Runtime evaluation/In an environment: Difference between revisions
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=={{header|Perl}}==
<lang perl>
{my $code = shift;
my $first = eval $code;
return eval($code) - $first;}
print eval_with_x('3 * $x', 5, 10), "\n"; # Prints "15
=={{header|PHP}}==
<lang php><?php
function eval_with_x($code, $a, $b) {
$x = $a;
$first = eval($code);
$x = $b;
$second = eval($code);
return $second - $first;
}
echo eval_with_x('return 3 * $x;', 5, 10), "\n"; # Prints "15".
?></lang>
=={{header|Python}}==
|
Revision as of 18:36, 14 March 2009
You are encouraged to solve this task according to the task description, using any language you may know.
Given a program in the language representing a function, evaluate it with the variable x (or another name if that is not valid) bound to a provided value, then evaluate it again with x bound to another provided value, then subtract the result of the first from the second and return or print it.
Preferably, do so in a way which does not involve string manipulation of source code, and is plausibly extensible to a runtime-chosen set of bindings.
For more general examples and language-specific details, see Eval.
Common Lisp
<lang lisp> (defun eval-with-x (program a b)
(let ((at-a (eval `(let ((x ',a)) ,program))) (at-b (eval `(let ((x ',b)) ,program)))) (- at-b at-a)))
</lang>
<lang lisp> (eval-with-x '(exp x) 0 1) => 1.7182817 </lang>
This version ensures that the program is compiled, once, for more efficient execution:
<lang lisp> (defun eval-with-x (program a b)
(let* ((f (compile nil `(lambda (x) ,program))) (at-a (funcall f a)) (at-b (funcall f b))) (- at-b at-a)))
</lang>
Perl
<lang perl>sub eval_with_x
{my $code = shift; my $x = shift; my $first = eval $code; $x = shift; return eval($code) - $first;}
print eval_with_x('3 * $x', 5, 10), "\n"; # Prints "15".</lang>
PHP
<lang php><?php function eval_with_x($code, $a, $b) {
$x = $a; $first = eval($code); $x = $b; $second = eval($code); return $second - $first;
}
echo eval_with_x('return 3 * $x;', 5, 10), "\n"; # Prints "15". ?></lang>
Python
<lang python>>>> def eval_with_x(code, a, b): return eval(code, {'x':b}) - eval(code, {'x':a})
>>> eval_with_x('2 ** x', 3, 5) 24</lang>
Python: for multiple names
A slight change allows the evaluation to take multiple names: <lang python>>>> def eval_with_args(code, **kwordargs): return eval(code, kwordargs)
>>> code = '2 ** x' >>> eval_with_args(code, x=5) - eval_with_args(code, x=3) 24 >>> code = '3 * x + y' >>> eval_with_args(code, x=5, y=2) - eval_with_args(code, x=3, y=1) 7 >>> </lang>
Ruby
<lang ruby>def getBinding(x)
binding
end
def eval_with_x(code, a, b)
eval(code, getBinding(b)) - eval(code, getBinding(a))
end
puts eval_with_x('2 ** x', 3, 5) # Prints "24"</lang>
Scheme
Almost identical to the Common Lisp version above. <lang scheme>(define (eval-with-x prog a b)
(let ((at-a (eval `(let ((x ',a)) ,prog))) (at-b (eval `(let ((x ',b)) ,prog)))) (- at-b at-a)))</lang>