2. Reading the case file

The case lives in a file, case.xml:

xml

<?xml version="1.0" encoding="UTF-8"?>
<!-- the case of a (pretend) flow solver: the input of the FoXy tutorial -->
<case name="cavity" version="2">
  <mesh>
    <grid ni="64" nj="32" nk="1"/>
    <domain>0.0 1.0 0.0 0.5 0.0 0.0</domain>
  </mesh>
  <time>
    <dt unit="s">1.0e-3</dt>
    <steps>500</steps>
    <adaptive>true</adaptive>
  </time>
  <fluid name="air">
    <density unit="kg/m3">1.225</density>
    <viscosity unit="Pa s">1.8e-5</viscosity>
  </fluid>
  <boundary side="west" type="inflow">
    <velocity>1.0 0.0 0.0</velocity>
  </boundary>
  <boundary side="east" type="outflow"/>
  <boundary side="north" type="wall"/>
  <boundary side="south" type="wall"/>
  <output every="100">results.xml</output>
  <notes>Lid-driven cavity &amp; inflow, after <![CDATA[<paper ref="1">]]>.</notes>
</case>

solver parses it and reads its settings as numbers, not as text.

f90

program solver
!< Tutorial, chapter 2: reading the case file.
use foxy
implicit none
type(xml_file)            :: case_file
integer(I4P)              :: iostat
character(len=256)        :: iomsg
real(R8P)                 :: dt, density
integer(I4P)              :: steps, ni, nj
logical                   :: adaptive
real(R8P),    allocatable :: domain(:)

call case_file%parse(filename='case.xml', iostat=iostat, iomsg=iomsg)
if (iostat/=0) then
   print '(A)', 'cannot read case.xml: '//trim(iomsg)
   stop 1
endif
call case_file%get_content(id=case_file%find(path='case/time/dt'), value=dt)
call case_file%get_content(id=case_file%find(path='case/time/steps'), value=steps)
call case_file%get_content(id=case_file%find(path='case/time/adaptive'), value=adaptive)
call case_file%get_content(id=case_file%find(path='case/mesh/domain'), values=domain)
call case_file%get_attribute(id=case_file%find(name='grid'), name='ni', value=ni)
call case_file%get_attribute(id=case_file%find(name='grid'), name='nj', value=nj)
print '(A,ES10.3,A,I0,A,L1)', 'dt = ', dt, ', steps = ', steps, ', adaptive = ', adaptive
print '(A,I0,A,I0,A,6F5.1)', 'grid ', ni, ' x ', nj, ', domain', domain
density = 1.0_R8P                                                           ! a default, kept on error
call case_file%get_content(id=case_file%find(path='case/fluid/densty'), value=density, iostat=iostat, iomsg=iomsg)
if (iostat==XML_NOT_FOUND) print '(A,F6.3)', 'not found, density kept: ', density
call case_file%get_attribute(id=case_file%find(name='density'), name='unit', value=steps, iostat=iostat, iomsg=iomsg)
if (iostat==XML_BAD_VALUE) print '(A)', trim(iomsg)
endprogram solver

parse(filename=) reads the file in chunks (1 MiB by default), so it is never read into one big string; the DOM still holds every tag and its text. For a file larger than your memory, use the index (chapter 6) or a handler (chapter 7). iostat is 0, the runtime I/O error (the file cannot be read), or XML_MALFORMED (see chapter 9). A UTF-16, UTF-32 or Latin-1 file is converted to UTF-8 as it is read.
get_content(id, value) converts the text of a tag into value: any integer kind, real(R4P), real(R8P) or logical (true, false, 1, 0). With values=, an allocatable array, it reads a blank-separated list.
get_attribute(id, name, value) does the same for an attribute.
On error the value is left unchanged, so a default set before the call survives: iostat is XML_NOT_FOUND for a missing tag or attribute (ID 0 included), XML_BAD_VALUE for a text that is not a value of that type; iomsg says which one and where. Without iostat, errors are silent.

Running it

$ solver
dt =  1.000E-03, steps = 500, adaptive = T
grid 64 x 32, domain  0.0  1.0  0.0  0.5  0.0  0.0
not found, density kept:  1.000
invalid character "k" in "kg/m3" in attribute "unit" of tag "density"

What you learned

Read files with parse(filename=), numbers and lists with get_content/get_attribute; a default plus iostat makes a setting optional. Reference: Typed values.

Next: 3. Walking the tree.

2. Reading the case file ​

Running it ​

2. Reading the case file

Running it