Between the creation of the Pythia object and the
init call for it, you may use several alternative
methods to modify some of the default values. The same variable
can be changed several times. If so, it is the last read value
that counts. The two special
";?>Tune:ee and
";?>Tune:pp modes and the
";?>Print:quiet flag
are expanded to change several settings in one go, but these obey
the same ordering rules.
a) Inside your main program you can directly set values with
pythia.readString(string)
where both the variable name and the value are contained inside
the character string, separated by blanks and/or a =, e.g.
pythia.readString("TimeShower:pTmin = 1.0");
The match of the name to the database is case-insensitive. Names
that do not match an existing variable are ignored. A warning is
printed, however. Strings beginning with a non-alphanumeric character,
like # or !, are assumed to be comments and are not processed at all.
Values below the minimum or above the maximum are set at
the respective border. In extreme cases, where it is necessary to
go outside the allowed range, "FORCE=" can replace
the normal "=" separator to force the requested value,
at own responsibility. For bool values, the following
notation may be used interchangeably:
true = on = yes = ok = 1, while everything else gives
false (including but not limited to
false, off, no and 0).
b) The Pythia readString(string) method
actually does not do changes itself, but sends on the string either
to the Settings class or to ParticleData.
The former holds if the string begins with a letter, the latter
if it begins with a digit. (The exception is if an input list has been
begun by an open brace { but no matching close brace } was present;
then all subsequent non-empty input is directed to Settings
until the close brace is found.) If desired, it is possible to communicate
directly with the corresponding Settings method:
pythia.settings.readString("TimeShower:pTmin = 1.0");
In this case, changes intended for ParticleData
would not be understood.
c) Underlying the settings.readString(string) method are
the settings-type-sensitive commands in the Settings, that
are split by names containing flag, mode,
parm or word. Thus, the example now reads
pythia.settings.parm("TimeShower:pTmin", 1.0);
Such a form could be convenient e.g. if a parameter is calculated
at the beginning of the main program, and thus is available as a
variable rather than as a character string.
Note that Boolean values must here be given as true or
false i.e. there is less flexibility than with the
previous methods.
At the same level, there are several different methods available.
These are included in the full description below, but normally the user
should have no need for them.
d) A simpler and more useful way is to collect all your changes
in a separate file, with one line per change, e.g.
TimeShower:pTmin = 1.0
Each line is read in as a string and processed with the methods already
introduced.
The file can be read by the
pythia.readFile(fileName);
method (or an istream instead of a fileName).
The file can freely mix commands to the Settings and
ParticleData classes, and so is preferable. Lines with
settings are handled by calls to the
pythia.settings.readString(string) method.
A file can make use of two extra features that are not available with the
readString(...) method. One is the possibility to provide
information for several distinct ";?>subruns.
The other is the possibility to comment out a section of lines in the file.
The first line of the commented section should then begin by /*
and the last begin by */. This is reminiscent of the convention
used in C++ and other languages, but is not as powerful, in that it is not
possible to comment in or out parts of lines. It is only the first two
non-blank characters of a line that are checked for a match, and a line
beginning with */ is counted as part of the commented section.
To avoid mistakes it is best to keep /* and */
on lines of their own, optionally followed by comments, but not by commands.