Monday, December 24, 2007

C#/Java/C++: Combining a variable assignment and evaluation

Pop Quiz! C#/Java/C++/Javascript/(probably others, too!) programmers, off the top of your head, what's the result of evaluating a variable assignment? In other words, to take a specific example, what is the output of this Java code snippet:

int n;
System.out.println(n = 50);
(Feel free to substitute in Console.Out.WriteLine (C#) or good old printf (C++) for the System.out.println in that snippet, depending on your language of choice.)

The answer is: 50. In Java and the other languages mentioned, the result of the evaluation of a variable assignment is the value being assigned.

I just came across this construct myself for the first time while I recently was doing a code review of a colleague's Java code. Somehow, prior to that code review, I had managed to go for over a decade of developing in these various languages without running across this!

The reason I hasn't run across this before may have to do with code readability. Doing two different things at once (in this case, a combined variable assignment and evaluation) often isn't very good for code readability (and therefore for ease of maintainability); in the general case, then, it probably makes the most sense for the variable assignment and evaluation to just be separated into two separate lines of code.

However, as my colleague's code demonstrated, combining an assignment with an evaluation can be useful when setting up a loop where the same statement is executed to assign a value to the loop variable both before the loop starts, and on each subsequent iteration of the loop. For example, here's a Java example of reading data from an input file a line at a time, using a

String inputLine;
while ((inputLine = bufferedReader.readLine()) != null)
    // Do something with inputLine...

The while statement in this case combines the assignment of the variable inputLine to the line of text read from the BufferedReader, with the check to stop looping when inputLine is null.

In the past, I've written the same logic in this manner:

String inputLine = bufferedReader.readLine();
while (inputLine != null)
    // Do something with inputLine...

    inputLine = bufferedReader.readLine();

I had always been kind of annoyed over the need to repeat the assignment (inputLine = bufferedReader.readLine()) in two different places.

For writing loops like this in the future, I'll have to think more about whether the gain in code brevity (and debatably, in elegance) from using the former approach (the combined assignment/evaluation in the while statement) is worth the potential cost for future maintainers in the readability of the code.

Wednesday, December 12, 2007

Java: Displaying negative percentage values in red and in parentheses

Recently, I was looking to write some code in Java for a financial application that would output percentage-format numbers, rounded to two decimal places, with negative numbers being displayed in parentheses and in red color. This was an internal application which only needed to work in the US/English locale.

For example, given the values 0.12345 and -0.12345, the formatted output needed to be, respectively:


Since for this application the output only needs to be formatted in the default locale, it can be done with one of the constructors of the Java DecimalFormat class. If the output will be displayed in a web browser (and only in a web browser), the HTML to display the red color for negative values can be embedded in the string passed to the NumberFormat constructors as well:

NumberFormat redNegativePercentTwoDecimalsFormat = new java.text.DecimalFormat(
  "0.00%;'<span style=\"color:#FF0000\">'(0.00%)'</span>'");

The created NumberFormat instance can then be used to output values in our desired format:


Which produces the desired output (when viewed in a web browser):


Monday, December 03, 2007

Finding album art quickly

Here's a quick series of steps that can be taken to find album art for an album (music CD) to paste into Windows Media Player or iTunes:
  • Open up Google Image Search. (Follow that link, or go to the main homepage and click the "Images" link there.)
  • From Google Image Search, search on the album name. (If the album name is something generic, try entering both the album and artist name in the search field.)
  • Click on one of the image results where the image size is a square and isn't tiny (e.g. 200x200).
  • If the image isn't immediately visible on the page that comes up (or even before the page finishes loading, if you don't want to wait), click the "See full-size image" link that Google puts at the top of the page.
  • Right-click the image and select the Copy option from the context menu that appears to put the image on your clipboard.
I found these steps useful when setting up my small music library in iTunes for the first time recently, having purchased my first iPod. These steps worked for even some of the non-mainstream CDs in my collection, such as Michigan Marching Band CDs and Black Mages CDs.