count-words-example command described in the preceding
section has two bugs, or rather, one bug with two manifestations.
First, if you mark a region containing only whitespace in the middle
of some text, the
count-words-example command tells you that the
region contains one word! Second, if you mark a region containing
only whitespace at the end of the buffer or the accessible portion of
a narrowed buffer, the command displays an error message that looks
Search failed: "\\w+\\W*"
If you are reading this in Info in GNU Emacs, you can test for these bugs yourself.
First, evaluate the function in the usual manner to install it.
If you wish, you can also install this keybinding by evaluating it:
(global-set-key "\C-c=" 'count-words-example)
To conduct the first test, set mark and point to the beginning and end of the following line and then type C-c = (or M-x count-words-example if you have not bound C-c =):
one two three
Emacs will tell you, correctly, that the region has three words.
Repeat the test, but place mark at the beginning of the line and place point just before the word ‘one’. Again type the command C-c = (or M-x count-words-example). Emacs should tell you that the region has no words, since it is composed only of the whitespace at the beginning of the line. But instead Emacs tells you that the region has one word!
For the third test, copy the sample line to the end of the *scratch* buffer and then type several spaces at the end of the line. Place mark right after the word ‘three’ and point at the end of line. (The end of the line will be the end of the buffer.) Type C-c = (or M-x count-words-example) as you did before. Again, Emacs should tell you that the region has no words, since it is composed only of the whitespace at the end of the line. Instead, Emacs displays an error message saying ‘Search failed’.
The two bugs stem from the same problem.
Consider the first manifestation of the bug, in which the command
tells you that the whitespace at the beginning of the line contains
one word. What happens is this: The
command moves point to the beginning of the region. The
tests whether the value of point is smaller than the value of
end, which it is. Consequently, the regular expression search
looks for and finds the first word. It leaves point after the word.
count is set to one. The
while loop repeats; but this
time the value of point is larger than the value of
loop is exited; and the function displays a message saying the number
of words in the region is one. In brief, the regular expression
search looks for and finds the word even though it is outside
the marked region.
In the second manifestation of the bug, the region is whitespace at
the end of the buffer. Emacs says ‘Search failed’. What happens
is that the true-or-false-test in the
while loop tests true, so
the search expression is executed. But since there are no more words
in the buffer, the search fails.
In both manifestations of the bug, the search extends or attempts to extend outside of the region.
The solution is to limit the search to the region—this is a fairly simple action, but as you may have come to expect, it is not quite as simple as you might think.
As we have seen, the
re-search-forward function takes a search
pattern as its first argument. But in addition to this first,
mandatory argument, it accepts three optional arguments. The optional
second argument bounds the search. The optional third argument, if
t, causes the function to return
nil rather than signal
an error if the search fails. The optional fourth argument is a
repeat count. (In Emacs, you can see a function's documentation by
typing C-h f, the name of the function, and then <RET>.)
count-words-example definition, the value of the end of
the region is held by the variable
end which is passed as an
argument to the function. Thus, we can add
end as an argument
to the regular expression search expression:
(re-search-forward "\\w+\\W*" end)
However, if you make only this change to the
definition and then test the new version of the definition on a
stretch of whitespace, you will receive an error message saying
What happens is this: the search is limited to the region, and fails as you expect because there are no word-constituent characters in the region. Since it fails, we receive an error message. But we do not want to receive an error message in this case; we want to receive the message that "The region does NOT have any words."
The solution to this problem is to provide
with a third argument of
t, which causes the function to return
nil rather than signal an error if the search fails.
However, if you make this change and try it, you will see the message
“Counting words in region ... ” and ... you will keep on seeing
that message ..., until you type C-g (
Here is what happens: the search is limited to the region, as before,
and it fails because there are no word-constituent characters in the
region, as expected. Consequently, the
nil. It does nothing else. In particular,
it does not move point, which it does as a side effect if it finds the
search target. After the
re-search-forward expression returns
nil, the next expression in the
while loop is evaluated.
This expression increments the count. Then the loop repeats. The
true-or-false-test tests true because the value of point is still less
than the value of end, since the
did not move point. ... and the cycle repeats ...
count-words-example definition requires yet another
modification, to cause the true-or-false-test of the
to test false if the search fails. Put another way, there are two
conditions that must be satisfied in the true-or-false-test before the
word count variable is incremented: point must still be within the
region and the search expression must have found a word to count.
Since both the first condition and the second condition must be true
together, the two expressions, the region test and the search
expression, can be joined with an
and special form and embedded in
while loop as the true-or-false-test, like this:
(and (< (point) end) (re-search-forward "\\w+\\W*" end t))
re-search-forward expression returns
t if the search
succeeds and as a side effect moves point. Consequently, as words are
found, point is moved through the region. When the search expression
fails to find another word, or when point reaches the end of the
region, the true-or-false-test tests false, the
exits, and the
count-words-example function displays one or
other of its messages.
After incorporating these final changes, the
works without bugs (or at least, without bugs that I have found!).
Here is what it looks like:
;;; Final version:
while(defun count-words-example (beginning end) "Print number of words in the region." (interactive "r") (message "Counting words in region ... ") ;;; 1. Set up appropriate conditions. (save-excursion (let ((count 0)) (goto-char beginning) ;;; 2. Run the while loop. (while (and (< (point) end) (re-search-forward "\\w+\\W*" end t)) (setq count (1+ count))) ;;; 3. Send a message to the user. (cond ((zerop count) (message "The region does NOT have any words.")) ((= 1 count) (message "The region has 1 word.")) (t (message "The region has %d words." count))))))