Real*8

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Ogier Dudley

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Aug 3, 2024, 12:30:05 PM8/3/24

to cacolnobunk

I thought that the 8 meant that 8 digits follow the decimal point. However, Wikipedia seems to say that single precision typically provides 6-9 digits whereas double precision typically provides 15-17 digits. Does this mean that the statement "64-bit precision" is inconsistent with real*8?

The "real4" statement specifies the variable names to be single precision 4-byte real numbers which has 7 digits of accuracy and a magnitude range of 10 from -38 to +38. The "real" statement is the same as "real4" statement in nearly all 32-bit computers.

The "real8" statement specifies the variable names to be double precision 8-byte real numbers which has 15 digits of accuracy and a magnitude range of 10 from -308 to +308. The "double precision" statement is the same as "real8" statement in nearly all 32-bit computers.

The new way is with "kinds". One should use the intrinsic functions to obtain the kind that has the precision that you need. Specifying the kind by specific numeric value is risky because different compilers use different values.

If applied to another type, it specifies the storage size in bytes. This should be avoided at any cost in Fortran 90+, where the concept of type KIND is introduced. Specifying storage size creates non-portable applications.