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
AppleScript
AppleScript's run script command allows to interpret and execute the string passed to it as an arbitrarily complex (or simple) script; such a string may thus be viewed as the "program" considered in the task description.
Each invocation of the run script command dynamically happens in a separate execution context, so there are no side-effects; on the other hand, this means that such an invocation is quite costly.
Arguments may be passed as a list of arbitrary values; this however requires the program to be written with an explicit run handler.
The result is the value (if any) returned by the program; any valid AppleScript value may be returned.
Given the above, the task may easily be implemented along these lines:
<lang AppleScript>
on task_with_x(pgrm, x1, x2)
local rslt1, rslt2
set rslt1 to run script pgrm with parameters {x1}
set rslt2 to run script pgrm with parameters {x2}
rslt2 - rslt1
end task_with_x
</lang>
Example usage (for legibility purposes, the program is stored into an intermediate variable):
<lang AppleScript>
set pgrm_with_x to "
on run {x}
2^x
end"
task_with_x(pgrm_with_x, 3, 5) </lang> The result is 24.0 (a real number).
AutoHotkey
AutoHotkey does not provide an API to the local symbol table. Local variables are also not supported within scopes outside functions. However, a local environment can be simulated by wrapping code in a temporary function. <lang AutoHotkey>msgbox % first := evalWithX("x + 4", 5) msgbox % second := evalWithX("x + 4", 6) msgbox % second - first return
evalWithX(expression, xvalue) { global script script = (
expression(){
x = %xvalue% ; := would need quotes
return %expression%
}
) renameFunction("expression", "") ; remove any previous expressions gosub load ; cannot use addScript inside a function yet exp := "expression" return %exp%() }
load: DllCall(A_AhkPath "\addScript","Str",script,"Uchar",0,"Cdecl UInt") return
renameFunction(funcName, newname){ static x%newname% := newname ; store newname in a static variable so its memory is not freed strput(newname, &x%newname%, strlen(newname) + 1) if fnp := FindFunc(funcName)
numput(&x%newname%, fnp+0, 0, "uint")
}</lang>
BBC BASIC
<lang bbcbasic> expression$ = "x^2 - 7"
one = FN_eval_with_x(expression$, 1.2)
two = FN_eval_with_x(expression$, 3.4)
PRINT two - one
END
DEF FN_eval_with_x(expr$, x)
= EVAL(expr$)</lang>
Bracmat
<lang>( ( eval-with-x
= code a b argument
. !arg:((=?code),?a,?b,?argument)
& (!b:?x&!code$!argument)
+ -1*(!a:?x&!code$!argument)
)
& out$(eval-with-x$((='(.$x^!arg)),3,5,2)) & out$(eval-with-x$((='(.$x^!arg)),12,13,2)) );</lang> Output:
16 25
Clojure
We must define x as global, but we use dynamic bindings. Only functions within the binding will see the newly bound value of x, before it re-establishes the bindings that existed before. <lang clojure>(def ^:dynamic x nil)
(defn eval-with-x [program a b]
(- (binding [x b] (eval program))
(binding [x a] (eval program))))</lang>
<lang clojure>(eval-with-x '(* x x) 4 9) => 65</lang>
Here's another version which avoids the global by compiling a local function with a single argument x (the ~'x is how to avoid automatic namespace qualification of symbols in a syntax quote): <lang clojure> (defn eval-with-x [program a b]
(let [func (eval `(fn [~'x] ~program))] (- (func b) (func a))))
</lang> <lang clojure>(eval-with-x '(* x x) 4 9) => 65</lang>
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>
Déjà Vu
<lang dejavu>local fib n: if <= n 1: n else: + fib - n 1 fib - n 2
local :code !compile-string dup "-- fib x" #one less than the xth fibonacci number
!run-blob-in { :fib @fib :x 4 } code !run-blob-in { :fib @fib :x 6 } code !. -</lang>
- Output:
5
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` # bind x and capture its resolver and the def x # 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>
EchoLisp
We evaluate prog in a new environment which is an association list ((x x-value)), and could be ((x x-value) (y y-value)....) <lang lisp> (define (eval-with-x prog x)
(eval prog (environment-new (list (list 'x x)))))
(define prog '( + 1 (* x x)))
(eval-with-x prog 10) → 101 (eval-with-x prog 1000) → 1000001 (- (eval-with-x prog 1000) (eval-with-x prog 10)) → 999900
- check x is unbound (no global)
x 😖️ error: #|user| : unbound variable : x </lang>
Elena
ELENA 5.0:
Using VM console client: <lang elena>import extensions; import extensions'scripting;
public program() {
var text := program_arguments[1];
var arg := program_arguments[2];
var program := lscript.interpret(text);
console.printLine(
text,",",arg," = ",program.eval(arg.toReal()));
}</lang>
- Output:
eval.exe "{ eval(x) { ^extensions'math'power(x,2); }}" 2
ELENA VM 5.0.4 (C)2005-2020 by Alex Rakov
Initializing...
Done...
{ eval(x) { ^extensions'math'power(x,2); }},2 = 4.0
Erlang
Functions below are used by dynamic variable names. Any changes here needs to be backwards compatible, or dynamic variable names must also be changed. <lang Erlang> -module( runtime_evaluation ).
-export( [evaluate_form/2, form_from_string/1, task/0] ).
evaluate_form( Form, {Variable_name, Value} ) -> Bindings = erl_eval:add_binding( Variable_name, Value, erl_eval:new_bindings() ), {value, Evaluation, _} = erl_eval:expr( Form, Bindings ), Evaluation.
form_from_string( String ) -> {ok, Tokens, _} = erl_scan:string( String ), {ok, [Form]} = erl_parse:parse_exprs( Tokens ), Form.
task() -> Form = form_from_string( "X." ), Variable1 = evaluate_form( Form, {'X', 1} ), io:fwrite( "~p~n", [Variable1] ), Variable2 = evaluate_form( Form, {'X', 2} ), io:fwrite( "~p~n", [Variable2] ), io:fwrite( "~p~n", [Variable2 - Variable1] ). </lang>
- Output:
14> runtime_evaluation:task(). 1 2 1
Factor
Being a stack-based language, there is usually no need to bind data stack objects to a variable name. This is the idiomatic way to do it, with eval referencing what it needs from the data stack:
<lang factor>USE: eval
- eval-bi@- ( a b program -- n )
tuck [ ( y -- z ) eval ] 2bi@ - ;</lang>
<lang factor>IN: scratchpad 9 4 "dup *" eval-bi@- .
65</lang>
Also note that, since programs are first-class denizens in Factor, the use cases for eval are few. Normally, you would pass in a program as a quotation:
<lang factor>: bi@- ( a b quot -- n ) bi@ - ; inline</lang>
<lang factor>IN: scratchpad 9 4 [ dup * ] bi@- .
65</lang>
However, we can adhere to the letter of the task. Although we are using a dynamic variable for x, it exists in a temporary, non-global namespace. As far as I can tell, eval is unaware of surrounding lexical scope.
<lang factor>SYMBOL: x
- eval-with-x ( a b program -- n )
tuck [ [ x ] dip [ ( -- y ) eval ] curry with-variable ] 2bi@ - ;</lang>
<lang factor>IN: scratchpad 9 4 "x get dup *" eval-with-x . 65 IN: scratchpad x get . f</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>
Go
<lang go>package main
import (
"bitbucket.org/binet/go-eval/pkg/eval" "fmt" "go/parser" "go/token"
)
func main() {
// an expression on x squareExpr := "x*x"
// parse to abstract syntax tree
fset := token.NewFileSet()
squareAst, err := parser.ParseExpr(squareExpr)
if err != nil {
fmt.Println(err)
return
}
// create an environment or "world"
w := eval.NewWorld()
// allocate a variable wVar := new(intV)
// bind the variable to the name x
err = w.DefineVar("x", eval.IntType, wVar)
if err != nil {
fmt.Println(err)
return
}
// bind the expression AST to the world
squareCode, err := w.CompileExpr(fset, squareAst)
if err != nil {
fmt.Println(err)
return
}
// directly manipulate value of variable within world
*wVar = 5
// evaluate
r0, err := squareCode.Run()
if err != nil {
fmt.Println(err)
return
}
// change value
*wVar--
// revaluate
r1, err := squareCode.Run()
if err != nil {
fmt.Println(err)
return
}
// print difference
fmt.Println(r0.(eval.IntValue).Get(nil) - r1.(eval.IntValue).Get(nil))
}
// int value implementation. type intV int64
func (v *intV) String() string { return fmt.Sprint(*v) } func (v *intV) Get(*eval.Thread) int64 { return int64(*v) } func (v *intV) Set(_ *eval.Thread, x int64) { *v = intV(x) } func (v *intV) Assign(t *eval.Thread, o eval.Value) {
*v = intV(o.(eval.IntValue).Get(t))
}</lang> Output:
9
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>
Explicit again
Or, using y as the free variable, instead of x: <lang J>EvalDiffWithY=: dyad define
-~/verb def x"_1 y
)</lang>
Example use:
<lang J> '^y' EvalDiffWithY 0 1 1.71828</lang>
This can be extended to support a user declared argument name:
<lang J>EvalDiffWithName=: adverb define
-~/m adverb def ('(m)=.y';x)"_1 y
)</lang>
This works by preceding the user provided expression with a statement which assigns the argument value to a local variable whose name was provided by the user. [Note that this implementation skirts the requirement that the implementation does not manipulate strings -- instead we manipulate a structure containing strings.]
Example use:
<lang J> '^George' 'George' EvalDiffWithName 0 1 1.71828
'Z + 2^Z' 'Z' EvalDiffWithName 2 3
5</lang>
Of course this could be re-defined such that the free variable declaration appears to the left of the expression ('Z' 'Z + 2^Z' Example 2 3). However, J's currying and precedence rules might make that less convenient to use, if this were ever used in a real program.
Java
Although Java is a compiled static language and expression evaluation is not intrinsic part of Java, we can still generate a class at run time and so emulate evaluation of string expressions. Java 1.6 provides some APIs for this sort of thing.
Issues:
- this is not thread-safe because it writes a generated class to the file system, then loads it
- the supplied code to evaluate is assumed to be an expression rather than a series of statements
- the supplied expression should evaluate to a number
- the same class is generated twice - never mind
- it's painfully verbose, but we're bending the language quite a bit
- the exception handling is minimal, but if something goes wrong you should get a stack dump and the exception might be helpful...
<lang java5>import java.io.File; import java.lang.reflect.Method; import java.net.URI; import java.util.Arrays; import javax.tools.JavaCompiler; import javax.tools.SimpleJavaFileObject; import javax.tools.ToolProvider;
public class Eval {
private static final String CLASS_NAME = "TempPleaseDeleteMe";
private static class StringCompiler
extends SimpleJavaFileObject {
final String m_sourceCode;
private StringCompiler( final String sourceCode ) {
super( URI.create( "string:///" + CLASS_NAME + Kind.SOURCE.extension ), Kind.SOURCE );
m_sourceCode = sourceCode;
}
@Override
public CharSequence getCharContent( final boolean ignoreEncodingErrors ) {
return m_sourceCode;
}
private boolean compile() {
final JavaCompiler javac = ToolProvider.getSystemJavaCompiler();
return javac.getTask( null, javac.getStandardFileManager( null, null, null ),
null, null, null, Arrays.asList( this )
).call();
}
private double callEval( final double x )
throws Exception {
final Class<?> clarse = Class.forName( CLASS_NAME );
final Method eval = clarse.getMethod( "eval", double.class );
return ( Double ) eval.invoke( null, x );
}
}
public static double evalWithX( final String code, final double x )
throws Exception {
final StringCompiler sc = new StringCompiler(
"class "
+ CLASS_NAME
+ "{public static double eval(double x){return ("
+ code
+ ");}}"
);
if ( ! sc.compile() ) throw new RuntimeException( "Compiler error" );
return sc.callEval( x );
}
public static void main( final String [] args )
throws Exception /* lazy programmer */ {
final String expression = args [ 0 ];
final double x1 = Double.parseDouble( args [ 1 ] );
final double x2 = Double.parseDouble( args [ 2 ] );
System.out.println(
evalWithX( expression, x1 )
- evalWithX( expression, x2 )
);
}
}</lang>
Example usage - calculating the difference of two squares (i.e. 9 - 2 = 7):
java Eval "Math.pow(x,2)" 3 1.414213562373095
Output:
7.0
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>
Jsish
From Javascript entry. <lang javascript>/* Runtime evaluation in an environment, in Jsish */ function evalWithX(expr, a, b) {
var x = a; var atA = eval(expr); x = b; var atB = eval(expr); return atB - atA;
}
- evalWithX('Math.exp(x)', 0, 1);
- evalWithX('Math.exp(x)', 1, 0);
/*
!EXPECTSTART!
evalWithX('Math.exp(x)', 0, 1) ==> 1.71828182845905 evalWithX('Math.exp(x)', 1, 0) ==> -1.71828182845905
!EXPECTEND!
- /</lang>
- Output:
prompt$ jsish -u runtimeEnvironmentEvaluation.jsi [PASS] runtimeEnvironmentEvaluation.jsi
Julia
<lang julia>macro evalwithx(expr, a, b)
return quote
x = $a
tmp = $expr
x = $b
return $expr - tmp
end
end
@evalwithx(2 ^ x, 3, 5) # raw expression (AST)</lang>
One can even perform the task without using macros:
<lang julia>function evalwithx(expr::Expr, a, b)
a = eval(quote let x = $a; return $expr end end) b = eval(quote let x = $b; return $expr end end) return b - a
end evalwithx(expr::AbstractString, a, b) = evalwithx(parse(expr), a, b)
evalwithx(:(2 ^ x), 3, 5) evalwithx("2 ^ x", 3, 5)</lang>
Kotlin
When you try to compile the following program, it will appear to the compiler that the local variable 'x' is assigned but never used and warnings will be issued accordingly. You can get rid of these warnings by compiling using the -nowarn flag. <lang scala>// Kotlin JS version 1.1.4-3
fun evalWithX(expr: String, a: Double, b: Double) {
var x = a val atA = eval(expr) x = b val atB = eval(expr) return atB - atA
}
fun main(args: Array<String>) {
println(evalWithX("Math.exp(x)", 0.0, 1.0))
}</lang>
- Output:
1.718281828459045
Liberty BASIC
<lang lb>
expression$ = "x^2 - 7"
Print (EvaluateWithX(expression$, 5) - EvaluateWithX(expression$, 3)) End
Function EvaluateWithX(expression$, x)
EvaluateWithX = Eval(expression$)
End Function </lang>
Lua
<lang lua> code = loadstring"return x^2" --this doesn't really need to be input, does it? val1 = setfenv(code, {x = io.read() + 0})() val2 = setfenv(code, {x = io.read() + 0})() print(val2 - val1) </lang>
In Lua 5.2 one can use the new load function to evaluate a string as Lua code and specify its environment:
<lang lua>env = {} f = load("return x", nil, nil, env) env.x = tonumber(io.read()) -- user enters 2 a = f() env.x = tonumber(io.read()) -- user enters 3 b = f() print(a + b) --> outputs 5</lang>
Mathematica/Wolfram Language
<lang Mathematica>Input source code is "10 x" , X is locally bound to 3 & 2 and the resulting expressions evaluated. (10 x /. x -> 3 ) - (10 x /. x -> 2 ) -> 10</lang>
MATLAB / 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;
end</lang> Usage:
p = 'x .* 2'; disp(calcit(p, [1:3], [4:6]));
Output:
6 6 6
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>
Nim
<lang nim>import macros, strformat
macro eval(s, x: static[string]): untyped =
parseStmt(&"let x={x}\n{s}")
echo(eval("x+1", "3.1"))</lang>
- Output:
4.1
ooRexx
The ooRexx interpret instruction executes dynamically created ooRexx code in the current variable context. <lang ooRexx> say evalWithX("x**2", 2) say evalWithX("x**2", 3.1415926)
- routine evalWithX
use arg expression, x
-- X now has the value of the second argument interpret "return" expression
</lang> Output:
4 9.86960406
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>
PARI/GP
There are many ways of doing this depending on the particular interpretation of the requirements. This code assumes that f is a string representing a GP closure. <lang parigp>test(f,a,b)=f=eval(f);f(a)-f(b); test("x->print(x);x^2-sin(x)",1,3)</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 * $x', 5, 10), "\n"; # Prints "15".</lang>
Phix
See Runtime_evaluation#Phix for more details, or the docs.
Evaluation occurs in a brand new context, and x must be explicitly provided/returned.
Note that nothing can escape the eval context: while x is not "global" here but could
trivially (/technically) be made one by prefixing its definition with "global", it would
still not escape the eval() context, and still have to be explicitly requested as a
return value, as per the second {"x"} parameter [aka rset] to eval().
without javascript_semantics requires("1.0.1") include eval.e function eval_with_x(string expr, integer a, b) return eval(expr,{"x"},{{"x",b}})[1] - eval(expr,{"x"},{{"x",a}})[1] end function ?eval_with_x("integer x = power(2,x)",3,5)
- Output:
(As in 25 - 23 === 32 - 8 === 24)
24
PHP
<lang php><?php function eval_with_x($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>
PicoLisp
<lang PicoLisp>(let Expression '(+ X (* X X)) # Local expression
(println
(+
(let X 3
(eval Expression) )
(let X 4
(eval Expression) ) ) )
(let Function (list '(X) Expression) # Build a local function
(println
(+
(Function 3)
(Function 4) ) ) ) )</lang>
Output:
32 32
Pike
Pike can only compile complete classes. therefore binding a value to a variable is only possible by string manipulation. even Pikes own interactive mode which seemingly evaluates expressions wraps them into a class and replaces variable references before compiling: <lang Pike>> int x=10; Result: 10 > x * 5; Result: 50 > dump wrapper Last compiled wrapper: 001: mapping(string:mixed) ___hilfe = ___Hilfe->variables; 002: # 1 003: mixed ___HilfeWrapper() { return (([mapping(string:int)](mixed)___hilfe)->x) * 5; ; } 004: ></lang> ___Hilfe is an object which stores all created variables;
to solve the problem in the task i would create a function that can take arguments: <lang Pike>string payload = "x * 5";
program demo = compile_string("string eval(mixed x){ " + payload + "; }");
demo()->eval(10); Result: 50 demo()->eval(20); Result: 100</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>
Racket
Same hack as the on in the CL/Scheme entries: <lang Racket>
- lang racket
(define ns (make-base-namespace)) (define (eval-with-x code a b)
(define (with v) (eval `(let ([x ',v]) ,code) ns)) (- (with b) (with a)))
</lang>
Better: a more direct use of eval with just the code (for example, this won't break if we use a namespace with a different meaning for let, which is very possible in Racket): <lang Racket>
- lang racket
(define ns (make-base-namespace)) (define (eval-with-x code a b)
(define (with v) (namespace-set-variable-value! 'x v #f ns) (eval code ns)) (- (with b) (with a)))
</lang>
Raku
(formerly Perl 6)
For security, you must explicitly allow use of 'EVAL'. <lang perl6>use MONKEY-SEE-NO-EVAL; sub eval_with_x($code, *@x) { [R-] @x.map: -> \x { EVAL $code } }
say eval_with_x('3 * x', 5, 10); # Says "15". say eval_with_x('3 * x', 5, 10, 50); # Says "105".</lang>
REBOL
<lang rebol>prog: [x * 2] fn: func [x] [do bind prog 'x] a: fn 2 b: fn 4 subtract b a</lang>
Result:
4
REXX
<lang rexx>/*REXX program demonstrates a run─time evaluation of an expression (entered at run─time)*/ say '──────── enter the 1st expression to be evaluated:'
parse pull x /*obtain an expression from the console*/ y.1= x /*save the provided expression for X. */ say
say '──────── enter the 2nd expression to be evaluated:'
parse pull x /*obtain an expression from the console*/ y.2= x /*save the provided expression for X. */
say say '──────── 1st expression entered is: ' y.1 say '──────── 2nd expression entered is: ' y.2 say
interpret 'say "──────── value of the difference is: "' y.2 "-" '('y.1")" /* ◄─────┐ */
/* │ */
/* │ */
/*subtract 1st exp. from the 2nd──►──┘ */
drop x /*X var. is no longer a global variable*/ exit 0 /*stick a fork in it, we're all done. */</lang>
- output
──────── enter the 1st expression to be evaluated: 42 + 2 ◄■■■■■■■■■ entered by the user ──────── enter the 2nd expression to be evaluated: 12**2 ◄■■■■■■■■■ entered by the user ──────── 1st expression entered is: 42 + 2 ──────── 2nd expression entered is: 12**2 ──────── value of the difference is: 100
Ring
<lang ring> expression = "return pow(x,2) - 7" one = evalwithx(expression, 1.2) two = evalwithx(expression, 3.4) see "one = " + one + nl + "two = " + two + nl
func evalwithx expr, x
return eval(expr)
</lang> Output:
one = -5.56 two = 4.56
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.
Scala
<lang Scala>object Eval extends App {
def evalWithX(expr: String, a: Double, b: Double)=
{val x = b; eval(expr)} - {val x = a; eval(expr)}
println(evalWithX("Math.exp(x)", 0.0, 1.0))
}</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>
Sidef
<lang ruby>func eval_with_x(code, x, y) {
var f = eval(code); x = y; eval(code) - f;
}
say eval_with_x(x: 3, y: 5, code: '2 ** x'); # => 24</lang>
Simula
<lang simula>BEGIN
CLASS ENV; BEGIN
CLASS ITEM(N, X); TEXT N; REAL X;
BEGIN
REF(ITEM) NEXT; NEXT :- HEAD; HEAD :- THIS ITEM;
END ITEM;
REF(ITEM) HEAD;
REF(ITEM) PROCEDURE LOOKUP(V); TEXT V;
BEGIN
REF(ITEM) I; BOOLEAN FOUND; I :- HEAD;
WHILE NOT FOUND DO
IF I == NONE OR ELSE I.N = V
THEN FOUND := TRUE
ELSE I :- I.NEXT;
LOOKUP :- I;
END LOOKUP;
REF(ENV) PROCEDURE SET(V, X); TEXT V; REAL X;
BEGIN
REF(ITEM) I; I :- LOOKUP(V);
IF I == NONE THEN I :- NEW ITEM(V, X) ELSE I.X := X;
SET :- THIS ENV;
END SET;
REAL PROCEDURE GET(V); TEXT V;
GET := LOOKUP(V).X;
END ENV;
CLASS EXPR(EV); REF(ENV) EV; BEGIN
REAL PROCEDURE POP;
BEGIN
IF STACKPOS > 0 THEN
BEGIN STACKPOS := STACKPOS - 1; POP := STACK(STACKPOS); END;
END POP;
PROCEDURE PUSH(NEWTOP); REAL NEWTOP;
BEGIN
STACK(STACKPOS) := NEWTOP;
STACKPOS := STACKPOS + 1;
END PUSH;
REAL PROCEDURE CALC(OPERATOR, ERR); CHARACTER OPERATOR; LABEL ERR;
BEGIN
REAL X, Y; X := POP; Y := POP;
IF OPERATOR = '+' THEN PUSH(Y + X)
ELSE IF OPERATOR = '-' THEN PUSH(Y - X)
ELSE IF OPERATOR = '*' THEN PUSH(Y * X)
ELSE IF OPERATOR = '/' THEN BEGIN
IF X = 0 THEN
BEGIN
EVALUATEDERR :- "DIV BY ZERO";
GOTO ERR;
END;
PUSH(Y / X);
END
ELSE IF OPERATOR = '^' THEN PUSH(Y ** X)
ELSE
BEGIN
EVALUATEDERR :- "UNKNOWN OPERATOR";
GOTO ERR;
END
END CALC;
PROCEDURE READCHAR(CH); NAME CH; CHARACTER CH;
BEGIN
IF T.MORE THEN CH := T.GETCHAR ELSE CH := EOT;
END READCHAR;
PROCEDURE SKIPWHITESPACE(CH); NAME CH; CHARACTER CH;
BEGIN
WHILE (CH = SPACE) OR (CH = TAB) OR (CH = CR) OR (CH = LF) DO
READCHAR(CH);
END SKIPWHITESPACE;
PROCEDURE BUSYBOX(OP, ERR); INTEGER OP; LABEL ERR;
BEGIN
CHARACTER OPERATOR;
REAL NUMBR;
BOOLEAN NEGATIVE;
SKIPWHITESPACE(CH);
IF OP = EXPRESSION THEN
BEGIN
NEGATIVE := FALSE;
WHILE (CH = '+') OR (CH = '-') DO
BEGIN
IF CH = '-' THEN NEGATIVE := NOT NEGATIVE;
READCHAR(CH);
END;
BUSYBOX(TERM, ERR);
IF NEGATIVE THEN
BEGIN
NUMBR := POP; PUSH(0 - NUMBR);
END;
WHILE (CH = '+') OR (CH = '-') DO
BEGIN
OPERATOR := CH; READCHAR(CH);
BUSYBOX(TERM, ERR); CALC(OPERATOR, ERR);
END;
END
ELSE IF OP = TERM THEN
BEGIN
BUSYBOX(FACTOR, ERR);
WHILE (CH = '*') OR (CH = '/') DO
BEGIN
OPERATOR := CH; READCHAR(CH);
BUSYBOX(FACTOR, ERR); CALC(OPERATOR, ERR)
END
END
ELSE IF OP = FACTOR THEN
BEGIN
BUSYBOX(POWER, ERR);
WHILE CH = '^' DO
BEGIN
OPERATOR := CH; READCHAR(CH);
BUSYBOX(POWER, ERR); CALC(OPERATOR, ERR)
END
END
ELSE IF OP = POWER THEN
BEGIN
IF (CH = '+') OR (CH = '-') THEN
BUSYBOX(EXPRESSION, ERR)
ELSE IF (CH >= '0') AND (CH <= '9') THEN
BUSYBOX(NUMBER, ERR)
ELSE IF (CH >= 'A') AND (CH <= 'Z') THEN
BUSYBOX(VARIABLE, ERR)
ELSE IF CH = '(' THEN
BEGIN
READCHAR(CH);
BUSYBOX(EXPRESSION, ERR);
IF CH = ')' THEN READCHAR(CH) ELSE GOTO ERR;
END
ELSE GOTO ERR;
END
ELSE IF OP = VARIABLE THEN
BEGIN
TEXT VARNAM;
VARNAM :- BLANKS(32);
WHILE (CH >= 'A') AND (CH <= 'Z')
OR (CH >= '0') AND (CH <= '9') DO
BEGIN
VARNAM.PUTCHAR(CH);
READCHAR(CH);
END;
PUSH(EV.GET(VARNAM.STRIP));
END
ELSE IF OP = NUMBER THEN
BEGIN
NUMBR := 0;
WHILE (CH >= '0') AND (CH <= '9') DO
BEGIN
NUMBR := 10 * NUMBR + RANK(CH) - RANK('0'); READCHAR(CH);
END;
IF CH = '.' THEN
BEGIN
REAL FAKTOR;
READCHAR(CH);
FAKTOR := 10;
WHILE (CH >= '0') AND (CH <= '9') DO
BEGIN
NUMBR := NUMBR + (RANK(CH) - RANK('0')) / FAKTOR;
FAKTOR := 10 * FAKTOR;
READCHAR(CH);
END;
END;
PUSH(NUMBR);
END;
SKIPWHITESPACE(CH);
END BUSYBOX;
BOOLEAN PROCEDURE EVAL(INP); TEXT INP;
BEGIN
EVALUATEDERR :- NOTEXT;
STACKPOS := 0;
T :- COPY(INP.STRIP);
READCHAR(CH);
BUSYBOX(EXPRESSION, ERRORLABEL);
IF NOT T.MORE AND STACKPOS = 1 AND CH = EOT THEN
BEGIN
EVALUATED := POP;
EVAL := TRUE;
GOTO NOERRORLABEL;
END;
ERRORLABEL:
EVAL := FALSE;
IF EVALUATEDERR = NOTEXT THEN
EVALUATEDERR :- "INVALID EXPRESSION: " & INP;
NOERRORLABEL:
END EVAL;
REAL PROCEDURE RESULT;
RESULT := EVALUATED;
TEXT PROCEDURE ERR;
ERR :- EVALUATEDERR;
INTEGER EXPRESSION, TERM, FACTOR, POWER, NUMBER, VARIABLE;
CHARACTER TAB, LF, CR, SPACE, EOT, CH;
REAL ARRAY STACK(0:31);
INTEGER STACKPOS;
REAL EVALUATED;
TEXT EVALUATEDERR, T;
EXPRESSION := 1;
TERM := 2;
FACTOR := 3;
POWER := 4;
NUMBER := 5;
VARIABLE := 6;
TAB := CHAR(9);
LF := CHAR(10);
CR := CHAR(13);
SPACE := CHAR(32);
EOT := CHAR(0);
END EXPR;
REF(EXPR) EXA, EXB;
EXA :- NEW EXPR(NEW ENV.SET("X", 3));
EXB :- NEW EXPR(NEW ENV.SET("X", 5));
IF EXA.EVAL("2 ^ X") THEN
BEGIN
IF EXB.EVAL("2 ^ X")
THEN OUTFIX(EXB.RESULT - EXA.RESULT, 3, 10)
ELSE OUTTEXT(EXB.ERR)
END ELSE OUTTEXT(EXA.ERR);
OUTIMAGE;
END. </lang>
- Output:
24.000
SNOBOL4
This program defines (at runtime) a function triple(), compiles it, and then executes it twice, with values x = 1 and then x = 3. The program subtracts the returned value from the first call from the value returned from the first call, and prints the result. In this example, the value x is passed as a parameter to the function triple().
<lang SNOBOL4> compiled = code(' define("triple(x)") :(a);triple triple = 3 * x :(return)') :<compiled> a x = 1
first = triple(x)
x = 3
output = triple(x) - first
end</lang>
Output:
6
If you specifically wanted to not pass x as a parameter but instead use it as a value from the environment, that's easy too:
<lang SNOBOL4> compiled = code(' define("triple()") :(a);triple triple = 3 * x :(return)') :<compiled> a x = 1
first = triple(x)
x = 3
output = triple(x) - first
end</lang>
The output is the same.
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 evaled, 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>
In 8.5, apply makes environments like this "first class":
<lang tcl>package require Tcl 8.5 proc eval_with {body a b} {
set lambda [list x $body]
expr {[apply $lambda $b] - [apply $lambda $a]}
}
eval_with {expr {2**$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.]
Transd
The (eval) function returns a pair, where the first element is boolean flag indicating success of running the code, and the second element is a string containing the output of code execution.
<lang Scheme>#lang transd
MainModule: {
code: "(+ 5 x)",
_start: (lambda (textout
(- (to-Int (with x 100 (snd (eval code))))
(to-Int (with x 1 (snd (eval code)))))
))
}</lang>
- Output:
99
TXR
In TXR's embedded Lisp dialect, we can implement the same solution as Lisp or Scheme: transform the code fragment by wrapping a let around it which binds a variable, and then evaluating the whole thing:
<lang txrlisp>(defun eval-subtract-for-two-values-of-x (code-fragment x1 x0)
(- (eval ^(let ((x ,x1)) ,code-fragment))
(eval ^(let ((x ,x0)) ,code-fragment))))
(eval-subtract-for-two-values-of-x 1 2) ;; yields -4.67077427047161</lang>
Cutting edge TXR code provides access to the environment manipulation functions, making this possible:
<lang txrlisp>(defun eval-subtract-for-two-values-of-x (code-fragment x1 x0)
(let ((e1 (make-env (list (cons 'x x1)))) ;; create two environments stuffed with binding for x
(e0 (make-env (list (cons 'x x0)))))
(- (eval code-fragment e1) ;; pass these environment to eval
(eval code-fragment e0))))
(eval-subtract-for-two-values-of-x '(exp x) 1 2)</lang>
Alternatively, empty environments can be made and extended with bindings:
<lang txrlisp>(defun eval-subtract-for-two-values-of-x (code-fragment x1 x0)
(let ((e1 (make-env))
(e0 (make-env)))
(env-vbind e1 'x x1)
(env-vbind e0 'x x0)
(- (eval code-fragment e1)
(eval code-fragment e0))))
(eval-subtract-for-two-values-of-x '(exp x) 1 2)</lang>
Explicit environment manipulation has the disadvantage of being hostile against compiling. (See notes about compilation in the Common Lisp example.)
there is an eval function which takes an environment parameter. However, currently there isn't any access to the manipulation of environment objects. It's probably a bad idea because run time tricks with lexical environments lead to programs that are not compilable.
Lastly, we can also solve this problem using dynamically scoped (a.k.a "special") variables. The problem description specifically says that the solution is not to use global variables. Though we must define the variables as global, we do not use the global bindings; we use dynamic bindings.
There is a hidden global variable, namely the dynamic environment itself. That's how eval is able to resolve the free-variable x occurring in code-fragment without receiving any environment parameter.
However, our two let constructs carefully save and restore the dynamic environment (and therefore any prior value of x), even in the face of exceptions, and
<lang txrlisp>(defvar x)
(defun eval-subtract-for-two-values-of-x (code-fragment x1 x0)
(- (let ((x x1)) (eval code-fragment))
(let ((x x0)) (eval code-fragment))))
(eval-subtract-for-two-values-of-x '(exp x) 1 2)</lang>
UNIX Shell
The backquotes ` ... ` capture the standard output of a subshell. Changes to parameter x in the subshell will not affect its parent shell.
<lang bash>eval_with_x() { set -- "`x=$2; eval "$1"`" "`x=$3; eval "$1"`" expr "$2" - "$1" }
eval_with_x ' # compute 2 ** $x p=1 while test $x -gt 0; do p=`expr $p \* 2` x=`expr $x - 1` done echo $p ' 3 5
- Prints '24'</lang>
Wren
Wren has an undocumented method called Meta.eval which can do runtime evaluation.
However, it only appears to work with module level variables. <lang ecmascript>import "meta" for Meta
var x Meta.eval("x = 2") System.print("First x = %(x)") var y = x // save this value
Meta.eval("x = 5") System.print("Second x = %(x)")
Meta.eval("x = x - y") System.print("Delta x = %(x)")</lang>
- Output:
First x = 2 Second x = 5 Delta x = 3
zkl
<lang zkl>fcn evalWithX(text,x) {
f:=Compiler.Compiler.compileText(text); f.x = x; // set free var in compiled blob f.__constructor(); // run blob vm.regX // compiler sets the VMs X register for cases like this
} const TEXT="var x; x*2"; // variables need to be declared evalWithX(TEXT,5) - evalWithX(TEXT,3) #--> 4</lang> This is not a complete solution but close.
Another way to do this is to create a class on the fly that contains the code to be run and reusing that class. The class just acts like as a container for x and a function: <lang zkl>var klass=Compiler.Compiler.compileText("var x; returnClass(x*2)"); (klass.__constructor(klass.x=5) - klass.__constructor(klass.x=3)).println();</lang> returnClass(x) is required in a constructor if you want to return something other than self. klass.x=y pokes y into the instance variable x. Running the constructor runs x*2.
- Output:
4
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