Info Catalog eintr: nth eintr: car cdr & cons eintr: setcdr

eintr: setcar

 
 7.5 ‘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.  Incidentally, when there is
 nothing on the line after the final parentheses, such as a comment,
 point can be on the next line.  Thus, if your cursor is in the first
 column of the next line, you do not need to move it.  Indeed, Emacs
 permits any amount of white space after the final parenthesis.)
 
    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 ‘antelope’ with ‘hippopotamus’; it
 _changed_ the list.
 

Info Catalog eintr: nth eintr: car cdr & cons eintr: setcdr