3. Lists and parameters

heat records the temperature at a probe (two coordinates), saves some fields, and lets the user override the physical parameters of the problem without editing an input file.

! exactly two values: the coordinates of a probe
call cli%add(switch='--probe', help='Probe coordinates', required=.false., act='store', nargs='2', def='0.5 0.5', &
             metavar='X')
! any number of values, each one a choice: the switch alone saves nothing
call cli%add(switch='--fields', help='Fields to save', required=.false., act='store', nargs='*', def='T', &
             choices='T,flux', metavar='NAME')
! KEY=VALUE pairs overriding the physical parameters
call cli%add(switch='--set', help='Physical parameters', required=.false., act='store', nargs='+', map=.true., &
             map_keys='alpha,t_end', def='alpha=1e-4')

• nargs='2': exactly two values, read into an array of two elements with get.
• nargs='*': any number of values, also none; read into an allocatable array with get_varying, which allocates it to the right size. choices apply to every value.
• map=.true.: KEY=VALUE pairs; map_keys lists the keys allowed. get_map_value reads one key, and leaves the variable untouched (found=.false.) when the key was not given: the built-in value stays.

call cli%get(switch='--probe', val=probe, error=error)           ; if (error /= 0) stop 1, quiet=.true.
call cli%get_varying(switch='--fields', val=fields, error=error) ; if (error /= 0) stop 1, quiet=.true.
alpha = 1.0d-4 ; t_end = 10.0d0                                   ! the built-in values
call cli%get_map_value(switch='--set', key='alpha', val=alpha, found=found, error=error)
call cli%get_map_value(switch='--set', key='t_end', val=t_end, found=found, error=error)

$ heat --probe 0.5 0.25 --fields T flux --set alpha=2e-5 t_end=60
probe at  0.50  0.25
saving 2 field(s): T, flux
alpha =  2.0E-05, t_end =  60.0

$ heat
probe at  0.50  0.50
saving 1 field(s): T
alpha =  1.0E-04, t_end =  10.0

--fields alone is an empty list, not the default:

$ heat --fields
probe at  0.50  0.50
saving 0 field(s): 
alpha =  1.0E-04, t_end =  10.0

A misspelled key is caught by parse, with a suggestion:

$ heat --set alfa=1
heat: error: unknown key "alfa" for "--set" (allowed: alpha, t_end)! Did you mean "alpha"?

Try 'heat --help' for help.
[exit status 1]

$ heat --help
usage: heat [--probe X#1 X#2] [--fields [NAME#1 NAME#2...]] [--set KEY=VALUE [KEY=VALUE...]] [--help] [--markdown] [--version]

Solve the 2D heat equation on a square plate

Optional switches:
  --probe X#1 X#2
      default value 0.5 0.5
      Probe coordinates
  --fields [NAME#1 NAME#2...]
      choices: T, flux
      default value T
      Fields to save
  --set KEY=VALUE [KEY=VALUE...]
      keys: alpha, t_end
      default value alpha=1e-4
      Physical parameters
  --help, -h
      Print this help message
  --markdown, -md
      Save this help message in a Markdown file
  --version, -v
      Print version

What you learned

Fixed and variable lists, get_varying, maps with get_map_value. Reference: List-valued arguments, Key=value options.

Next: 4. Commands.