## 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.

### Concepts

We distinguish four 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, so that all four kinds are represented by four-letter type names.
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.

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

- The variable name, of the form `class:name` (or `file:name`, usually these agree), e.g. `TimeShower:pTmin`. The class/file part usually identifies the `.xml` file where the variable is defined, and the part of the program where it is used, but such a connection cannot be strictly upheld, since e.g. the same variable may be used in a few different cases (even if most of them are not).
- The default value, set in the original declaration, and intended to represent a reasonable choice.
- The current value, which differs from the default when the user so requests.
- An allowed range of values, represented by meaningful minimum and maximum values. This has no sense for a `flag` or a `word` (and is not used there), is usually rather well-defined for a `mode`, but less so for a `parm`. Often the allowed range exaggerates the degree of our current knowledge, so as not to restrict too much what the user can do. One may choose not to set the lower or upper limit, in which case the range is open-ended.

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

### Operation

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` or `<word` are identified, and the information on such a line is used to define a new flag, mode, parameter or word. 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 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. 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

           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.

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

           pythia.settings.listAll();

   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

           pythia.settings.listChanged();

   where you will only get those variables that differ from their defaults. Or you can use

           pythia.settings.list("string");

   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

           pythia.settings.writeFile(fileName);

   This file could then directly be read in by `readFile(fileName)` in a subsequent (identical) run. Some variants of this command are listed below.

### Methods

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(...)`.

Settings::Settings()
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)
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)
add an entry of the respective kind to the database. The name and default value always has to be supplied, for `Mode` and `Word` 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)
return the current value of the respective setting. If the name does not exist in the database, a value `false`, `0`, `0.` and `" "` is returned, respectively.

map Settings::getFlagMap(string match)
map Settings::getModeMap(string match)
map Settings::getParmMap(string match)
map Settings::getWordMap(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)
change the current value of the respective setting to the provided new value. 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)
as above, but do not check lower and upper limits, so that the current value 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)
reset the current value to the default one.