Fortran String to Number

This is an initiative in order improvement string to number conversion of Fortran following the discussions in the fortran community. Which is still very much a work in progress.

The key elements are:

• Transforming the individual strings into its ASCII code integer equivalent.
• Add up the values respecting their respective power of 10th position.
• For float numbers: determine the positions of the decimal place marker and exponent caracters.

Intended use cases:

• Interpreting a single string:

use str2num_m, only: str2real
real(8) :: r
r = str2real("1.234")
r = str2real(" -0.2331726067853964E-01 ")

• Interpreting an array of strings:

use str2num_m, only: str2real
real(8), allocatable :: r(:)
...
! Loop approach
do i = 1, n
   rval(i) = str2real ( strs(i) )
enddo
! or array approach
rval(:) = str2real ( strs(:) )

• stream-lining through a chain of numbers in one string:

use str2num_m, only: str2real_p !> pointer version of the str2real function
character(:),allocatable,target :: strs_seq !> Original string
character(len=:), pointer :: ps !> Working pointer
real(8), allocatable :: r(:)
...
ps => strs_seq(1:)
do i = 1, n
   rval(i) = str2real_p ( ps ) !> the pointer is shifted within the function
enddo
! OR
do i = 1, n
   ps => strs(i)(1:)
   rval(i) =  str2real_p( ps )
enddo

(a str2int and str2int_p equivalents are also available in the module, which will return only positive integers)

Performance

This library comes with a test for correctness and a benchmark for execution time. The benchmark only contains one format, the speedup will be different for shorter/other formats. All tests are a comparison between formatted read and str2real

Latest test show this implemantation is about 20 times faster than the standard read. (compiler and hardward dependent)

test correctness:

fpm test test

benchmark execution time:

fpm test bench --flag "-O3 -march=native"

fpm test main_test --flag "-O3 -march=native"

Footnote

Authors of the original project Carl Burkert and collaborators: Jacob Williams and Beliavsky

Further help from Tran Quoc Viet