind
implicit none
integer,parameter :: dc = kind(' ')
integer,parameter :: dl = kind(.true.)

   print *, "The default character kind is ", dc
   print *, "The default logical kind is ", dl

end program demo_kind

Results:

    The default character kind is            1
    The default logical kind is            4

Standard#

Fortran 95

out_of_range#

Name#

out_of_range(3) - [TYPE:NUMERIC] Whether a value cannot be converted safely.

Synopsis#

    result = out_of_range (x, mold [, round])

     elemental logical function(x, mold, round)

      TYPE,kind=KIND),intent(in) :: x
      TYPE,kind=KIND),intent(in) :: mold
      logical,intent(in),optional     :: round

Characteristics#

x is of type integer or real.
mold is an integer or real scalar.
round is a logical scalar.
the result is a default logical.

Description#

out_of_range(3) determines whether a value x can be converted safely to a real or integer variable the same type and kind as mold.

For example, if int8 is the kind value for an 8-bit binary integer type, out_of_range(-128.5, 0_int8) will have the value false and out_of_range(-128.5, 0_int8, .true.) will have the value .true. because the value will be truncated when converted to an integer and -128 is a representable value on a two’s complement machine in eight bits even though +128 is not.

Options#

x
a scalar to be tested for whether it can be stored in a variable of the type and kind of mold
mold and kind are queried to determine the characteristics of what needs to be fit into.
round
flag whether to round the value of xx before validating it as an integer value like mold.

round can only be present if x is of type real and mold is of type integer.

Result#

From the standard:

Case (i): If mold is of type integer, and round is absent or present with the value false, the result is true if and only if the value of X is an IEEE infinity or NaN, or if the integer with largest magnitude that lies between zero and X inclusive is not representable by objects with the type and kind of mold.

Case (ii): If mold is of type integer, and round is present with the value true, the result is true if and only if the value of X is an IEEE infinity or NaN, or if the integer nearest X, or the integer of greater magnitude if two integers are equally near to X, is not representable by objects with the type and kind of mold.

Case (iii): Otherwise, the result is true if and only if the value of X is an IEEE infinity or NaN that is not supported by objects of the type and kind of mold, or if X is a finite number and the result of rounding the value of X (according to the IEEE rounding mode if appropriate) to the extended model for the kind of mold has magnitude larger than that of the largest finite number with the same sign as X that is representable by objects with the type and kind of mold.

NOTE

mold is required to be a scalar because the only information taken from it is its type and kind. Allowing an array mold would require that it be conformable with x. round is scalar because allowing an array rounding mode would have severe performance difficulties on many processors.

Examples#

Sample program:

program demo_out_of_range
use, intrinsic :: iso_fortran_env, only : int8, int16, int32, int64
use, intrinsic :: iso_fortran_env, only : real32, real64, real128
implicit none
integer            :: i
integer(kind=int8) :: i8, j8

    ! compilers are not required to produce an error on out of range.
    ! here storing the default integers into 1-byte integers
    ! incorrectly can have unexpected results
    do i=127,130
       i8=i
       j8=-i
       ! OUT_OF_RANGE(3f) can let you check if the value will fit
       write(*,*)i8,j8,' might have expected',i,-i, &
        & out_of_range( i,i8), &
        & out_of_range(-i,i8)
    enddo
    write(*,*) 'RANGE IS ',-1-huge(0_int8),'TO',huge(0_int8)
    ! the real -128.5 is truncated to -128 and is in range
    write(*,*) out_of_range (  -128.5, 0_int8)         ! false

    ! the real -128.5 is rounded to -129 and is not in range
    write(*,*) out_of_range (  -128.5, 0_int8, .true.) ! true

end program demo_out_of_range

Results:

  >  127 -127  might have expected         127        -127 F F
  > -128 -128  might have expected         128        -128 T F
  > -127  127  might have expected         129        -129 T T
  > -126  126  might have expected         130        -130 T T
  > RANGE IS         -128 TO  127
  > F
  > T

Standard#

FORTRAN 2018

selected_char_kind#

Name#

selected_char_kind(3) - [KIND] Select character kind such as “Unicode”

Synopsis#

    result = selected_char_kind(name)

     integer function selected_char_kind(name)

      character(len=*),intent(in) :: name

Characteristics#

name is a default character scalar
the result is a default integer scalar

Description#

selected_char_kind(3) returns a kind parameter value for the character set named name.

If a name is not supported, -1 is returned. Otherwise the result is a value equal to that kind type parameter value.

The list of supported names is processor-dependent except for “DEFAULT”.

If name has the value “DEFAULT”, then the result has a value equal to that of the kind type parameter of default character. This name is always supported.
If name has the value “ASCII”, then the result has a value equal to that of the kind type parameter of ASCII character.
If name has the value “ISO_10646”, then the result has a value equal to that of the kind type parameter of the ISO 10646 character kind (corresponding to UCS-4 as specified in ISO/IEC 10646).
If name is a processor-defined name of some other character kind supported by the processor, then the result has a value equal to that kind type parameter value. Pre-defined names include “ASCII” and “ISO_10646”.

The NAME is interpreted without respect to case or trailing blanks.

Options#

name
A name to query the processor-dependent kind value of, and/or to determine if supported. name, interpreted without respect to case or trailing blanks.

Currently, supported character sets include “ASCII” and “DEFAULT” and “ISO_10646” (Universal Character Set, UCS-4) which is commonly known as “Unicode”. Supported names other than “DEFAULT” are processor dependent.

Result#

Examples#

Sample program:

Linux
program demo_selected_char_kind
use iso_fortran_env
implicit none

intrinsic date_and_time,selected_char_kind

! set some aliases for common character kinds
! as the numbers can vary from platform to platform

integer, parameter :: default = selected_char_kind ("default")
integer, parameter :: ascii =   selected_char_kind ("ascii")
integer, parameter :: ucs4  =   selected_char_kind ('ISO_10646')
integer, parameter :: utf8  =   selected_char_kind ('utf-8')

! assuming ASCII and UCS4 are supported (ie. not equal to -1)
! define some string variables
character(len=26, kind=ascii ) :: alphabet
character(len=30, kind=ucs4  ) :: hello_world
character(len=30, kind=ucs4  ) :: string

   write(*,*)'ASCII     ',&
    & merge('Supported    ','Not Supported',ascii /= -1)
   write(*,*)'ISO_10646 ',&
    & merge('Supported    ','Not Supported',ucs4 /= -1)
   write(*,*)'UTF-8     ',&
    & merge('Supported    ','Not Supported',utf8 /= -1)

   if(default.eq.ascii)then
       write(*,*)'ASCII is the default on this processor'
   endif

  ! for constants the kind precedes the value, somewhat like a
  ! BOZ constant
   alphabet = ascii_"abcdefghijklmnopqrstuvwxyz"
   write (*,*) alphabet

   hello_world = ucs4_'Hello World and Ni Hao -- ' &
                 // char (int (z'4F60'), ucs4)     &
                 // char (int (z'597D'), ucs4)

  ! an encoding option is required on OPEN for non-default I/O
   if(ucs4 /= -1 )then
      open (output_unit, encoding='UTF-8')
      write (*,*) trim (hello_world)
   else
      write (*,*) 'cannot use utf-8'
   endif

   call create_date_string(string)
   write (*,*) trim (string)

contains

! The following produces a Japanese date stamp.
subroutine create_date_string(string)
intrinsic date_and_time,selected_char_kind
integer,parameter :: ucs4 = selected_char_kind("ISO_10646")
character(len=1,kind=ucs4),parameter :: &
       nen =   char(int( z'5e74' ),ucs4), & ! year
       gatsu = char(int( z'6708' ),ucs4), & ! month
       nichi = char(int( z'65e5' ),ucs4)    ! day
character(len= *, kind= ucs4) string
integer values(8)
   call date_and_time(values=values)
   write(string,101) values(1),nen,values(2),gatsu,values(3),nichi
 101 format(*(i0,a))
end subroutine create_date_string

end program demo_selected_char_kind

Results:

The results are very processor-dependent

 >  ASCII     Supported
 >  ISO_10646 Supported
 >  UTF-8     Not Supported
 >  ASCII is the default on this processor
 >  abcdefghijklmnopqrstuvwxyz
 >  Hello World and Ni Hao -- 你好
 >  2022年10月15日

Standard#

Fortran 2003

selected_int_kind#

Name#

selected_int_kind(3) - [KIND] Choose integer kind

Synopsis#

    result = selected_int_kind(r)

    integer function selected_int_kind(r)

     integer(kind=KIND),intent(in) :: r

Characteristics#

r is an integer scalar.
the result is an default integer scalar.

Description#

selected_int_kind(3) return the kind value of the smallest integer type that can represent all values ranging from -10**r (exclusive) to 10**r (exclusive). If there is no integer kind that accommodates this range, selected_int_kind returns -1.

Options#

r
The value specifies the required range of powers of ten that need supported by the kind type being returned.

Result#

The result has a value equal to the value of the kind type parameter of an integer type that represents all values in the requested range.

if no such kind type parameter is available on the processor, the result is -1.

If more than one kind type parameter meets the criterion, the value returned is the one with the smallest decimal exponent range, unless there are several such values, in which case the smallest of these kind values is returned.

Examples#

Sample program:

program demo_selected_int_kind
implicit none
integer,parameter :: k5 = selected_int_kind(5)
integer,parameter :: k15 = selected_int_kind(15)
integer(kind=k5) :: i5
integer(kind=k15) :: i15

    print *, huge(i5), huge(i15)

    ! the following inequalities are always true
    print *, huge(i5) >= 10_k5**5-1
    print *, huge(i15) >= 10_k15**15-1
end program demo_selected_int_kind

Results:

  >   2147483647  9223372036854775807
  >  T
  >  T

Standard#

Fortran 95

selected_real_kind#

Name#

selected_real_kind(3) - [KIND] Choose real kind

Synopsis#

    result = selected_real_kind([p] [,r] [,radix] )

    integer function selected_int_kind(r)

     real(kind=KIND),intent(in),optional :: p
     real(kind=KIND),intent(in),optional :: r
     real(kind=KIND),intent(in),optional :: radix

Characteristics#

p is an integer scalar
r is an integer scalar
radix is an integer scalar
the result is an default integer scalar

Description#

selected_real_kind(3) return the kind value of a real data type with decimal precision of at least p digits, exponent range of at least r, and with a radix of radix. That is, if such a kind exists

+ it has the decimal precision as returned by **precision**(3) of at
  least **p** digits.
+ a decimal exponent range, as returned by the function **range**(3)
  of at least **r**
+ a radix, as returned by the function **radix**(3) , of **radix**,

If the requested kind does not exist, -1 is returned.

At least one argument shall be present.

Options#

p
the requested precision
r
the requested range
radix
the desired radix

Before Fortran 2008, at least one of the arguments r or p shall be present; since Fortran 2008, they are assumed to be zero if absent.

Result#

selected_real_kind returns the value of the kind type parameter of a real data type with decimal precision of at least p digits, a decimal exponent range of at least R, and with the requested radix.

If p or r is absent, the result value is the same as if it were present with the value zero.

If the radix parameter is absent, there is no requirement on the radix of the selected kind and real kinds with any radix can be returned.

If more than one real data type meet the criteria, the kind of the data type with the smallest decimal precision is returned. If no real data type matches the criteria, the result is

-1
if the processor does not support a real data type with a precision greater than or equal to p, but the r and radix requirements can be fulfilled
-2
if the processor does not support a real type with an exponent range greater than or equal to r, but p and radix are fulfillable
-3
if radix but not p and r requirements are fulfillable
-4
if radix and either p or r requirements are fulfillable
-5
if there is no real type with the given radix

Examples#

Sample program:

program demo_selected_real_kind
implicit none
integer,parameter :: p6 = selected_real_kind(6)
integer,parameter :: p10r100 = selected_real_kind(10,100)
integer,parameter :: r400 = selected_real_kind(r=400)
real(kind=p6) :: x
real(kind=p10r100) :: y
real(kind=r400) :: z

   print *, precision(x), range(x)
   print *, precision(y), range(y)
   print *, precision(z), range(z)
end program demo_selected_real_kind

Results:

  >            6          37
  >           15         307
  >           18        4931

Standard#

Fortran 95 ; with RADIX - Fortran 2008

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Types and kinds#

Fortran Data Types#

Implicit Typing#

aimag#

Name#

Synopsis#

Characteristics#

Description#

Options#

Result#

Examples#

Standard#

See Also#

cmplx#

Name#

Synopsis#

Characteristics#

Description#

cmplx(3) and double precision#

NOTE for I/O#

Options#

Result#

Examples#

Standard#

See Also#

int#

Name#

Synopsis#

Characteristics#

Description#

Options#

Result#

Examples#

Standard#

See Also#

nint#

Name#

Synopsis#

Characteristics#

Description#

Options#

Result#

Examples#

Standard#

See Also#

real#

Name#

Synopsis#

Characteristics#

Description#

Options#

Result#

Examples#

Standard#

See Also#

dble#

Name#

Synopsis#

Characteristics#

Description#

Options#

Result#

Examples#

Standard#

See also#

transfer#

Name#

Synopsis#

Characteristics#

Description#

Options#

Result#

Examples#

Comments#

Standard#

See also#

logical#

Name#

Synopsis#