Runtime evaluation/In an environment
You are encouraged to solve this task according to the task description, using any language you may know.
Given a program in the language (as a string or AST) with a free variable named x (or another name if that is not valid syntax), evaluate it with x 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.
Do so in a way which:
- does not involve string manipulation of the input source code
- is plausibly extensible to a runtime-chosen set of bindings rather than just x
- does not make x a global variable
or note that these are impossible.
See also
- For more general examples and language-specific details, see Eval.
- Dynamic variable names is a similar task.
ALGOL 68
Variable names are generally not visible at run time with classic compilers. However ALGOL 68G is an interpretor and it retains this ability. Note that evaluate returns a string. <lang algol68>PROC eval_with_x = (STRING code, INT a, b)STRING: (INT x=a; evaluate(code) ) + (INT x=b; evaluate(code)); print((eval_with_x("2 ** x", 3, 5), new line))</lang> Output:
+8 +32
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>
E
<lang e># Constructing an environment has to be done by way of evaluation for historical reasons which will hopefully be entirely eliminated soon. def bindX(value) {
def [resolver, env] := e` def x # bind x and capture its resolver and the resulting environment `.evalToPair(safeScope) resolver.resolve(value) # set the value return env
}
def evalWithX(program, a, b) {
def atA := program.eval(bindX(a)) def atB := program.eval(bindX(b)) return atB - atA
}</lang>
<lang e>? evalWithX(e`(x :float64).exp()`, 0, 1)
- value: 1.7182818284590455</lang>
Forth
EVALUATE invokes the Forth interpreter on the given string. <lang forth>: f-" ( a b snippet" -- )
[char] " parse ( code len ) 2dup 2>r evaluate swap 2r> evaluate - . ;
2 3 f-" dup *" \ 5 (3*3 - 2*2)</lang> This can be used to treat a data stream as code, or to provide a lightweight macro facility when used in an IMMEDIATE word. <lang forth>: :macro ( "name <char> ccc<char>" -- )
: [CHAR] ; PARSE POSTPONE SLITERAL POSTPONE EVALUATE POSTPONE ; IMMEDIATE
- macro times 0 do ;
- test 8 times ." spam " loop ;
see test
- test
8 0 DO .\" spam " LOOP ; ok</lang>
Genyris
One way is to use a macro. In genyris, macros are lazy functions which execute twice, the return value is also evaluated in the caller's environment: <lang genyris>defmacro add100() (+ x 100)
var x 23 var firstresult (add100) x = 1000 print
+ firstresult (add100)</lang>
This prints 1223.
Another way is to use dynamically scoped variables. In Genyris, symbols prefixed with a period are looked up in the caller's environment, not the lexical environment of the closure. When a dictionary is the first element of the expression, an environment is created and the &rest is evaluated.
<lang genyris>def add100() (+ .x 100)
(dict) # create an environment capable of holding dynamic bindings
var .x 23 # create a binding in the dictionary var firstresult (add100) .x = 1000 print + firstresult (add100)</lang>
Dictionaries can hold bindings to dynamic symbols. To minimize the danger of dynamic scope there is no recursive ascent in the binding lookup. <lang genyris>(dict)
var .x 23 (dict) print .x # fails</lang>
Groovy
The solution: <lang groovy>def cruncher = { x1, x2, program ->
Eval.x(x1, program) - Eval.x(x2, program)
}</lang>
Test Program: <lang groovy>def fibonacciProgram = x < 1 ? 0 : x == 1 ? 1 : (2..x).inject([0,1]){i, j -> [i[1], i[0]+i[1]]}[1]
println "F(${10}) - F(${5}) = ${Eval.x(10, fibonacciProgram)} - ${Eval.x(5, fibonacciProgram)} = " + cruncher(10, 5, fibonacciProgram)</lang>
Output:
F(10) - F(5) = 55 - 5 = 50
J
Explicit
The following satisfies the requirements: <lang j> EvalWithX=. monad : 0
'CODE V0 V1'=. y (". CODE [ x=. V1) - (". CODE [ x=. V0) ) EvalWithX '^x';0;1
1.71828183</lang>
Tacit
However, it is easier via point-free coding: <lang j> (0&({::) -~&>/@:(128!:2&.>) 1 2&{) '^';0;1 1.71828183</lang>
JavaScript
eval uses the environment from the calling function.
<lang javascript>function evalWithX(expr, a, b) {
var x = a; var atA = eval(expr); x = b; var atB = eval(expr); return atB - atA;
}</lang>
<lang javascript>evalWithX('Math.exp(x)', 0, 1) // returns 1.718281828459045</lang>
Metafont
<lang metafont>vardef evalit(expr s, va, vb) = save x,a,b; x := va; a := scantokens s; x := vb; b := scantokens s; a-b enddef;
show(evalit("2x+1", 5, 3)); end</lang>
Octave
In Octave, undeclared variables are local.
<lang octave>function r = calcit(f, val1, val2)
x = val1; a = eval(f); x = val2; b = eval(f); r = b-a;
endfunction
p = "x .* 2"; disp(calcit(p, [1:3], [4:6]));</lang>
Output:
6 6 6
Oz
<lang oz>declare
fun {EvalWithX Program A B} {Compiler.evalExpression Program env('X':B) _} - {Compiler.evalExpression Program env('X':A) _} end
in
{Show {EvalWithX "{Exp X}" 0.0 1.0}}</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>
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>
R
We can set up thing so that the "unbound" variable can be any accepted symbol for variables. <lang R>evalWithAB <- function(expr, var, a, b) {
env <- new.env() # provide a separate env, so that the choosen assign(var, a, envir=env) # var name do not collide with symbols inside # this function (e.g. it could be even "env") atA <- eval(parse(text=expr), env) # and then evaluate the expression inside this # ad hoc env-ironment assign(var, b, envir=env) atB <- eval(parse(text=expr), env) return(atB - atA)
}
print(evalWithAB("2*x+1", "x", 5, 3)) print(evalWithAB("2*y+1", "y", 5, 3)) print(evalWithAB("2*y+1", "x", 5, 3)) # error: object "y" not found</lang>
Ruby
<lang ruby>def bind_x_to_value(x)
binding
end
def eval_with_x(code, a, b)
eval(code, bind_x_to_value(b)) - eval(code, bind_x_to_value(a))
end
puts eval_with_x('2 ** x', 3, 5) # Prints "24"</lang>
The magic here is how the binding
method works with the bind_x_to_value(x)
method.
When bind_x_to_value
is called, it sets its local variable x
to the value passed.
The binding
method then returns a reference to the current context (or stack frame) to the caller.
eval
can then use the local variable x
in this context.
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>
Tcl
<lang tcl>proc eval_twice {func a b} {
set x $a set 1st [expr $func] set x $b set 2nd [expr $func] expr {$2nd - $1st}
}
puts [eval_twice {2 ** $x} 3 5] ;# ==> 24</lang>
Here's another take, similar to other answers. It passes a code block to be eval
ed, not just an expression for expr
<lang tcl>proc eval_with_x {code val1 val2} {
expr {[set x $val2; eval $code] - [set x $val1; eval $code]}
} eval_with_x {expr {2**$x}} 3 5 ;# ==> 24</lang>
TI-89 BASIC
<lang ti89b>evalx(prog, a, b) Func
Local x,eresult1,eresult2 a→x expr(prog)→eresult1 b→x expr(prog)→eresult2 Return eresult2-eresult1
EndFunc
■ evalx("ℯ^x", 0., 1)
1.71828</lang>
There are no facilities for control over the environment; expr() evaluates in the same environment as the caller, including local variables. [Someone please verify this statement.]