SLSH Library Reference (version 2.2.0): Reading Text-formated Data Files

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## 5. Reading Text-formated Data Files

These functions are defined in `readascii.sl`.

## 5.1 readascii

Synopsis

  Read data from a text file

Usage

  `Int_Type readascii (file, &v1,...&vN ; qualifiers)`

Description

  This function may be used to read formatted data from a text (as opposed to binary) file and stores the values as arrays in the specified variables `v1,..., vN` (passed as references). It returns the number of lines read from the file that matched the format (implicit or specified by a qualifier).

  The file parameter may be a string that gives the filename to read, a `File_Type` object representing an open file pointer, or an array of lines to be scanned.

Qualifiers

  The following qualifiers are supported by the function

  > `

  >        format=string        The sscanf format string to be used when
  >                               parsing lines.
  >        nrows=integer        The maximum number of matching rows to handle.
  >        ncols=integer        If a single reference is passed, it will be
  >                               assigned an array of ncols arrays of data values.
  >        skip=integer         Skip the specified number of lines before
  >                               scanning
  >        maxlines=integer     Read no more than this many lines.
  >        cols=Array_Type      Read only the specified (1-based) columns.
  >                               Used with an implict format
  >        delim=string         For an implicit format, use this as a field
  >                               separator.  The default is whitespace.
  >        type=string          For an implicit format, use this sscanf type
  >                               specifier.  Default is %lf (Double_Type).
  >        size=integer         Use this value as the initial size for the
  >                               arrays.
  >        dsize=integer        Use this value as an increment when
  >                               reallocating the arrays.
  >        stop_on_mismatch     Stop reading input when a line does not match
  >                               the format
  >        lastline=&v          Assign the last line read to the variable v.
  >        lastlinenum=&v       Assign the last line number (1-based to v)
  >        comment=string       Lines beginning with this string are ignored.
  >        as_list              If present, then return data in lists rather
  >                               than arrays.

  > `

Example

  As a simple example, consider a file called `imped.dat` containing

  > `

  >         # Distance    Zr   Zi
  >         0.0          50.2    0.1
  >         1.0          47.3  -12.2
  >         2.0          43.9  -15.8

  > `

  The 3 columns may be read and stored in the variables `x`, `zr`, `zi` using

  > `

  >          n = readascii ("imped.dat", &x, &zr, &zi);

  > `

  After return, The value of `x` will be set to `[0.0,1.0,2.0]`, `zr` to `[50.2,47.3,43.9]`, `zi` to `[0.1,-12.2,-15.8]`, and `n` to 3.

  Another way to read the same data is to use

  > `

  >         n = readascii ("imped.dat", &data; ncols=3);

  > `

  In this case, `data` will be `Array_Type[3]`, with each element of the array containing the array of data values for the corresponding column. As before, `n` will be 3.

  As a more complex example, Consider a file called `score.dat` that contains:

  > `

  >         Name      Score     Date          Flags
  >         Bill       73.2     03-Nov-2046    1
  >         James      22.9     03-Nov-2046    1
  >         Lucy       89.1     04-Nov-2046    3

  > `

  This file may be read using

  > `

  >          n = readascii ("score.dat", &name, &score, &date, &flags;
  >                         format="%s %lf %s %d");

  > `

  In this case, `n` will be 3, `name` and `date` will be `String_Type` arrays, `score` will be a `Double_Type` array, and `flags` will be an `Int_Type` array.

  Now suppose that only the score and flags column are of interest. The `name` and `date` fields may be ignored using

  > `

  >          n = readascii ("score.dat", &score, &flags";
  >                         format="%*s %lf %*s %d");

  > `

  Here, `%*s` indicates that the field is to be parsed as a string, but not assigned to a variable.

  Consider the task of reading columns from a file called `books.dat` that contain quoted strings such as:

  > `

  >          # Year  Author Title
  >          "1605" "Miguel de Cervantes"  "Don Quixote de la Mancha"
  >          "1885" "Mark Twain" "The Adventures of Huckleberry Finn"
  >          "1955" "Vladimir Nabokov" "Lolita"

  > `

  Such a file may be read using

  > `

  >          n = readascii ("books.dat", &year, &author, &title;
  >                         format="\"%[^\"]\" \"%[^\"]\" \"%[^\"]\"");

  > `

Notes

  This current version of this function does not handle missing data.

  By default, lines not matching the expected format are assumed to be comments and are skipped. So normally the `comment` qualifier is not needed. However, it is useful in conjunction with the `stop_on_mismatch` qualifier to force the parser to skip lines beginning with the comment string and continue scanning.

See Also

  `sscanf, atof, fopen, fgets, fgetslines`

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