Create a variable name from a string in Lisp

Question

I'm trying to take a string, and convert it into a variable name. I though (make-symbol) or (intern) would do this, but apparently it's not quite what I want, or I'm using it incorrectly.

For example:

> (setf (intern (string "foo")) 5)
> foo
  5

Here I would be trying to create a variable named 'foo' with a value of 5. Except, the above code gives me an error. What is the command I'm looking for?

WReach · Accepted Answer · 2011-03-15 02:36:10Z

15

There are a number of things to consider here:

SETF does not evaluate its first argument. It expects a symbol or a form that specifies a location to update. Use SET instead.
Depending upon the vintage and settings of your Common Lisp implementation, symbol names may default to upper case. Thus, the second reference to foo may actually refer to a symbol whose name is "FOO". In this case, you would need to use (intern "FOO").
The call to STRING is harmless but unnecessary if the value is already a string.

Putting it all together, try this:

> (set (intern "FOO") 5)
> foo
  5

answered Mar 15, 2011 at 2:36

WReach

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2 Comments

Jeff Over a year ago

Hey, that worked! Well, sort of. I'm actually not trying to assign values using (set) but using another function called (add-dm) which I didn't create. Apparently, (add-dm) has the same limitation as (setf), where the first expression is not evaluated, so I'll have to figure out another way. But that did shed some light on things, so thanks! (Also, I do realize that (string) is unnecessary, though my ultimate goal is to create a symbol from concatenated strings, so I kept it in just in case it made a difference).

WReach Over a year ago

@Jeff If ADD-DM is a macro, it might expand to an exported function that can be used in the same manner as SET. Try inspecting the result of (macroexpand-1 '(add-dm ...)) to see if you are in luck.

Rainer Joswig · Accepted Answer · 2011-03-15 21:17:47Z

9

Use:

CL-USER 7 > (setf (SYMBOL-VALUE (INTERN "FOO")) 5) 
5

CL-USER 8 > foo
5

This also works with a variable:

CL-USER 11 > (let ((sym-name "FOO"))
               (setf (SYMBOL-VALUE (INTERN sym-name)) 3))
3

CL-USER 12 > foo
3

Remember also that by default symbols are created internally as uppercase. If you want to access a symbol via a string, you have to use an uppercase string then.

answered Mar 15, 2011 at 21:17

Rainer Joswig

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