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(emacs-lisp-intro.info)setcar

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`setcar'
========

   As you might guess from their names, the `setcar' and `setcdr'
functions set the CAR or the CDR of a list to a new value.  They
actually change the original list, unlike `car' and `cdr' which leave
the original list as it was.  One way to find out how this works is to
experiment.  We will start with the `setcar' function.

   First, we can make a list and then set the value of a variable to the
list, using the `setq' function.  Here is a list of animals:

     (setq animals '(antelope giraffe lion tiger))

If you are reading this in Info inside of GNU Emacs, you can evaluate
this expression in the usual fashion, by positioning the cursor after
the expression and typing `C-x C-e'.  (I'm doing this right here as I
write this.  This is one of the advantages of having the interpreter
built into the computing environment.)

   When we evaluate the variable `animals', we see that it is bound to
the list `(antelope giraffe lion tiger)':

     animals
          => (antelope giraffe lion tiger)

Put another way, the variable `animals' points to the list `(antelope
giraffe lion tiger)'.

   Next, evaluate the function `setcar' while passing it two arguments,
the variable `animals' and the quoted symbol `hippopotamus'; this is
done by writing the three element list `(setcar animals 'hippopotamus)'
and then evaluating it in the usual fashion:

     (setcar animals 'hippopotamus)

After evaluating this expression, evaluate the variable `animals'
again.  You will see that the list of animals has changed:

     animals
          => (hippopotamus giraffe lion tiger)

The first element on the list, `antelope' is replaced by `hippopotamus'.

   So we can see that `setcar' did not add a new element to the list as
`cons' would have; it replaced `giraffe' with `hippopotamus'; it
_changed_ the list.