The Settings Scheme

The Settings class keeps track of all the flags, modes, parameters and words used during the event generation. As such, it serves all the Pythia program elements from one central repository. Accessing it allows the user to modify the generator behaviour.

Each Pythia object has a public member settings of the Settings class. Therefore you access the settings methods as pythia.settings.command(argument), assuming that pythia is an instance of the Pythia class. Further, for the most frequent user tasks, Pythia methods have been defined, so that pythia.command(argument) would work, see further below.

The central section on this page is the Operation one. The preceding concepts section is there mainly to introduce the basic structure and the set of properties that can be accessed. The subsequent sections provide a complete listing of the existing public methods, which most users probably will have little interaction with.


We distinguish seven kinds of user-modifiable variables, by the way they have to be stored:
  1. Flags are on/off switches, and are stored as bool.
  2. Modes corresponds to a finite enumeration of separate options, and are stored as int.
  3. Parameters take a continuum of values, and are stored as double. The shorthand notation parm is used in the C++ code and XML tags.
  4. Words are simple character strings and are stored as string. No blanks or double quotation marks (") may appear inside a word, the former to simplify parsing of an input file and the latter not to cause conflicts with XML attribute delimiters. Currently the main application is to store file names.
  5. Vectors of flags take a variable length, and are stored as vector<bool>. The shorthand notation fvec is used in the C++ code and XML tags. When the vector is input as a string, all the values have to be given as a comma-separated list with no blanks, to simplify parsing.
  6. Vectors of modes take a variable length, and are stored as vector<int>. The shorthand notation mvec is used in the C++ code and XML tags. When the vector is input as a string, all the values have to be given as a comma-separated list with no blanks, to simplify parsing.
  7. Vectors of parameters take a variable length and for each element a continuum of values, and are stored as vector<double>. The shorthand notation pvec is used in the C++ code and XML tag. When the vector is input as a string, all the values have to be given as a comma-separated list with no blanks, to simplify parsing.
Note that all shorthands have been chosen four letters long.

In general, each variable stored in Settings is associated with four kinds of information:

Technically, the Settings class is implemented with the help of seven separate maps, one for each kind of variable, with the variable name used as key.


The normal flow of setting values is:

  1. When a Pythia object pythia is created, the member pythia.settings is asked to scan the files listed in the Index.xml file in the xmldoc subdirectory.

    In all of the files scanned, lines beginning with <flag, <mode, <parm, <word, <fvec, <mvec or <pvec are identified, and the information on such a line is used to define a new flag, mode, parameter, word, or vector of flags, modes or parameters. To exemplify, consider a line

    <parm name="TimeShower:pTmin" default="0.5" min="0.1" max="2.0"> 
    which appears in the TimeShower.xml file, and there defines a parameter TimeShower:pTmin with default value 0.5 GeV and allowed variation in the range 0.1 - 2.0 GeV. The min and max values are optional.
    Important: the values in the .xml files should not be changed, except by the PYTHIA authors. Any changes should be done with the help of the methods described below.
  2. 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

    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. 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. 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
    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.

  3. In the pythia.init(...) call, many of the various other program elements are initialized, making use of the current values in the database. Once initialized, the common Settings database is likely not consulted again by these routines. It is therefore not productive to do further changes in mid-run: at best nothing changes, at worst you may set up inconsistencies.

    A routine reInit(fileName) is provided, and can be used to zero all the maps and reinitialize them from scratch. Such a call might be useful if several subruns are to be made with widely different parameter sets - normally the maps are only built from scratch once, namely when the Pythia() object is created. A more economical alternative is offered by resetAll(), however, which sets all variables back to their default values.

  4. You may at any time obtain a listing of all variables in the database by calling
    The listing is strictly alphabetical, which at least means that names from the same file are kept together, but otherwise may not be so well-structured: important and unimportant ones will appear mixed. A more relevant alternative is
    where you will only get those variables that differ from their defaults. Or you can use
    where only those variables with names that contain the string (case-insensitive match) are listed. Thus, with a string shower, the shower-related variables would be shown.
  5. The above listings are in a tabular form that cannot be read back in. Assuming you want to save all changed settings (maybe because you read in changes from several files), you can do that by calling
    This file could then directly be read in by readFile(fileName) in a subsequent (identical) run. Some variants of this command are listed below.


The complete list of methods and arguments is as follows. Most of the ones of interest to the user have already been mentioned above. Others can be used, but the same functionality is better achieved by higher-level routines. Some are part of the internal machinery, and should not be touched by user.

Note that there is no Settings::readFile(...) method. The intention is that you should use Pythia::readFile(...). It parses and decides which individual lines should be sent on to Settings::readString(...).

the constructor, which takes no arguments. Internal.

bool Settings::initPtr(Info* infoPtrIn)  
initialize pointer to error-message database. Internal.

bool Settings::init(string startFile = "../xmldoc/Index.xml", bool append = false, ostream& os = cout)  
read in the settings database.
argument startFile (default = "../xmldoc/Index.xml") : read in the settings from all the files listed in this file, and assumed to be located in the same subdirectory.
argument append (default = false) : By default nothing is done if the method has already been called once. If true the further settings read in are added to the current database.
argument os (default = cout) : stream for error printout.
Note: The method returns false if it fails.

bool Settings::reInit(string startFile = "../xmldoc/Index.xml", ostream& os = cout)  
overwrite the existing database.
argument startFile (default = "../xmldoc/Index.xml") : read in the settings from all the files listed in this file, and assumed to be located in the same subdirectory.
argument os (default = cout) : stream for error printout.
Note: The method returns false if it fails.

bool Settings::readString(string line, bool warn = true, ostream& os = cout)  
read in a string, and change the relevant quantity in the database. It is normally used indirectly, via Pythia::readString(...) and Pythia::readFile(...).
argument line : the string to be interpreted as an instruction.
argument warn (default = true) : write a warning message or not whenever the instruction does not make sense, e.g. if the variable does not exist in the databases.
argument os (default = cout) : stream for error printout.
Note: the method returns false if it fails to make sense out of the input string.

bool Settings::writeFile(string toFile, bool writeAll = false)  
bool Settings::writeFile(ostream& os = cout, bool writeAll = false)  
write current settings to a file or to an ostream.
argument toFile, os : file or stream on which settings are written.
argument writeAll (default = false) : normally only settings that have been changed are written, but if true then all settings are output.
Note: the method returns false if it fails.

void Settings::listAll(ostream& os = cout)  
void Settings::listChanged(ostream& os = cout)  
void Settings::list(string match, ostream& os = cout)  
list all or changed settings, or a group of them.
argument match : list all those settings where the name contains the match (sub)string (case-insensitive).
argument os (default = cout) : output stream for the listing.

void Settings::resetAll()  
reset all current values to their defaults.

bool Settings::isFlag(string key)  
bool Settings::isMode(string key)  
bool Settings::isParm(string key)  
bool Settings::isWord(string key)  
bool Settings::isFVec(string key)  
bool Settings::isMVec(string key)  
bool Settings::isPVec(string key)  
return true if an entry of the given name and kind exists, else false.

void Settings::addFlag(string key, bool default)  
void Settings::addMode(string key, int default, bool hasMin, bool hasMax, int min, int max)  
void Settings::addParm(string key, double default, bool hasMin, bool hasMax, double min, double max)  
void Settings::addWord(string key, string default)  
void Settings::addFVec(string key, vector<bool> default)  
void Settings::addMVec(string key, vector<int> default, bool hasMin, bool hasMax, int min, int max)  
void Settings::addPVec(string key, vector<double> default, bool hasMin, bool hasMax, double min, double max)  
add an entry of the respective kind to the database. The name and default value(s) always has to be supplied, for Mode, Parm, MVec and PVec additionally if lower and/or upper limits are to be imposed and, if so, what those limit are.

bool Settings::flag(string key)  
int Settings::mode(string key)  
double Settings::parm(string key)  
string Settings::word(string key)  
vector<bool> Settings::fvec(string key)  
vector<int> Settings::mvec(string key)  
vector<double> Settings::pvec(string key)  
return the current value(s) of the respective setting. If the name does not exist in the database, a value false, 0, 0., " ", or a vector of length 1 and value false, 0 or 0., respectively, is returned.

bool Settings::flagDefault(string key)  
int Settings::modeDefault(string key)  
double Settings::parmDefault(string key)  
string Settings::wordDefault(string key)  
vector<bool> Settings::fvecDefault(string key)  
vector<int> Settings::mvecDefault(string key)  
vector<double> Settings::pvecDefault(string key)  
return the default value(s) of the respective setting. If the name does not exist in the database, a value false, 0, 0., " ", or a vector of length 1 and value false, 0 or 0., respectively, is returned.

map Settings::getFlagMap(string match)  
map Settings::getModeMap(string match)  
map Settings::getParmMap(string match)  
map Settings::getWordMap(string match)  
map Settings::getFVecMap(string match)  
map Settings::getMVecMap(string match)  
map Settings::getPVecMap(string match)  
return a map of all settings of the respective type that contain the string "match" in its name.

void Settings::flag(string key, bool now)  
void Settings::mode(string key, int now)  
void Settings::parm(string key, double now)  
void Settings::word(string key, string now)  
void Settings::fvec(string key, vector<bool> now)  
void Settings::mvec(string key, vector<int> now)  
void Settings::pvec(string key, vector<double> now)  
change the current value(s) of the respective setting to the provided new value(s). If lower or upper limits have been set, input values outside the allowed range are reinterpreted as being a the nearest limit.

void Settings::forceMode(string key, int now)  
void Settings::forceParm(string key, double now)  
void Settings::forceMVec(string key, vector<int> now)  
void Settings::forcePVec(string key, vector<double> now)  
as above, but do not check lower and upper limits, so that the current value(s) can be put outside the intended borders.

void Settings::resetFlag(string key)  
void Settings::resetMode(string key)  
void Settings::resetParm(string key)  
void Settings::resetWord(string key)  
void Settings::resetFVec(string key)  
void Settings::resetMVec(string key)  
void Settings::resetPVec(string key)  
reset the current value to the default one.