GNU Octave: Character Strings in Oct-Files

A character string in Octave is just a special Array class. Consider the example:

#include <octave/oct.h>

DEFUN_DLD (stringdemo, args, , "String Demo")
{
  octave_value_list retval;
  int nargin = args.length ();

  if (nargin != 1)
    print_usage ();
  else
    {
      charMatrix ch = args(0).char_matrix_value ();

      if (! error_state)
        {
          retval(1) = octave_value (ch, '\'');  // Single Quote String

          octave_idx_type nr = ch.rows ();
          for (octave_idx_type i = 0; i < nr / 2; i++)
            {
              std::string tmp = ch.row_as_string (i);
              ch.insert (ch.row_as_string (nr-i-1).c_str (), i, 0);
              ch.insert (tmp.c_str (), nr-i-1, 0);
            }
          retval(0) = octave_value (ch, '"');  // Double Quote String
        }
    }
  return retval;
}

s0 = ["First String"; "Second String"];
[s1,s2] = stringdemo (s0)
⇒ s1 = Second String
        First String

⇒ s2 = First String
        Second String

typeinfo (s2)
⇒ sq_string
typeinfo (s1)
⇒ string

One additional complication of strings in Octave is the difference between single quoted and double quoted strings. To find out if an octave_value contains a single or double quoted string use one of the predicate tests shown below.

if (args(0).is_sq_string ())
  octave_stdout << "First argument is a single quoted string\n";
else if (args(0).is_dq_string ())
  octave_stdout << "First argument is a double quoted string\n";

Note, however, that both types of strings are represented by the charNDArray type, and so when assigning to an octave_value, the type of string should be specified. For example:

octave_value_list retval;
charNDArray ch;
…
// Create single quoted string
retval(1) = octave_value (ch);        // default constructor is sq_string
           OR
retval(1) = octave_value (ch, '\'');  // explicitly create sq_string

// Create a double quoted string
retval(0) = octave_value (ch, '"');

A.1.3 Character Strings in Oct-Files