DEXP or EXP for exponential function in fortran?

I have two very short questions:

1 - I just read that DEXP() is the archaic form of EXP(). Does it mean that it should not be u开发者_运维知识库sed anymore? I always thought that DEXP() was the double precision equivalent to EXP().

2 - What is the range of the exponential function? Is it compiler dependent?

Question number 1:

In modern Fortran it is always better to use the generic functions, such as EXP(), in preference to the outdated type specific equivalents, such as DEXP().

In the old (really old) versions of Fortran (before FORTRAN 77), a different function was required for each data type. So if you wanted the exponential function would need: EXP() for single precision numbers, DEXP() for double precision numbers, or CEXP() for complex numbers. Fortran now has function overloading, so a single function will work for any standard type.

Question number 2.

In principle the possible range of the exponent can be processor and compiler dependent. However, most modern processors and compilers will use the IEEE standard.

If needed, it is possible to specify the required range of a variable when declaring it. The function to use is SELECTED_REAL_KIND([P,R]).

For example, suppose you to make sure that x is of a type with decimal precision of at least 10 digits and a decimal exponent range of at least 100.

INTEGER, PARAMETER :: mytype = SELECTED_REAL_KIND(10, 100)
REAL(KIND=mytype) :: x

For more information: SELECTED_REAL_KIND

In practice, if you are writing a program that requires a given accuracy, and which may be run on exotic or old systems, it is a very good idea to define your types in this way. Some common definitions are shown here: Real Precision

"exp" is a generic function, that returns the same type as its argument -- precision of real or complex. It should be used in preference to the older form "dexp" because with "exp" the compiler will automatically return the correct type. The generic names were added in Fortran 77.

The answer to part 2 of your question is that the range of the exponential function is the set of all positive real numbers. In Fortran terms that means the set of all REAL numbers greater than 0. Yes it is, according to the Fortran standards, compiler dependent, but in practice you won't go far wrong if you take it to be the set of all positive IEEE floating-point numbers, single or double precision as you wish. But to be strict you need to be familiar with the KINDs of real numbers that your compiler supports which will almost certainly include the IEEE f-p numbers, but may include others too.