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#### 4.1.1.7 ASCII

In many cases tables are stored in some sort of unstructured plain text format, with cells separated by spaces or some other delimiters. There is a wide variety of such formats depending on what delimiters are used, how columns are identified, whether blank values are permitted and so on. It is impossible to cope with them all, but the ASCII handler attempts to make a good guess about how to interpret a given ASCII file as a table, which in many cases is successful. In particular, if you just have columns of numbers separated by something that looks like spaces, you should be just fine.

Despite the name, by default it reads Unicode characters in the UTF-8 encoding. It can if necessary be restricted to ASCII characters only by setting the `encoding` option.

Here are the detailed rules for how the ASCII-format tables are interpreted:

- Bytes in the file are interpreted as Unicode text (by default UTF-8 encoded)
- Each table row is represented by a single line of text
- Lines are terminated by one or more contiguous line termination characters: line feed (0x0A) or carriage return (0x0D)
- Within a line, fields are separated by one or more whitespace characters: space (" ") or tab (0x09)
- A field is either an unquoted sequence of non-whitespace characters, or a sequence of non-newline characters between matching single (') or double (") quote characters - spaces are therefore allowed in quoted fields
- Within a quoted field, whitespace characters are permitted and are treated literally
- Within a quoted field, any character preceded by a backslash character ("\") is treated literally. This allows quote characters to appear within a quoted string.
- An empty quoted string (two adjacent quotes) or the string "`null`" (unquoted) represents the null value
- All data lines must contain the same number of fields (this is the number of columns in the table)
- The data type of a column is guessed according to the fields that appear in the table. If all the fields in one column can be parsed as integers (or null values), then that column will turn into an integer-type column. The types that are tried, in order of preference, are: `Boolean`, `Short` `Integer`, `Long`, `Float`, `Double`, `String`
- Some special values are permitted for floating point columns: `NaN` for not-a-number, which is treated the same as a null value for most purposes, and `Infinity` or `inf` for infinity (with or without a preceding +/- sign). These values are matched case-insensitively.
- Empty lines are ignored
- Anything after a hash character "#" (except one in a quoted string) on a line is ignored as far as table data goes; any line which starts with a "!" is also ignored. However, lines which start with a "#" or "!" at the start of the table (before any data lines) will be interpreted as metadata as follows:
  - The last "#"/"!"-starting line before the first data line may contain the column names. If it has the same number of fields as there are columns in the table, each field will be taken to be the title of the corresponding column. Otherwise, it will be taken as a normal comment line.
  - Any comment lines before the first data line not covered by the above will be concatenated to form the "description" parameter of the table.

If the list of rules above looks frightening, don't worry, in many cases it ought to make sense of a table without you having to read the small print. Here is an example of a suitable ASCII-format table:

        #
        # Here is a list of some animals.
        #
        # RECNO  SPECIES         NAME         LEGS   HEIGHT/m
          1      pig             "Pigling Bland"  4  0.8
          2      cow             Daisy        4      2
          3      goldfish        Dobbin       ""     0.05
          4      ant             ""           6      0.001
          5      ant             ""           6      0.001
          6      ant             ''           6      0.001
          7      "queen ant"     'Ma\'am'     6      2e-3
          8      human           "Mark"       2      1.8

In this case it will identify the following columns:

        Name       Type
        ----       ----
        RECNO      Short
        SPECIES    String
        NAME       String
        LEGS       Short
        HEIGHT/m   Float

It will also use the text "`Here is a list of some animals`" as the Description parameter of the table. Without any of the comment lines, it would still interpret the table, but the columns would be given the names `col1`..`col5`.

The handler behaviour may be modified by specifying one or more comma-separated name=value configuration options in parentheses after the handler name, e.g. "`ascii(maxSample=100000,notypes=short;float)`". The following options are available:

`maxSample = <int>`
  Controls how many rows of the input file are sampled to determine column datatypes. This file format provides no header information about column type, so the handler has to look at the column data to see what type of value appears to be present in each column, before even starting to read the data in. By default it goes through the whole table when doing this, which can be time-consuming for large tables. If this value is set, it limits the number of rows that are sampled in this data characterisation pass, which can reduce read time substantially. However, if values near the end of the table differ in apparent type from those near the start, it can also result in getting the datatypes wrong. (Default: `0`)
`notypes = <type>[;<type>...]`
  Specifies a semicolon-separated list of names for datatypes that will not appear in the columns of the table as read. Type names that can be excluded are `blank`, `boolean`, `short`, `int`, `long`, `float`, `double`, `date`, `hms` and `dms`. So if you want to make sure that all integer and floating-point columns are 64-bit (i.e. `long` and `double` respectively) you can set this value to "`short;int;float`".
`encoding = ASCII|UTF-8|UTF-16|...`
  Specifies the character encoding used to interpret the input file. (Default: `UTF-8`)

This format cannot be automatically identified by its content, so in general it is necessary to specify that a table is in ASCII format when reading it. However, if the input file has the extension "`.txt`" (case insensitive) an attempt will be made to read it using this format.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk