Question: I'm just starting out in learning Ruby and I've written a program that generates some numbers and assigns them to variables @one, @two, @three etc. The user can then specify a variable to change by inputting it's name (e.g one). I then need to do something like '@[valueofinout] = asd'. How would I do this, and is there a better way as the way I'm thinking of seems to be discouraged? I've found

```
x = "myvar"
myvar = "hi"
eval(x) -> "hi"

```

but I don't completely understand why the second line is needed. In my case would I use something like

```
@one = "21"
input = "one"
input = "@" + input
changeto = "22"
eval(input) -> changeto

```

Answer: 
Use `instance_variable_set` ([rubydoc](http://ruby-doc.org/core/classes/Object.html#M000368))

```
instance_variable_set("@" + varname, value)

```

In most cases though, you should separate your normal Ruby variables from the variables your user is interacting with. How about creating a Hash of user variables, e.g.

```
@uservars = { 'one' => 1, 'two' => 2 }
two = @uservars['two']   # Look up 'two' variable

varname = "myvar"
@uservars[varname] = 5   # Set a variable by name
value = @uservars[varname]  # Get a variable by name 

```
Instance variables can be retrieved via this method:

```
input = instance_variable_get("@one")

```

After this, in your case you'll have `input` equal to "21".