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# Evaluation

Normally, you evaluate expressions simply by typing them at the Octave prompt, or by asking Octave to interpret commands that you have saved in a file.

Sometimes, you may find it necessary to evaluate an expression that has been computed and stored in a string, or use a string as the name of a function to call. The `eval` and `feval` functions allow you to do just that, and are necessary in order to evaluate commands that are not known until run time, or to write functions that will need to call user-supplied functions.

Built-in Function: eval (command)
  Parse the string command and evaluate it as if it were an Octave program, returning the last value computed. The command is evaluated in the current context, so any results remain available after `eval` returns. For example,

      eval ("a = 13")
           -| a = 13
           => 13

  In this case, the value of the evaluated expression is printed and it is also returned returned from `eval`. Just as with any other expression, you can turn printing off by ending the expression in a semicolon. For example,

      eval ("a = 13;")
           => 13

  In this example, the variable `a` has been given the value 13, but the value of the expression is not printed. You can also turn off automatic printing for all expressions executed by `eval` using the variable `default_eval_print_flag`.

Built-in Variable: default_eval_print_flag
  If the value of this variable is nonzero, Octave prints the results of commands executed by `eval` that do not end with semicolons. If it is zero, automatic printing is suppressed. The default value is 1.

Built-in Function: feval (name, ...)
  Evaluate the function named name. Any arguments after the first are passed on to the named function. For example,

      feval ("acos", -1)
           => 3.1416

  calls the function `acos` with the argument ``-1'`.

  The function `feval` is necessary in order to be able to write functions that call user-supplied functions, because Octave does not have a way to declare a pointer to a function (like C) or to declare a special kind of variable that can be used to hold the name of a function (like `EXTERNAL` in Fortran). Instead, you must refer to functions by name, and use `feval` to call them.

Here is a simple-minded function using `feval` that finds the root of a user-supplied function of one variable using Newton's method.

    function result = newtroot (fname, x)

    # usage: newtroot (fname, x)
    #
    #   fname : a string naming a function f(x).
    #   x     : initial guess

      delta = tol = sqrt (eps);
      maxit = 200;
      fx = feval (fname, x);
      for i = 1:maxit
        if (abs (fx) < tol)
          result = x;
          return;
        else
          fx_new = feval (fname, x + delta);
          deriv = (fx_new - fx) / delta;
          x = x - fx / deriv;
          fx = fx_new;
        endif
      endfor

      result = x;

    endfunction

Note that this is only meant to be an example of calling user-supplied functions and should not be taken too seriously. In addition to using a more robust algorithm, any serious code would check the number and type of all the arguments, ensure that the supplied function really was a function, etc. See See section Predicates for Numeric Objects, for example, for a list of predicates for numeric objects, and See section Status of Variables, for a description of the `exist` function.

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