Kind parameters
| What kind means | |
| selected_real_kind | |
| iso_fortran_env kinds | |
| The working-precision idiom |
What kind means
A kind parameter selects one variant of a type: how many digits a real holds or how large an integer can grow.
Write it in parentheses after the type name, as in real(8) or integer(4), but prefer named constants over raw numbers.
program kinddemo
implicit none
print *, kind(1.0)
print *, kind(1.0d0)
end program kinddemo
The kind function reports the kind number of any value, so these two lines show the default real kind and the double precision kind.
selected_real_kind
The selected_real_kind function asks for a real kind by required decimal digits, which keeps code portable across compilers.
Store the answer in a named integer constant and use that constant as the kind for all working variables.
program seldemo
implicit none
integer, parameter :: sp = selected_real_kind(6)
real(sp) :: x
x = 1.5_sp
print *, x
end program seldemo
iso_fortran_env kinds
The iso_fortran_env module provides ready made kind names such as int32, int64, real32, and real64.
Load only the names needed with the only clause, then use them exactly like kinds from selected_real_kind.
program envdemo
use iso_fortran_env, only : int32, real64
implicit none
integer(int32) :: n
real(real64) :: y
n = 7_int32
y = 2.5_real64
print *, n, y
end program envdemo
The working-precision idiom
Many projects define one double precision constant named dp and use real(dp) for every float in the program.
Changing one line then changes the precision of the whole model, which is ideal for experiments and teaching.
program dpdemo
implicit none
integer, parameter :: dp = kind(1.0d0)
real(dp) :: pi
pi = 3.141592653589793_dp
print *, pi
end program dpdemo
Next: Operators
Article author: Arthur Isaev
