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Flat File Reader/Writer for Lua

This module reads and writes tables as files with fixed-width records.


open, mode)

Creates a new object for reading or writing a file. The source is either an open file object or a filename to be opened. The mode is either "r", "w", or "a" to read, write, or append to the file. When passing an open file as the source, the mode must be compatible with the existing handle. A handle to be used for appending must allow both reading and writing.

The new file object will not have any columns defined. You must use columns or header to set the column widths.



Define the keys of the table. The definitions may be:

  1. Column numbers only. Each field is defined by a pair of numbers: the first and last columns in the file. Records are returned with integer keys. This format completely defines the columns.

  2. Names only. Each field is identified by name. The columns will be determined when the header is read. This column definition is incomplete.

  3. Names and numbers. Each field is identified by name and the start and end columns in the file. This format completely defines the columns.

A field name will be used as the key in the output table. Any character that is printable is allowed in the field name, including spaces. Spaces at the beginning or end of the name will not be included in the key.

All defined fields must be present in each record. When reading by name, optional or unknown fields are allowed. Add a "?" to the end of a field name to make it optional. That field will be included if it is found in the header, or skipped if not. If you need to read a field that ends with "?" then add a space character after the question mark. A field name that is just "?" indicates that unknown fields should be returned.

The use of optional or unknown fields makes the column definitions incomplete. The header must be read. By default, a newly opened file will read all fields in the header. This is the same as if you called reader:columns("?").

header (reading)

    reader:header([skip] [,columnname])

Read the existing header from a file and set the column definitions from it. The function may be called with either, both, or neither argument.

The skip argument is the number of lines at the beginning of the file that will be ignored. These lines will be returned as a list of values.

The columnname argument is a field name that must be in the file header. Lines from the file (after those ignored) will be read until this string is found. The found line will then be used to determine the names and column widths of fields in the file. The function will return an error message if the header can't be found.

It is neccessary to call this function if the columns are not completely defined by the columns function. If the columns were defined by number only, then the columnname argument will be used to locate the header, but the header will not be parsed as the programmed columns will be assumed correct. If any names were given in the call to columns, then the columnname argument will be ignored. The header function will then search for the defined field names and fail if not all non-optional names are in the file.

This form of the header function should also be used when appending to a file.

header (writing)


Write the header with optional lines. This function writes a line with the defined field names at the appropriate column widths. The fields must have been completely defined by a call to columns. The arguments to this function are lines that will be written before the header. You do not need to include newline characters, they will be added automatically.



Returns an iterator for reading rows from the file. Each record will be returned as a table with the defined keys. If the expand argument is given, then the iterator function will return the fields as individual values.


    reader:read([what] [,expand])

Read records from the file. The what argument is either "row" to read one record, or "all" to read the entire file. A single record will be returned as a table with the defined keys, or if expand is true as a list of values. Reading multiple records will return a table of tables and the expand argument is ignored.

Optional fields that are missing from the file will be nil in the returned table. When returning the values without a table, missing fields will be an empty string.


    reader:readinto(destination [,expand])

Read a record and pass the result to destination which can be a table or function. If a table, the fields will be stored as keys by the field name. If the destination is a function, then a table of fields will be passed as the argument. If expand is true, then the fields will be passed to the function as individual arguments. The return value of readinto is the destination table or the return value from the function.



Write a row to the file with the passed values as fields. The arguments must be strings or numbers. A single table may be passed to read the fields from the keys of the table.


Fixed width text file reader for Lua







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