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