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. For such files, the `csv_readcol` function might be a better choice.

  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, csv_readcol`

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