TOPIC: C
18th October 2007
The time-honoured syntax for a for loop in a UNIX script is what you see below, and that is what works with the default shell in Sun's Solaris UNIX operating system, ksh88.
for i in 1 2 3 4 5 6 7 8 9 10
do
if [[ -d dir$i ]]
then
:
else
mkdir dir$i
fi
done
However, there is a much nicer syntax supported since the advent of ksh93. It follows C language conventions found in all sorts of places like Java, Perl, PHP and so on. Here is an example:
for (( i=1; i<11; i++ ))
do
if [[ -d dir$i ]]
then
:
else
mkdir dir$i
fi
done
20th June 2007
This is a SAS error that can be seen from time to time:
WARNING: The quoted string currently being processed has become more than 262 characters long. You may have unbalanced quotation marks.
In the days before SAS version 8, this was something that needed to be immediately corrected. In these days of SAS character variables extending beyond 200 characters in length, it becomes a potential millstone around a SAS programmer's neck. If you run a piece of code like this:
data _null_;
x="aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
run;
What you get back is the warning message at the heart of the matter. While the code is legitimate and works fine, the spurious error is returned because SAS hasn't found a closing quote by the required position and the 262-character limit is a hard constraint that cannot be extended. There is another way, though: the new QUOTELENMAX option in SAS9. Setting it as follows removes the messages in most situations (yes, I did find one where it didn't play ball):
options noquotelenmax;
This does, however, beg the question as to how you check for unbalanced quotes in SAS logs these days; clearly, looking for a closing quote is an outmoded approach. Thanks to code highlighting, it is far easier to pick them out before the code gets submitted. The other question that arises is why you would cause this to happen anyway, but there are occasions where you assign the value of a macro variable to a data set one and the string is longer than the limit set by SAS. Here's some example code:
data _null_;
length y $400;
y=repeat("f",400);
call symput("y",y)
run;
data _null_;
x="&y";
run;
My own weakness is where I use PROC SQL to combine strings into a macro variable, a lazy man's method of combining all distinct values for a variable into a delimited list like this:
proc sql noprint;
select distinct compress(string_var) into :vals separated by " " from dataset;
quit;
Of course, creating a long delimited string using the CATX (new to SAS9) function avoids the whole situation and there are other means, but there may be occasions, like the use of system macro variables, where it is unavoidable and NOQUOTELENMAX makes a much better impression when these arise.
9th June 2007
I am in the process of further linking the content of hillwalking blog into the fabric of other parts of my website. To date, this has included adding a list of all the posts to the site map and a dossier of the latest entries to the site's welcome page. One thing that started to crop up were a series of warnings of the form:
Cannot modify header information - headers already sent by...
The cause of this was my calling a PHP script that was part of my hillwalking blog installation, and this caused Bad Behaviour to be invoked. The result was that an HTTP header was being sent to create a cookie after one already had been sent to display a web page. A spot of conditional coding and the use of an extra flag resolved the problem.
Apparently, there is another surefire way of getting the same result: whitespace before or after the <?php ... ?> tag in an included script. The cookie issue is one that I can understand, but it does seem strange that an attempt is made to send HTTP header information when the latter arises. It causes loads of questions, though...
8th June 2007
When you open up a SAS macro catalogue so that its entries for use by other programs, it has a major impact on the ability to change the library reference used to access the catalogue after it has apparently been unlocked.
options mstored sasmstore=bld_v001;
Using the line above will open the catalogue for reading, but there is no way to close it to change the library reference or unassign it until the SAS session is shut down. Even this line will not do the trick:
options nomstored sasmstore='';
What it means in practice is that if you have a standard macro setting up access to a number of standard macro libraries, then that setup macro needs to check for any library references used and not try to reassign them, causing errors in the process.
3rd April 2007
One of the nice things about eBooks is the saving that you can make on buying one instead of the dead tree edition. And if you get one from Apress, it is the full article that you get, and they keep it available so that you can download another version if you need it. You can also print the thing off if you want too, yet a laser printer producing double-sided prints is an asset if you don’t want your space invaded by a hoard of lever arch binders. Having a copious supply of inexpensive toner helps too, as does cheap paper. Otherwise, you could spend your savings on printing the thing yourself.
The ever pervasive Safari does things a little differently from the likes of Apress. Mind you, the emphasis there is on the library aspect of the operation and not eBook selling. The result is that you can only ever download chapters, so no index or overall table of contents. You still can buy all the chapters for a particular book, though some publishers don’t seem to allow this for some reason, but finding anything in there after you have had a read becomes an issue, especially when it’s the hard copy that you are using. Take yesterday, for instance, when trying to relocate the formatting parameters for the UNIX date function. Though I eventually found them in the chapters of UNIX in a Nutshell that I have downloaded and printed off, I spent rather longer looking in Learning the Korn Shell than I should have done. Even if I know that you can search in the PDF’s themselves, that is more laborious when there are a number of files to search rather than just the one. I suppose that the likes of O’Reilly prefers you to buy paper copies of its books for more extensive use, and they have a point, even if having the electronic version all in one file does make life so much easier.
12th February 2007
Over the weekend, I have been updating the theme on my other blog, HennessyBlog. It has been a task that projected me onto a learning curve with the WordPress 2.1 codebase. Thus, I have collected what I encountered, so I know that it’s out there on the web for you (and I) to use and peruse. It took some digging to get to know some of what you find below. Since any function used to power WordPress takes some finding, I need to find one place on the web where the code for WordPress is more fully documented. The sites presenting tutorials on how to use WordPress are more often than not geared towards non-techies rather than code cutters like myself. Then again, they might be waiting for someone to do it for them…
The changes made are as follows:
Tweaks to the interface
These are subtle, with the addition of navigation controls to the sidebar and the change in location of the post metadata being the most obvious enhancements. “Decoration” with solid and dashed lines (using CSS border attributes rather than the deprecated hr tagset) and standards compliance links.
Standards compliance
Adding standards compliance links does mean that you’d better check that all is in order; it was then that I discovered that there was work to be done. There is an issue with the WordPress wpautop function (it lives in the formatting.php file) in that it sometimes doesn’t add closing tags. Finding out that it was this function that is implicated took a trip to the WordPress.org website; while a good rummage in the wp-includes folder does a lot, it can’t achieve everything.
Like many things in the WordPress code, the wpautop function isn’t half buried. The the_content function (see template-functions-post.php) used to output blog entries calls the get_content function (also in template-functions-post.php) to extract the data from MySQL. The add_filter function (in plugin.php) associates the wpautop function and others with the get_the_content function to add the p tags to the output.
To return to the non-ideal behaviour that caused me to start out on the above quest, an example is where you have an img tag enclosed by div tags. The required substitution involves the use of regular expressions that work most of the time but get confused here. So adding a hack to the wpautop function was needed to change the code so that the p end tag got inserted. I’ll be keeping an eye out for any more scenarios like this that slip through the net and for any side effects. Otherwise, compliance is just making sure that all those img tags have their alt attributes completed.
Tweaks to navigation code
Most of my time has been spent on tweaking of the PHP code supporting the navigation. Because different functions were being called in different places, I wanted to harmonise things. To accomplish this, I created new functions in the functions.php for my theme and needed to resolve a number of issues along the way. Not least among these were regular expressions used for subsetting with the preg_match function; these were not Perl-compliant to my eyes, as would be implied by the choice of function. Now that I have found that PCRE’s in PHP use a more pragmatic syntax, there appeared to be issues with the expressions that were being used. These seemed to behave OK in their native environment but fell out of favour within the environs of my theme. Being acquainted with Perl, I went for a more familiar expression style and the issue has been resolved.
Along the way, I broke the RSS feed. This was on my off-line test blog so no one, apart from myself, that is, would have noticed. After a bit of searching, I realised that some stray white-space from the end of a PHP file (wp-config.php being a favourite culprit), after the PHP end tag in the script file as it happens, was finding its way into the feed and causing things to fall over. Feed readers don’t take too kindly to the idea of the XML declaration not making an appearance on the first line of the file. Some confusion was caused by the refusal of Firefox to refresh things as it should before I realised that a forced refresh of the feed display was needed. Sometimes, it takes a while for an addled brain to think of these kinds of things.