Dynamic variable names

Create a variable with a user-defined name.

Task

The variable name should not be written in the program text, but should be taken from the user dynamically.

See also

Eval in environment is a similar task.

Arturo

<lang rebol>name: strip input "enter a variable name: " value: strip input "enter a variable value: "

let name value

print ["the value of variable" name "is:" var name]</lang>

Output:

enter a variable name: myvar
enter a variable value: 2021
the value of variable myvar is: 2021

AutoHotkey

<lang AutoHotkey>InputBox, Dynamic, Variable Name %Dynamic% = hello ListVars MsgBox % %dynamic% ; says hello</lang>

AWK

syntax: GAWK -f DYNAMIC_VARIABLE_NAMES.AWK
Variables created in GAWK's internal SYMTAB (symbol table) can only be accessed via SYMTAB[name]

BEGIN {

   PROCINFO["sorted_in"] = "@ind_str_asc"
   show_symbol_table()
   while (1) {
     printf("enter variable name? ")
     getline v_name
     if (v_name in SYMTAB) {
       printf("name already exists with a value of '%s'\n",SYMTAB[v_name])
       continue
     }
     if (v_name ~ /^$/) {
       printf("name is null\n")
       continue
     }
     if (v_name !~ /^[A-Za-z][A-Za-z0-9_]*$/) {
       printf("name illegally constructed\n")
       continue
     }
     break
   }
   printf("enter value? ")
   getline v_value
   SYMTAB[v_name] = v_value
   printf("variable '%s' has been created and assigned the value '%s'\n\n",v_name,v_value)
   show_symbol_table()
   exit(0)

} function show_symbol_table( count,i) {

   for (i in SYMTAB) {
     printf("%s ",i)
     if (isarray(SYMTAB[i])) { count++ }
   }
   printf("\nsymbol table contains %d names of which %d are arrays\n\n",length(SYMTAB),count)

} </lang>

Output:

ARGC ARGIND ARGV BINMODE CONVFMT ENVIRON ERRNO FIELDWIDTHS FILENAME FNR FPAT FS IGNORECASE LINT NF NR OFMT OFS ORS PREC PROCINFO RLENGTH ROUNDMODE RS RSTART RT SUBSEP TEXTDOMAIN v_name v_value
symbol table contains 30 names of which 3 are arrays

enter variable name? FPAT
name already exists with a value of '[^[:space:]]+'
enter variable name? 0
name illegally constructed
enter variable name?
name is null
enter variable name? animal
enter value? zebra
variable 'animal' has been created and assigned the value 'zebra'

ARGC ARGIND ARGV BINMODE CONVFMT ENVIRON ERRNO FIELDWIDTHS FILENAME FNR FPAT FS IGNORECASE LINT NF NR OFMT OFS ORS PREC PROCINFO RLENGTH ROUNDMODE RS RSTART RT SUBSEP TEXTDOMAIN animal v_name v_value
symbol table contains 31 names of which 3 are arrays

BASIC

Works with: Beta BASIC version 3.0

,

Works with: SAM BASIC

<lang basic>10 INPUT "Enter a variable name", v$ 20 KEYIN "LET "+v$+"=42"</lang>

Batch File

<lang DOS>@echo off setlocal enableDelayedExpansion

set /p "name=Enter a variable name: " set /p "value=Enter a value: "

Create the variable and set its value

set "%name%=%value%"

Display the value without delayed expansion

call echo %name%=%%%name%%%

Display the value using delayed expansion

echo %name%=!%name%!</lang>

BBC BASIC

<lang bbcbasic> INPUT "Enter a variable name: " name$

     INPUT "Enter a numeric value: " numeric$
     dummy% = EVAL("FNassign("+name$+","+numeric$+")")
     PRINT "Variable " name$ " now has the value "; EVAL(name$)
     END
     
     DEF FNassign(RETURN n, v) : n = v : = 0</lang>

Bracmat

<lang bracmat>( put$"Enter a variable name: " & get$:?name & whl

 ' ( put$"Enter a numeric value: "
   & get$:?numeric:~#
   )

& !numeric:?!name & put$(str$("Variable " !name " now has the value " !!name \n)) );</lang>

C#

Works with: C sharp version 5

Not exactly a variable, but ExpandoObject allows adding properties at runtime. <lang csharp>using System; using System.Dynamic; using System.Collections.Generic;

public class Program {

   public static void Main()
   {
       string varname = Console.ReadLine();
       //Let's pretend the user has entered "foo"
       dynamic expando = new ExpandoObject();
       var map = expando as IDictionary<string, object>;
       map.Add(varname, "Hello world!");

       Console.WriteLine(expando.foo);
   }

}</lang>

Output:

Hello world!

Clojure

<lang clojure>(eval `(def ~(symbol (read)) 42))</lang>

Common Lisp

The short answer is this: <lang lisp> (setq var-name (read)) ; reads a name into var-name (set var-name 1) ; assigns the value 1 to a variable named as entered by the user </lang>

The academic answer is this:

In Common Lisp, symbol objects name variables; symbols are produced from strings by way of read (general syntax) or intern (specificially retrieving or making a symbol).

Symbols are grouped into packages — roughly namespaces — and any time symbols are created at runtime it is usually good to explicitly specify what package they are created in, outside of user/developer tools for working from the REPL (interactive mode) where the current package *package* is appropriate.

Within the standard, every variable is either lexical or special (dynamic scope). There is no global lexical environment, so in order to "create a variable", we must either create our own mechanism to remember it for lexical binding in a later evaluation, or create a special variable. It is unspecified what happens when a symbol not lexically bound or declared special is used as a variable.

Every symbol has a value slot — a field which, roughly, contains its current value considered as a special variable.

Therefore, there are two parts to dynamically creating a variable: we must declare it special, and give it a value. The first part is accomplished by the proclaim function for making declarations at run-time. The second part is simply assigning to the value slot. <lang lisp>(defun rc-create-variable (name initial-value)

 "Create a global variable whose name is NAME in the current package and which is bound to INITIAL-VALUE."
 (let ((symbol (intern name)))
   (proclaim `(special ,symbol))
   (setf (symbol-value symbol) initial-value)
   symbol))</lang>

<lang lisp>CL-USER> (rc-create-variable "GREETING" "hello") GREETING

CL-USER> (print greeting) "hello"</lang> Things to note:

Once a symbol has been declared special, it cannot be used as a lexical variable. Because of this potentially-surprising behavior, it is conventional to give all symbols naming special variables distinguished names, typically by asterisks as in *greeting*, so that lexical variables will not accidentally be given those names.

Some implementations do, to some extent, support global non-special variables; in these, because of the preceding problem, it is better to simply set the value slot and not proclaim it special. However, this may provoke undefined-variable warnings since the compiler or interpreter has no information with which to know the symbol is intended to be a variable.

Common Lisp, by default, is case-insensitive; however it accomplishes this by canonicalizing read input to uppercase; there is syntax to denote a lower or mixed-case symbol name, |Foo| or F\o\o. intern does not go through the input path (reader), so we must provide the name in uppercase to make an "ordinary" variable name.

Déjà Vu

In Déjà Vu, variable names are idents, which are completely separate from strings, and cannot easily be created from them. The way around that is to invoke the compiler: <lang dejavu>local :var-name !run-blob !compile-string dup concat( ":" !prompt "Enter a variable name: " ) local var-name 42

Assuming the user types THISISWEIRD, otherwise this'll error

!. THISISWEIRD</lang>

Output:

Enter a variable name: THISISWEIRD
42

E

In E, there are no global variables, and there is no modification of the local (lexical) environment. However, it is possible to construct a program which binds any given variable name. <lang e>def makeNounExpr := <elang:evm.makeNounExpr>

def dynVarName(name) {

   def variable := makeNounExpr(null, name, null)

   return e`{
   
       def a := 1
       def b := 2
       def c := 3
   
       {
           def $variable := "BOO!"
           [a, b, c]
       }
   
   }`.eval(safeScope)

}

? dynVarName("foo")

value: [1, 2, 3]

? dynVarName("b")

value: [1, "BOO!", 3]

? dynVarName("c")

value: [1, 2, "BOO!"]</lang>

It is also possible to capture the environment object resulting from the evaluation of the constructed program and use it later; this is done by bindX in Eval in environment#E (except for the program being constant, which is independent).

Elena

Dynamic variables are not supported by the language. But it is possible to set a dynamic property.

ELENA 5.0 : <lang elena>import system'dynamic; import extensions;

class TestClass {

   object theVariables;

   constructor()
   {
       theVariables := new DynamicStruct()
   }
   
   function()
   {
       auto prop := new MessageName(console.write:"Enter the variable name:".readLine());
       (prop.setPropertyMessage())(theVariables,42);
       
       console.printLine(prop.toPrintable(),"=",(prop.getPropertyMessage())(theVariables)).readChar()     
   }

}

public program = new TestClass();</lang>

Output:

Enter the variable name:a
a=42

Emacs Lisp

A variable is a symbol. A name can be read from the user as a string and interned to a symbol.

<lang Lisp>(set (intern (read-string "Enter variable name: ")) 123)</lang>

This example deliberately doesn't use any temporary variables so their names won't clash with what the user might enter. A set like this hits any let dynamic binding or buffer-local setting in the usual way.

Erlang

This task uses functions from Runtime evaluation. <lang Erlang> -module( dynamic_variable_names ).

-export( [task/0] ).

task() ->

   {ok,[Variable_name]} = io:fread( "Variable name? ",	"~a" ),
   Form = runtime_evaluation:form_from_string( erlang:atom_to_list(Variable_name) ++ "." ),
   io:fwrite( "~p has value ~p~n", [Variable_name, runtime_evaluation:evaluate_form(Form, {Variable_name, 42})] ).

</lang>

Output:

12> dynamic_variable_names:task().
Variable name? Asd
'Asd' has value 42

Factor

By convention, variable names are usually symbols, but because dynamic variables are implemented via implicit association lists, any object can be used as the key for a value. In this case, we use the string the user enters. <lang factor>42 readln set</lang>

Forth

<lang forth>s" VARIABLE " pad swap move ." Variable name: " pad 9 + 80 accept pad swap 9 + evaluate</lang> Of course, it is easier for the user to simply type VARIABLE name at the Forth console.

FreeBASIC

FreeBASIC is a statically typed, compiled language and so it is not possible to create new variables, dynamically, at run time. However, you can make it look to the user like you are doing so with code such as the following. Ideally, a 'map' should be used for an exercise such as this but, as there isn't one built into FB, I've used a dynamic array instead which is searched linearly for the variable name.

<lang freebasic>' FB 1.05.0 Win64

Type DynamicVariable

 As String name
 As String value

End Type

Function FindVariableIndex(a() as DynamicVariable, v as String, nElements As Integer) As Integer

 v = LCase(Trim(v))
 For i As Integer = 1 To nElements
   If a(i).name = v Then Return i
 Next
 Return 0

End Function

Dim As Integer n, index Dim As String v Cls

Do

Input "How many variables do you want to create (max 5) "; n

Loop Until n > 0 AndAlso n < 6

Dim a(1 To n) As DynamicVariable Print Print "OK, enter the variable names and their values, below"

For i As Integer = 1 to n

 Print
 Print "  Variable"; i
 Input "    Name  : ", a(i).name 
 a(i).name = LCase(Trim(a(i).name)) ' variable names are not case sensitive in FB
 If i > 0 Then
    index = FindVariableIndex(a(), a(i).name, i - 1)
    If index > 0 Then
      Print "  Sorry, you've already created a variable of that name, try again"
      i -= 1
      Continue For
    End If 
 End If
 Input "    Value : ", a(i).value
 a(i).value = LCase(Trim(a(i).value))

 Print
 Input "Which variable do you want to inspect "; v
 If v = "q" OrElse v = "Q" Then Exit Do
 index = FindVariableIndex(a(), v, n)
 If index = 0 Then
   Print "Sorry there's no variable of that name, try again"
 Else
   Print "It's value is "; a(index).value
 End If

Loop End</lang>

Sample input/output :

Output:

How many variables do you want to create (max 5) ? 3

OK, enter the variable names and their values, below

  Variable 1
    Name  : a
    Value : 1

  Variable 2
    Name  : b
    Value : 2

  Variable 3
    Name  : b
  Sorry, you've already created a variable of that name, try again

  Variable 3
    Name  : c
    Value : 4

Press q to quit

Which variable do you want to inspect ? b
It's value is 2

Which variable do you want to inspect ? c
It's value is 4

Which variable do you want to inspect ? a
It's value is 1

Which variable do you want to inspect ? q

GAP

<lang gap># As is, will not work if val is a String Assign := function(var, val) Read(InputTextString(Concatenation(var, " := ", String(val), ";"))); end;</lang>

Genyris

The intern function creates a symbol from an arbitrary string. Defvar creates a binding. Weird symbols are quoted with pipe characters. <lang genyris>defvar (intern 'This is not a pipe.') 42 define |<weird>| 2009</lang>

Go

Translation of: FreeBASIC

Go is in the same boat as other statically typed, compiled languages here in that variables cannot be created dynamically at runtime. However, we can use the built-in map type to associate names input at runtime with values which, in practice, is just as good. <lang go>package main

import (

   "bufio"
   "fmt"
   "log"
   "os"
   "strconv"
   "strings"

)

func check(err error) {

   if err != nil {
       log.Fatal(err)
   }

}

func main() {

   scanner := bufio.NewScanner(os.Stdin)
   n := 0
   for n < 1 || n > 5 {
       fmt.Print("How many integer variables do you want to create (max 5) : ")
       scanner.Scan()
       n, _ = strconv.Atoi(scanner.Text())
       check(scanner.Err())
   }
   vars := make(map[string]int)
   fmt.Println("OK, enter the variable names and their values, below")
   for i := 1; i <= n; {
       fmt.Println("\n  Variable", i)
       fmt.Print("    Name  : ")
       scanner.Scan()
       name := scanner.Text()
       check(scanner.Err())
       if _, ok := vars[name]; ok {
           fmt.Println("  Sorry, you've already created a variable of that name, try again")
           continue
       }
       var value int
       var err error
       for {
           fmt.Print("    Value : ")
           scanner.Scan()
           value, err = strconv.Atoi(scanner.Text())
           check(scanner.Err())
           if err != nil {
               fmt.Println("  Not a valid integer, try again")
           } else {
               break
           }
       }
       vars[name] = value
       i++
   }
   fmt.Println("\nEnter q to quit")
   for {
       fmt.Print("\nWhich variable do you want to inspect : ")
       scanner.Scan()
       name := scanner.Text()
       check(scanner.Err())
       if s := strings.ToLower(name); s == "q" {
           return
       }
       v, ok := vars[name]
       if !ok {
           fmt.Println("Sorry there's no variable of that name, try again")
       } else {
           fmt.Println("It's value is", v)
       }
   }

}</lang>

Output:

Sample input/output:

How many integer variables do you want to create (max 5) : 3
OK, enter the variable names and their values, below

  Variable 1
    Name  : pip
    Value : 1

  Variable 2
    Name  : squeak
    Value : 2

  Variable 3
    Name  : pip
  Sorry, you've already created a variable of that name, try again

  Variable 3
    Name  : wilfred 
    Value : 3

Enter q to quit

Which variable do you want to inspect : squeak
It's value is 2

Which variable do you want to inspect : auntie
Sorry there's no variable of that name, try again

Which variable do you want to inspect : wilfred
It's value is 3

Which variable do you want to inspect : q

Groovy

Solution: <lang groovy>def varname = 'foo' def value = 42

new GroovyShell(this.binding).evaluate("${varname} = ${value}")

assert foo == 42</lang>

Haskell

<lang haskell>data Var a = Var String a deriving Show main = do

   putStrLn "please enter you variable name"
   vName <- getLine
   let var = Var vName 42
   putStrLn $ "this is your variable: " ++ show var</lang>

Tasks listed here have been marked as "un-implementable" in Unicon. Solutions may be impossible to do, too complex to be of any valuable instruction in Unicon, or prohibited by the task definition.

But hey; if you think you can prove us wrong, go for it. :-)

J

This code was written for J6.02. In J8.04 you will need to replace require'misc' with require'general/misc/prompt'

<lang j>require 'misc' (prompt 'Enter variable name: ')=: 0</lang>

For example: <lang j> require 'misc'

  (prompt 'Enter variable name: ')=: 0

Enter variable name: FOO

FOO

0</lang> Or, if the name is defined in the variable 'userDefined' <lang j> userDefined=: 'BAR'

  (userDefined)=: 1
  BAR

1</lang>

Java

Java does not support dynamic naming of variables. Therefore, HashMap has been implemented here. It is similar to an array but allows the searching of values by String keys instead of simply index numbers. <lang java>public static void main(String... args){

       HashMap<String, Integer> vars = new HashMap<String, Integer>();
       //The variable name is stored as the String. The var type of the variable can be
       //changed by changing the second data type mentiones. However, it must be an object
       //or a wrapper class.
       vars.put("Variable name", 3); //declaration of variables
       vars.put("Next variable name", 5);
       Scanner sc = new Scanner(System.in);
       String str = sc.next();
       vars.put(str, sc.nextInt()); //accpeting name and value from user
       
       System.out.println(vars.get("Variable name")); //printing of values
       System.out.println(vars.get(str));

} </lang>

JavaScript

<lang javascript>var varname = 'foo'; // pretend a user input that var value = 42; eval('var ' + varname + '=' + value);</lang> Alternatively, without using eval: <lang javascript>var varname = prompt('Variable name:'); var value = 42; this[varname] = value;</lang>

jq

jq does not have variables in the usual sense, but in practice the key/value pairs of JSON objects can be used as variable/value bindings. Using this approach, the given task can be accomplished using the following program:

<lang jq>"Enter a variable name:", (input as $var

| ("Enter a value:" ,
   (input as $value | { ($var) : $value })))</lang>

Transcript

$ jq -nrR -f program.jq
Enter a variable name:
abracadabra
Enter a value:
magic
{
  "abracadabra": "magic"
}

Julia

Works with: Julia version 0.6

Julia has powerful macros:

<lang julia>print("Insert the variable name: ")

variable = Symbol(readline(STDIN)) expression = quote

   $variable = 42
   println("Inside quote:")
   @show $variable

end

eval(expression)

println("Outside quote:") @show variable println("If I named the variable x:") @show x</lang>

Output:

Insert the variable name: 
x

Inside quote:
x = 42
Outside quote:
variable = :x
x = 42

Kotlin

Kotlin is a statically typed, compiled language and so it is not possible to create new variables, dynamically, at run time. However, you can make it look to the user like you are doing so with code such as the following which uses a map:

Translation of: FreeBASIC

<lang scala>// version 1.1.4

fun main(args: Array<String>) {

   var n: Int
   do {
       print("How many integer variables do you want to create (max 5) : ")
       n = readLine()!!.toInt()
   }
   while (n < 1 || n > 5)

   val map = mutableMapOf<String, Int>()
   var name: String
   var value: Int
   var i = 1
   println("OK, enter the variable names and their values, below")
   do {
       println("\n  Variable $i")
       print("    Name  : ")
       name = readLine()!!
       if (map.containsKey(name)) {
           println("  Sorry, you've already created a variable of that name, try again")
           continue
       }
       print("    Value : ")
       value = readLine()!!.toInt()
       map.put(name, value)
       i++
   }
   while (i <= n)

   println("\nEnter q to quit")
   var v: Int?
   while (true) {
       print("\nWhich variable do you want to inspect : ")
       name = readLine()!!
       if (name.toLowerCase() == "q") return
       v = map[name]
       if (v == null) println("Sorry there's no variable of that name, try again")
       else println("It's value is $v")
   }

}</lang> Sample input/output:

Output:

How many integer variables do you want to create (max 5) : 3
OK, enter the variable names and their values, below

  Variable 1
    Name  : faith
    Value : 1

  Variable 2
    Name  : hope
    Value : 2

  Variable 3
    Name  : hope
  Sorry, you've already created a variable of that name, try again

  Variable 3
    Name  : charity
    Value : 3

Enter q to quit

Which variable do you want to inspect : chastity
Sorry there's no variable of that name, try again

Which variable do you want to inspect : charity
It's value is 3

Which variable do you want to inspect : q

Lasso

Thread vars in Lasso 9 can have dynamic names, but local variables cannot.

The example below outputs a random decimal that was assigned to the variable name entered as part of the GET params. <lang Lasso>local(thename = web_request->param('thename')->asString) if(#thename->size) => {^ var(#thename = math_random) var(#thename) else '<a href="?thename=xyz">Please give the variable a name!</a>' ^}</lang>

Lingo

<lang lingo>-- varName might contain a string that was entered by a user at runtime

-- A new global variable with a user-defined name can be created at runtime like this: (the globals)[varName] = 23 -- or (the globals).setProp(varName, 23)

-- An new instance variable (object property) with a user-defined name can be created at runtime like this: obj[varName] = 23 -- or obj.setProp(varName, 23)</lang>

Logo

<lang logo>? make readword readword julie 12 ? show :julie 12</lang>

Logtalk

Logtalk objects can be create or compiled such that new predicates can be added at runtime. A simple example: <lang logtalk> | ?- create_object(Id, [], [set_logtalk_flag(dynamic_declarations,allow)], []),

    write('Variable name:  '), read(Name),
    write('Variable value: '), read(Value),
    Fact =.. [Name, Value],
    Id::assertz(Fact).

Variable name: foo. Variable value: 42. Id = o1, Name = foo, Value = 42, Fact = foo(42).

?- o1::current_predicate(foo/1). true.

| ?- o1::foo(X). X = 42. </lang>

Lua

<lang lua>_G[io.read()] = 5 --puts 5 in a global variable named by the user</lang>

M2000 Interpreter

<lang M2000 Interpreter> Module DynamicVariable {

     input "Variable Name:", a$
     a$=filter$(a$," ,+-*/^~'\({=<>})|!$&"+chr$(9)+chr$(127))
     While a$ ~ "..*"  {a$=mid$(a$, 2)}
     If len(a$)=0 then Error "No name found"
     If chrcode(a$)<65 then Error "Not a Valid name"
     Inline a$+"=1000"
     Print eval(a$)=1000
     \\ use of a$ as pointer to variable
     a$.+=100
     Print eval(a$)=1100
     \\ list of variables
     List

} Keyboard "George"+chr$(13) DynamicVariable </lang>

M4

<lang M4>Enter foo, please. define(`inp',esyscmd(`echoinp')) define(`trim',substr(inp,0,decr(len(inp)))) define(trim,42) foo</lang>

DOS batch file echoinp.bat:

@echo off
set /p Input=
echo %Input%

Mathematica / Wolfram Language

<lang Mathematica>varname = InputString["Enter a variable name"]; varvalue = InputString["Enter a value"]; ReleaseHold[ Hold[Set["nameholder", "value"]] /. {"nameholder" -> Symbol[varname], "value" -> varvalue}]; Print[varname, " is now set to ", Symbol[varname]]</lang>

Example output:

-> testvar is now set to 86

Maxima

<lang maxima>/* Use :: for indirect assignment */ block([name: read("name?"), x: read("value?")], name :: x);</lang>

min

Works with: min version 0.19.3

<lang min>42 "Enter a variable name" ask define</lang>

MUMPS

This is done in immediate mode so you can see the variable is created, although you will have to reference it through the indirection operator, "@". <lang mumps>USER>KILL ;Clean up workspace

USER>WRITE ;show all variables and definitions

USER>READ "Enter a variable name: ",A Enter a variable name: GIBBERISH USER>SET @A=3.14159

USER>WRITE

A="GIBBERISH" GIBBERISH=3.14159</lang>

Nanoquery

<lang Nanoquery>print "Enter a variable name: " name = input()

print name + " = " exec(name + " = 42") exec("println " + name)</lang>

Output:

Enter a variable name: test
test = 42

Nim

Nim is a compiled language, with powerful Templating and Macros, which are compile-time rather than run-time.

This solution emulates dynamic variables by mapping a string to a pointer to a variable (using a table). <lang Nim>import tables

var

 theVar: int = 5
 varMap = initTable[string, pointer]()

proc ptrToInt(p: pointer): int =

 result = cast[ptr int](p)[]

proc main() =

 write(stdout, "Enter a var name: ")
 let sVar = readLine(stdin)
 varMap[$svar] = theVar.addr
 echo "Variable ", sVar, " is ", ptrToInt(varMap[$sVar])

when isMainModule:

 main()</lang>

Output:

Enter a var name: varZ
Variable varZ is 5

Octave

<lang octave>varname = input ("Enter variable name: ", "s"); value = input ("Enter value: ", "s"); eval([varname,"=",value]);</lang>

Oforth

<lang oforth>: createVar(varname)

  "tvar: " varname + eval ;

"myvar" createVar

12 myvar put myvar at .</lang>

PARI/GP

<lang parigp>eval(Str(input(), "=34"))</lang>

Pascal

Free Pascal

Works with: Free Pascal version version 3.2.0

<lang Pascal> PROGRAM ExDynVar; {$mode objfpc}{$H+}{$J-}{R+} (*

*  Free Pascal Compiler version 3.2.0 [2020/06/14] for x86_64
*  The free and readable alternative
*  compiles natively to almost any platform, including raspberry PI
*
*  This demo uses a dictionary because it is compiled: it cannot  make
*  dynamic variables at runtime.
*)

USES

   Generics.Collections,
               SysUtils,
               Variants;

TYPE

   Tdict = specialize TDictionary<string, variant>;

VAR

   VarName:         string;
   strValue:        string;
   VarValue:       variant;
   D:                Tdict;

   FUNCTION SetType ( strVal: string ): variant;
   (* If the value is numeric, store it as numeric, otherwise store it as string *)
       BEGIN
           TRY
               SetType := StrToFloat ( strVal ) ;
           EXCEPT
               SetType := strVal ;
           END;
       END;

BEGIN

   D := TDict.Create;
   REPEAT
       Write  ( 'Enter variable name : '  ) ;
       ReadLn ( VarName  ) ;
       Write  ( 'Enter variable Value : ' ) ;
       ReadLn ( strValue ) ;
       VarValue := SetType ( strValue ) ;
       TRY
           BEGIN
               D.AddOrSetValue ( VarName, VarValue ) ;
               Write                     ( VarName ) ;
               Write                     (  ' = '  ) ;
               WriteLn             ( D [ VarName ] ) ;
           END;
       EXCEPT
           WriteLn ( 'Something went wrong.. Try again' ) ;
       END;
   UNTIL ( strValue =  ) ;
   D.Free;

END. </lang>JGAD 2021/05/13

Perl

<lang perl>print "Enter a variable name: "; $varname = <STDIN>; # type in "foo" on standard input chomp($varname); $$varname = 42; # when you try to dereference a string, it will be

               # treated as a "symbolic reference", where they
               # take the string as the name of the variable

print "$foo\n"; # prints "42"</lang> If you are operating in a strict environment, this isn't possible. You need to use 'eval' in this case <lang perl>use strict;

print "Enter a variable name: "; my $foo; my $varname = <STDIN>; # type in "foo" on standard input chomp($varname); my $varref = eval('\$' . $varname); $$varref = 42; print "$foo\n"; # prints "42"</lang>

Phix

dictionary

In a sense, this is kinda faking it: <lang Phix>constant globals = new_dict()

while 1 do

   string name = prompt_string("Enter name or press Enter to quit:")
   if length(name)=0 then exit end if
   bool bExists = (getd_index(name,globals)!=NULL)
   string prompt = iff(not bExists?"No such name, enter a value:"
                  :sprintf("Already exists, new value[%s]:",{getd(name,globals)}))
   string data = prompt_string(prompt)
   if length(data) then
       setd(name,data,globals)
   end if

end while</lang>

Output:

Enter name or press Enter to quit:fred
No such name, enter a value:35
Enter name or press Enter to quit:fred
Already exists, new value[35]:
Enter name or press Enter to quit:james
No such name, enter a value:1
Enter name or press Enter to quit:fred
Already exists, new value[35]:
Enter name or press Enter to quit:james
Already exists, new value[1]:
Enter name or press Enter to quit:

dynamic classes

Library: Phix/Class

<lang Phix>requires("0.8.2") class dc dynamic -- public string fred = "555" -- (predefine some fields if you like) end class dc d = new()

while 1 do

   string name = prompt_string("Enter name or press Enter to quit:")
   if length(name)=0 then exit end if
   bool bExists = (get_field_type(d,name)!=NULL)

-- bool bExists = string(d[name]) -- alt...

   string prompt = iff(not bExists?"No such name, enter a value:"
                  :sprintf("Already exists, new value[%s]:",{d[name]}))
   string data = prompt_string(prompt)
   if length(data) then
       d[name] = data
   end if

end while</lang> Same output as above (for the same input)
Note you would get a fatal crash were that predefined fred not made public, and you entered that, which you could I suppose avoid by testing the result of get_field_flags(d,name) for SF_PRIVATE (and then skipping the prompt and any attempt to store).

PHP

PicoLisp

<lang PicoLisp>(de userVariable ()

  (prin "Enter a variable name: ")
  (let Var (line T)                                  # Read transient symbol
     (prin "Enter a value: ")
     (set Var (read))                                # Set symbol's value
     (println 'Variable Var 'Value (val Var)) ) )    # Print them</lang>

Output:

Enter a variable name: Tom
Enter a value: 42
Variable "Tom" Value 42
-> 42

PowerShell

<lang powershell>$variableName = Read-Host New-Variable $variableName 'Foo' Get-Variable $variableName</lang>

ProDOS

<lang>editvar /newvar /value=a /userinput=1 /title=Enter a variable name: editvar /newvar /value=b /userinput=1 /title=Enter a variable title: editvar /newvar /value=-a- /title=-b-</lang>

Python

Works with: Python version 2.x

<lang python>>>> name = raw_input("Enter a variable name: ") Enter a variable name: X >>> globals()[name] = 42 >>> X 42</lang>

Works with: Python version 3.x

<lang python>>>> name = input("Enter a variable name: ") Enter a variable name: X >>> globals()[name] = 42 >>> X 42</lang> Note: most of the time when people ask how to do this on newsgroups and other forums, on investigation, it is found that a neater solution is to map name to value in a dictionary.

R

<lang R># Read the name in from a command prompt varname <- readline("Please name your variable >")

Make sure the name is valid for a variable

varname <- make.names(varname) message(paste("The variable being assigned is '", varname, "'"))

Assign the variable (with value 42) into the user workspace (global environment)

assign(varname, 42)

Check that the value has been assigned ok

ls(pattern=varname) get(varname)</lang>

Racket

This works on the Racket REPL:

<lang Racket> -> (begin (printf "Enter some name: ")

         (namespace-set-variable-value! (read) 123))

Enter some name: bleh -> bleh 123 </lang>

Raku

(formerly Perl 6)

Works with: Rakudo version 2018.03

You can interpolate strings as variable names:

<lang perl6>our $our-var = 'The our var'; my $my-var = 'The my var';

my $name = prompt 'Variable name: '; my $value = $::('name'); # use the right sigil, etc

put qq/Var ($name) starts with value ｢$value｣/;

$::('name') = 137;

put qq/Var ($name) ends with value ｢{$::('name')}｣/; </lang>

REBOL

<lang REBOL>REBOL [ Title: "Dynamic Variable Name" URL: http://rosettacode.org/wiki/Dynamic_variable_names ]

Here, I ask the user for a name, then convert it to a word and
assign the value "Hello!" to it. To read this phrase, realize that
REBOL collects terms from right to left, so "Hello!" is stored for
future use, then the prompt string "Variable name? " is used as the
argument to ask (prompts user for input). The result of ask is
converted to a word so it can be an identifier, then the 'set' word
accepts the new word and the string ("Hello!") to be assigned.

set to-word ask "Variable name? " "Hello!"</lang>

Session output:

Variable name? glister
== "Hello!"
>> glister
== "Hello!"

Retro

<lang Retro>:newVariable ("-) s:get var ;

newVariable: foo</lang>

REXX

Checks could've been made to:

check for the minimum number of arguments
check for a legitimate REXX variable name

<lang rexx>/*REXX program demonstrates the use of dynamic variable names & setting a val.*/ parse arg newVar newValue say 'Arguments as they were entered via the command line: ' newVar newValue say call value newVar, newValue say 'The newly assigned value (as per the VALUE bif)------' newVar value(newVar)

                                      /*stick a fork in it,  we're all done. */</lang>

output for the input: abc 456

Arguments as they were entered via the command line = abc 45678.1

The newly assigned value (as per the VALUE bif)------ abc 45678.1

Ring

<lang ring> See "Enter the variable name: " give cName eval(cName+"=10") See "The variable name = " + cName + " and the variable value = " + eval("return "+cName) + nl </lang> Output <lang ring> Enter the variable name: test The variable name = test and the variable value = 10 </lang>

RLaB

In RLaB all the objects are located in a global list $$. To create a variable dynamically, one writes a new entry into the global list. Consider the following example: <lang RLaB>>> s = "myusername" myusername >> $$.[s] = 10; >> myusername

 10</lang>

Ruby

<lang ruby>p "Enter a variable name" x = "@" + gets.chomp! instance_variable_set x, 42 p "The value of #{x} is #{instance_variable_get x}" </lang>

Example output:

"Enter a variable name"
hello
"The value of @hello is 42"

Scheme

<lang scheme>=> (define (create-variable name initial-val)

    (eval `(define ,name ,initial-val) (interaction-environment)))

=> (create-variable (read) 50) <hello

=> hello 50</lang>

Sidef

It is not possible to create a new lexical variable at run-time, but there are other various ways to do something similar.

<lang ruby>var name = read("Enter a variable name: ", String); # type in 'foo'

class DynamicVar(name, value) {

   method init {
       DynamicVar.def_method(name, ->(_) { value })
   }

}

var v = DynamicVar(name, 42); # creates a dynamic variable say v.foo; # retrieves the value</lang>

Slate

Slate symbols are objects that name methods and slots. "Variable definition" is like defining a method which holds the value of a slot, and "variable access" is just method-call to get that value back. <lang slate>define: #name -> (query: 'Enter a variable name: ') intern. "X" define: name -> 42. X print.</lang>

Smalltalk

Works with: Pharo

Define a block-temporary variable with name specified by user input. Set that variable to 42. Print that variable's name and value. <lang smalltalk>| varName | varName := FillInTheBlankMorph request: 'Enter a variable name'. Compiler evaluate:('| ', varName, ' | ', varName, ' := 42. Transcript show: value of ', varName, '; show: is ; show: ', varName).</lang> the above creates a variable which is visible only inside the evaluated block (which is considered good style). It will not be visible outside.

A bad style alternative which creates a globally visible variable in the Smalltalk namespace (and is therefore visible everywhere) is: <lang smalltalk>| varName | varName := Stdin request: 'Enter a global variable name:'. Smalltalk at:varName asSymbol put:42. expr := Stdin request:'Enter an expression:'. (Compiler evaluate:expr) printCR </lang> Be reminded again: this is considered *very very bad style*, and every programmer doing this should be fired.

Output:

Enter a global variable name: abc

Enter an expression: abc squared + 5

1769

SNOBOL4

Indirect string reference of variables is a basic feature of Snobol, using the $ operator. trim( ) is needed for Snobol4+. <lang SNOBOL4>* # Get var name from user

       output = 'Enter variable name:'
       invar = trim(input)

# Get value from user, assign

       output = 'Enter value:'
       $invar = trim(input)

Display

       output = invar ' == ' $invar

end</lang>

Output:

Enter variable name:
pi
Enter value:
3.14159
pi == 3.14159

Stata

Here a scalar variable is created, but one could create a dataset variable, a matrix... Notice the name of the variable is not "s", but the name stored in the global macro "s".

<lang stata>display "Name?" _request(s) scalar $s=10 display $s</lang>

Tcl

<lang Tcl>puts "Enter a variable name:" gets stdin varname set $varname 42 puts "I have set variable $varname to [set $varname]"</lang> Note that it is more normal to use the user's name to index into a Tcl associative array, as the syntax gets easier to work with in that case: <lang tcl>puts -nonewline "Enter an element name: "; flush stdout gets stdin elemname set ary($elemname) [expr int(rand()*100)] puts "I have set element $elemname to $ary($elemname)"</lang> Another common method for working with dynamic variables is to make an alias to the variable with a fixed name: <lang tcl>puts -nonewline "Enter a variable name: "; flush stdout gets stdin varname upvar 0 $varname v; # The ‘0’ for “current scope” set v [expr int(rand()*100)] puts "I have set variable $varname to $v (see for yourself: [set $varname])"</lang>

TI-89 BASIC

<lang ti89b>Local varName,value InputStr "Variable name", varName Prompt value value → #varName</lang>

TUSCRIPT

<lang tuscript> $$ MODE TUSCRIPT ASK "Enter variablename": name="" ASK "Enter value": value="" TRACE +@name @name=$value PRINT @name </lang> Output:

Enter variablename >test
Enter value >Hello World!
TRACING     Scratch-Datei -*TUSTEP.EDT
   5    00  TRACE +@name
test         = Hello World!
Hello World!

UNIX Shell

<lang bash>read name declare $name=42 echo "${name}=${!name}"</lang>

Wren

Although Wren is dynamically typed, it is not possible to create new variables at run time. We therefore follow the example of some of the statically typed languages here and use a map instead. <lang ecmascript>import "io" for Stdin, Stdout

var userVars = {} System.print("Enter three variables:") for (i in 0..2) {

   System.write("\n  name : ")
   Stdout.flush()
   var name = Stdin.readLine()
   System.write("  value: ")
   Stdout.flush()
   var value = Num.fromString(Stdin.readLine())
   userVars[name] = value

}

System.print("\nYour variables are:\n") for (kv in userVars) {

   System.print("  %(kv.key) = %(kv.value)")

}</lang>

Output:

Sample session:

Enter three variables:

  name : pip
  value: 3

  name : squeak
  value: 4

  name : wilfred
  value: 5

Your variables are:

  pip = 3
  squeak = 4
  wilfred = 5

zkl

zkl doesn't support adding vars to an existing class but can create a new class with new vars: <lang zkl>vname:="foo"; // or vname:=ask("var name = "); klass:=Compiler.Compiler.compileText("var %s=123".fmt(vname))(); // compile & run the constructor klass.vars.println(); klass.foo.println(); klass.setVar(vname).println(); // setVar(name,val) sets the var</lang>

Output:

L(L("foo",123))
123
123

Zsh

<lang zsh>read name typeset $name=42</lang>