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 java.io.BufferedReader:
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.
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