# 2.1. The input file¶

## 2.1.1. Using the input file¶

By default, the run time expects to find user input in a file input in the current working directory. If a different file name is required, its name should be provided as the sole command line argument, e.g.,

./Ludwig.exe input_file_name


If the input file is not located in the current working directory the code will terminate immediately with an error message.

When an input file is located, its content is read by a single MPI task, and its contents then broadcast to all MPI relevant tasks. The format of the file is plain ASCII text, and its contents are parsed on a line by line basis. Lines may contain the following:

# Comments are introduced by a # and are ignored
# Here is a key value pair
key value


Blank lines are treated as comments. The behaviour of the code is determined by a set of key value pairs. Any given key may appear only once in the input file. If the key value pairs are not correctly formed, the code will usually terminate with an error message and indicate the offending input line.

## 2.1.2. Key value pairs¶

Key value pairs are made up of a key — an alphanumeric string with no white space — and corresponding value following white space. Values may take on the follow forms:

key_string           value_string

key_integer_scalar   1
key_integer_vector   1_2_3

key_double_scalar    1.0
key_double_vector    1.0_2.0_3.0

# keys intended for logical switches may take a number of different values
key_switch_off       [0 | no  | off]
key_switch_on        [1 | yes | on ]


Values which are strings should contain no white space. Scalar parameters may be integer values, or floating point values with a decimal point (scientific notation is also allowed). Vector parameters are introduced by a set of three values (to be interpreted as $$x,y,z$$ components of the vector in Cartesian coordinates) separated by an underscore. The identity of the key will specify what type of value is expected. Keys and (string) values are case sensitive.

Most keys have an associated default value which will be used if the key is not present. Some keys must be specified: an error will occur if they are missing. The remainder of this part of the guide details the various choices for key value pairs, along with any default values, and any relevant constraints.

## 2.1.3. Unused key value pairs¶

Key value pairs which appear in the input file, but are not used by the code at run time, are reported at the end of the run. This may indicate that the key string is incorrect or unrecognised.