Re: Iterating over an array style question

Eric Sosman <esosman@ieee-dot-org.invalid>
Wed, 24 Nov 2010 20:37:18 -0500
On 11/24/2010 2:20 PM, Alex Mentis wrote:

Eric Sosman wrote:

     BufferedReader rdr = ...;
    for (String line; (line = rdr.readLine()) != null; ) // BZZZT!

    Pattern patt = ...;
    Matcher match = patt.matcher(...);
    if (match.matches()) // BZZZT!

... and so on, and so on. Sounds like a silly school.

As for your code above, why not use:

BufferedReader rdr = ...;

String line = rdr.readLine();
while (line != null)
    line = rdr.readLine();

That seems much clearer to me.

     It also (1) gives `line' a wider scope than the original,
and (2) writes the same piece of code twice, giving subsequent
programmers a chance to change one without changing the other
to match. (1) could be addressed by introducing another block:

        String line = rdr.readLine();
        while (line != null) {
            line = rdr.readLine();

.... or by rearranging the duplication:

    for (String line = rdr.readLine(); line != null;
         line = rdr.readLine()) {

.... but I see no way to dismiss (2) -- the more serious objection --
without relying on side-effects. (Or on throwing an exception to
break out of a "normal" control flow, which is also Bad.)

Your second example doesn't appear to have a side-effect, so I'm not
sure what your point is there. match.matches() simply returns a
Boolean, which is what is expected in the condition part of the if
statement. It's not checking a condition and also changing some
variable before or after it's done.

     Quoth the Javadoc, "More information about a successful match can
be obtained by querying the state of the matcher." The updating of that
"state" is a side-effect of match.matches(). If you don't think so,
pray explain why all of match.end(),, and so on behave
differently before match.matches() than afterward.

Eric Sosman

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