The Settings Scheme

The Settings class keeps track of all the flags, modes, parameters and words in the program. As such, it serves the other program elements from one central repository. Accessing it allows the user to modify the behaviour of the program. The Settings class is purely static, i.e. you can interact with it directly by Settings::command(argument). However, a settings object of the Settings class is a public member of the Pythia class, so an alternative notation would be 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.

Concepts

We distinguish four kinds of user-modifiable variables, by the way they have to be stored:

Flags are on/off switches, and are stored as bool.
Modes corresponds to a finite enumeration of separate options, and are stored as int.
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.
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:

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.

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.
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, unless an optional second argument false is used. 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. 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); 
```
Boolean values should here be given as true or false i.e. there is less flexibility in the lower-level methods.
At the same level, there are several different methods available. We here show the ones for mode, but corresponding methods exist for flag, parm and word, with obvious restrictions where min and max are not defined. Again name comparisons are case-insensitive.
method mode( name)
gives the current value,
method mode( name, value)
sets the current value,
method isMode( name)
tells whether a mode has been defined or not,
method addMode( name, default, min, max)
defines a new mode,
method forceMode( name, value)
sets the value, also when outside the recommended bounds (and it is completely up to you to face the consequences),
method resetMode( name)
resets the current value to the default one.
Normally the user should have no need for these methods. The main exception is when some of the variables defined on the Main-Program Settings page are used to set run-specific information (like the CM energy or the number of events to generate) in an external file (see 2d below) and these variables are to be read into the main program. Then the flag(name), mode(name) parm(name) and word(name) methods are to be used, see e.g. the main programs in the examples subdirectory to find out how it works.
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. 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. Again, an optional second argument false allows you to switch off warning messages for unknown variables.

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 from scratch. Such a call might be required if several Pythia objects are created in the same run, and requested to have different values - by default the init() call is only made the first time. However, a more economical solution is then offered by resetAll(), which sets all variables to their default values.

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.

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. A second argument true would print all settings, not only the changed ones. Further, the first argument can be replaced by (a reference to) an ostream, by default cout.