(octave.info)Variables

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7 Variables
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Variables let you give names to values and refer to them later.  You
have already seen variables in many of the examples.  The name of a
variable must be a sequence of letters, digits and underscores, but it
may not begin with a digit.  Octave does not enforce a limit on the
length of variable names, but it is seldom useful to have variables with
names longer than about 30 characters.  The following are all valid
variable names

     x
     x15
     __foo_bar_baz__
     fucnrdthsucngtagdjb

However, names like ‘__foo_bar_baz__’ that begin and end with two
underscores are understood to be reserved for internal use by Octave.
You should not use them in code you write, except to access Octave’s
documented internal variables and built-in symbolic constants.

   Case is significant in variable names.  The symbols ‘a’ and ‘A’ are
distinct variables.

   A variable name is a valid expression by itself.  It represents the
variable’s current value.  Variables are given new values with
“assignment operators” and “increment operators”.  Note: Assignment
Expressions.

   There is one built-in variable with a special meaning.  The ‘ans’
variable always contains the result of the last computation, where the
output wasn’t assigned to any variable.  The code ‘a = cos (pi)’ will
assign the value -1 to the variable ‘a’, but will not change the value
of ‘ans’.  However, the code ‘cos (pi)’ will set the value of ‘ans’ to
-1.

   Variables in Octave do not have fixed types, so it is possible to
first store a numeric value in a variable and then to later use the same
name to hold a string value in the same program.  Variables may not be
used before they have been given a value.  Doing so results in an error.

 -- Automatic Variable: ans
     The most recently computed result that was not explicitly assigned
     to a variable.

     For example, after the expression

          3^2 + 4^2

     is evaluated, the value returned by ‘ans’ is 25.

 -- : isvarname (NAME)
     Return true if NAME is a valid variable name.

     See also: Note: iskeyword, Note: exist,
     Note: who.

 -- : VARNAME = genvarname (STR)
 -- : VARNAME = genvarname (STR, EXCLUSIONS)
     Create valid unique variable name(s) from STR.

     If STR is a cellstr, then a unique variable is created for each
     cell in STR.

          genvarname ({"foo", "foo"})
            ⇒
               {
                 [1,1] = foo
                 [1,2] = foo1
               }

     If EXCLUSIONS is given, then the variable(s) will be unique to each
     other and to EXCLUSIONS (EXCLUSIONS may be either a string or a
     cellstr).

          x = 3.141;
          genvarname ("x", who ())
            ⇒ x1

     Note that the result is a char array or cell array of strings, not
     the variables themselves.  To define a variable, ‘eval()’ can be
     used.  The following trivial example sets ‘x’ to 42.

          name = genvarname ("x");
          eval ([name " = 42"]);
            ⇒ x =  42

     This can be useful for creating unique struct field names.

          x = struct ();
          for i = 1:3
            x.(genvarname ("a", fieldnames (x))) = i;
          endfor
            ⇒ x =
               {
                 a =  1
                 a1 =  2
                 a2 =  3
               }

     Since variable names may only contain letters, digits, and
     underscores, ‘genvarname’ will replace any sequence of disallowed
     characters with an underscore.  Also, variables may not begin with
     a digit; in this case an ‘x’ is added before the variable name.

     Variable names beginning and ending with two underscores "__" are
     valid, but they are used internally by Octave and should generally
     be avoided; therefore, ‘genvarname’ will not generate such names.

     ‘genvarname’ will also ensure that returned names do not clash with
     keywords such as "for" and "if".  A number will be appended if
     necessary.  Note, however, that this does *not* include function
     names such as "sin".  Such names should be included in EXCLUSIONS
     if necessary.

     See also: Note: isvarname, *note iskeyword:
     XREFiskeyword, Note: exist, Note: who, Note:
     tempname, Note: eval.

 -- : namelengthmax ()
     Return the MATLAB compatible maximum variable name length.

     Octave is capable of storing strings up to 2^{31} - 1 in length.
     However for MATLAB compatibility all variable, function, and
     structure field names should be shorter than the length returned by
     ‘namelengthmax’.  In particular, variables stored to a MATLAB file
     format (‘*.mat’) will have their names truncated to this length.

Global Variables

Persistent Variables

Status of Variables