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1 @c Copyright (C) 1996, 1997 John W. Eaton |
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2 @c This is part of the Octave manual. |
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3 @c For copying conditions, see the file gpl.texi. |
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4 |
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5 @node Evaluation, Statements, Expressions, Top |
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6 @chapter Evaluation |
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7 |
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8 Normally, you evaluate expressions simply by typing them at the Octave |
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9 prompt, or by asking Octave to interpret commands that you have saved in |
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10 a file. |
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11 |
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12 Sometimes, you may find it necessary to evaluate an expression that has |
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13 been computed and stored in a string, or use a string as the name of a |
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14 function to call. The @code{eval} and @code{feval} functions allow you |
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15 to do just that, and are necessary in order to evaluate commands that |
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16 are not known until run time, or to write functions that will need to |
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17 call user-supplied functions. |
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18 |
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19 @deftypefn {Built-in Function} {} eval (@var{command}) |
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20 Parse the string @var{command} and evaluate it as if it were an Octave |
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21 program, returning the last value computed. The @var{command} is |
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22 evaluated in the current context, so any results remain available after |
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23 @code{eval} returns. For example, |
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24 |
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25 @example |
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26 @group |
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27 eval ("a = 13") |
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28 @print{} a = 13 |
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29 @result{} 13 |
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30 @end group |
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31 @end example |
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32 |
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33 In this case, the value of the evaluated expression is printed and it is |
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34 also returned returned from @code{eval}. Just as with any other |
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35 expression, you can turn printing off by ending the expression in a |
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36 semicolon. For example, |
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37 |
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38 @example |
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39 eval ("a = 13;") |
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40 @result{} 13 |
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41 @end example |
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42 |
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43 In this example, the variable @code{a} has been given the value 13, but |
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44 the value of the expression is not printed. You can also turn off |
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45 automatic printing for all expressions executed by @code{eval} using the |
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46 variable @code{default_eval_print_flag}. |
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47 @end deftypefn |
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48 |
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49 @defvr {Built-in Variable} default_eval_print_flag |
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50 If the value of this variable is nonzero, Octave prints the results of |
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51 commands executed by @code{eval} that do not end with semicolons. If it |
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52 is zero, automatic printing is suppressed. The default value is 1. |
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53 @end defvr |
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54 |
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55 @deftypefn {Built-in Function} {} feval (@var{name}, @dots{}) |
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56 Evaluate the function named @var{name}. Any arguments after the first |
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57 are passed on to the named function. For example, |
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58 |
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59 @example |
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60 feval ("acos", -1) |
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61 @result{} 3.1416 |
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62 @end example |
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63 |
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64 @noindent |
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65 calls the function @code{acos} with the argument @samp{-1}. |
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66 |
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67 The function @code{feval} is necessary in order to be able to write |
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68 functions that call user-supplied functions, because Octave does not |
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69 have a way to declare a pointer to a function (like C) or to declare a |
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70 special kind of variable that can be used to hold the name of a function |
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71 (like @code{EXTERNAL} in Fortran). Instead, you must refer to functions |
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72 by name, and use @code{feval} to call them. |
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73 @end deftypefn |
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74 |
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75 Here is a simple-minded function using @code{feval} that finds the root |
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76 of a user-supplied function of one variable using Newton's method. |
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77 |
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78 @example |
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79 @group |
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80 @cindex Fordyce, A. P. |
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81 @findex newtroot |
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82 function result = newtroot (fname, x) |
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83 |
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84 # usage: newtroot (fname, x) |
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85 # |
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86 # fname : a string naming a function f(x). |
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87 # x : initial guess |
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88 |
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89 delta = tol = sqrt (eps); |
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90 maxit = 200; |
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91 fx = feval (fname, x); |
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92 for i = 1:maxit |
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93 if (abs (fx) < tol) |
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94 result = x; |
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95 return; |
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96 else |
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97 fx_new = feval (fname, x + delta); |
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98 deriv = (fx_new - fx) / delta; |
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99 x = x - fx / deriv; |
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100 fx = fx_new; |
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101 endif |
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102 endfor |
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103 |
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104 result = x; |
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105 |
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106 endfunction |
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107 @end group |
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108 @end example |
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109 |
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110 Note that this is only meant to be an example of calling user-supplied |
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111 functions and should not be taken too seriously. In addition to using a |
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112 more robust algorithm, any serious code would check the number and type |
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113 of all the arguments, ensure that the supplied function really was a |
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114 function, etc. See @xref{Predicates for Numeric Objects}, for example, |
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115 for a list of predicates for numeric objects, and @xref{Status of |
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116 Variables}, for a description of the @code{exist} function. |
