Runtime evaluation/In an environment: Difference between revisions
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=={{header|REXX}}== |
=={{header|REXX}}== |
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<lang rexx>/*REXX program to demonstrate some run-time evaulations. */ |
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<lang rexx> |
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a= |
a=fact(3) |
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b= |
b=fact(4) |
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say b-a |
say b-a |
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exit |
exit |
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⚫ | |||
'''output''' |
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⚫ | |||
</lang> |
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Output: |
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<pre> |
<pre> |
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18 |
18 |
Revision as of 17:56, 19 June 2012
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
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>
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` # 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>
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
Java is compiled static language and expression evaluation is not intrinsic part of Java or the standard libraries. In scripting languages for example - it makes sense to have eval method, as interpreter can interpret string and include its contents as part of the surrounding methods, with complete access to local variables.
Clearly there are lots of ways to get around the problem, but as of Java 6, Java can contain scripting environments. For example Mozilla Rihno JavaScript engine is contained in JDK 6 Bean Scripting Framework.
Example:
<lang java5>ScriptEngine js = new ScriptEngineManager().getEngineByName("js"); System.out.println(js.eval("function D(x){return x*2;} var x=3; x=D(x);"));</lang>
Output:
6.0
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>
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>
Mathematica
<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>
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
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 * $x', 5, 10), "\n"; # Prints "15".</lang>
PHP
<lang php><?php function eval_with_x($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>
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 to demonstrate some run-time evaulations. */ a=fact(3) b=fact(4) say b-a exit
fact: procedure; arg n; !=1; do j=2 to n; !=!*j; end; return !</lang> output
18
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>
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 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.]
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>
- Programming Tasks
- Solutions by Programming Task
- ALGOL 68
- AutoHotkey
- BBC BASIC
- Common Lisp
- E
- Forth
- Genyris
- Go
- Groovy
- J
- Java
- Java examples needing attention
- Examples needing attention
- JavaScript
- Liberty BASIC
- Lua
- Mathematica
- Metafont
- Octave
- OoRexx
- Oz
- PARI/GP
- Perl
- PHP
- PicoLisp
- Pike
- Python
- R
- REBOL
- REXX
- Ruby
- Scheme
- SNOBOL4
- Tcl
- TI-89 BASIC
- TI-89 BASIC examples needing attention
- UNIX Shell
- Ada/Omit
- C/Omit
- D/Omit
- Haskell/Omit
- GUISS/Omit