Function: delete

delete is a function defined in fns.c.

Signature

(delete ELT SEQ)

Documentation

Delete members of SEQ which are equal to ELT, and return the result.

SEQ must be a sequence (i.e. a list, a vector, or a string). The return value is a sequence of the same type.

If SEQ is a list, this behaves like delq, except that it compares with equal instead of eq. In particular, it may remove elements by altering the list structure.

If SEQ is not a list, deletion is never performed destructively; instead this function creates and returns a new vector or string.

Write (setq foo (delete element foo)) to be sure of correctly changing the value of a sequence foo. See also remove, which does not modify the argument.

Other relevant functions are documented in the list group.

View in manual

Probably introduced at or before Emacs version 21.1.

Shortdoc

;; list
(delete 2 (list 1 2 3 4))
    => (1 3 4)
  (delete "a" (list "a" "b" "c" "d"))
    => ("b" "c" "d")

Source Code

// Defined in /usr/src/emacs/src/fns.c
{
  if (VECTORP (seq))
    {
      ptrdiff_t n = 0;
      ptrdiff_t size = ASIZE (seq);
      USE_SAFE_ALLOCA;
      Lisp_Object *kept = SAFE_ALLOCA (size * sizeof *kept);

      for (ptrdiff_t i = 0; i < size; i++)
	{
	  kept[n] = AREF (seq, i);
	  n += NILP (Fequal (AREF (seq, i), elt));
	}

      if (n != size)
	seq = Fvector (n, kept);

      SAFE_FREE ();
    }
  else if (STRINGP (seq))
    {
      if (!CHARACTERP (elt))
	return seq;

      ptrdiff_t i, ibyte, nchars, nbytes, cbytes;
      int c;

      for (i = nchars = nbytes = ibyte = 0;
	   i < SCHARS (seq);
	   ++i, ibyte += cbytes)
	{
	  if (STRING_MULTIBYTE (seq))
	    {
	      c = STRING_CHAR (SDATA (seq) + ibyte);
	      cbytes = CHAR_BYTES (c);
	    }
	  else
	    {
	      c = SREF (seq, i);
	      cbytes = 1;
	    }

	  if (c != XFIXNUM (elt))
	    {
	      ++nchars;
	      nbytes += cbytes;
	    }
	}

      if (nchars != SCHARS (seq))
	{
	  Lisp_Object tem;

	  tem = make_uninit_multibyte_string (nchars, nbytes);
	  if (!STRING_MULTIBYTE (seq))
	    STRING_SET_UNIBYTE (tem);

	  for (i = nchars = nbytes = ibyte = 0;
	       i < SCHARS (seq);
	       ++i, ibyte += cbytes)
	    {
	      if (STRING_MULTIBYTE (seq))
		{
		  c = STRING_CHAR (SDATA (seq) + ibyte);
		  cbytes = CHAR_BYTES (c);
		}
	      else
		{
		  c = SREF (seq, i);
		  cbytes = 1;
		}

	      if (c != XFIXNUM (elt))
		{
		  unsigned char *from = SDATA (seq) + ibyte;
		  unsigned char *to   = SDATA (tem) + nbytes;
		  ptrdiff_t n;

		  ++nchars;
		  nbytes += cbytes;

		  for (n = cbytes; n--; )
		    *to++ = *from++;
		}
	    }

	  seq = tem;
	}
    }
  else
    {
      Lisp_Object prev = Qnil, tail = seq;

      FOR_EACH_TAIL (tail)
	{
	  if (!NILP (Fequal (elt, XCAR (tail))))
	    {
	      if (NILP (prev))
		seq = XCDR (tail);
	      else
		Fsetcdr (prev, XCDR (tail));
	    }
	  else
	    prev = tail;
	}
      CHECK_LIST_END (tail, seq);
    }

  return seq;
}