Question

I have a do-while loop that needs to log a message once (so it doesn’t clutter the log) each time its status (e.g. pass/fail) changes, but still has to do other things each time it goes through the loop. Using a simple boolean variable can basically tell you if you’ve already logged that message, which works once you’re in a known condition. However, if you want the message to be printed the first time in either case (pass/fail), you have to account for that. For example, if you default your condition to true, and it is, in fact, true the first time, it won’t log the ‘True’ message b/c it thinks it was already true (and vice-versa for i.c. false).

This seems like it would be a good place for a nullable boolean with i.c.=Null, but in languages where those aren’t present, what’s one to do?

The simplest solution I could think of would be to use an extra boolean variable like ‘firstTime = True’, but using that always bothers me as an elementary workaround when I feel like there should be a more delicate way to handle it. Another option is to use the breakout condition of the do-while as your initial condition for whatever variable you’re using as your conditional, but that can be confusing when someone reads int status = STATUS_QUIT, and it certainly requires more explanatory comments than bool firstTime = true. A third option would be to use an enum instead of a bool and have {firstTime, true, false} or something.

Are there other reasons for using one over the other, or are there better ways of doing this?

Code example with two options I came up with:

Using bool firsttime:

bool firstTime = true, infoFound = false;
do
{
    if (getInfo())
    {
        if (!infoFound)
        {
            // log it (ONCE)(important)
            infoFound = true;
        }
        // use info (every time)
    }
    else if (infoFound || firstTime)
    {
        // log it (ONCE)(important)
        infoFound = false;
        firstTime = false;
    }
// FYI, WaitForStatusUpdate is a blocking call...
} while (STATUS_QUIT != WaitForStatusUpdate());

Use the while loop ‘break-out condition’ as the initial condition for a check variable:
(status is updated at the end of the do-while, so the do section will not be executed ever again if status == breakOutCondition; we can use this to our advantage here and set status = breakOutContition initially – the first time through it will be breakOutCondition but any subsequent loop will be something else… Still not sure I like this as it’s kind of a hack…

bool infoFound = false;
int status = STATUS_QUIT;
do
{
    if (getInfo())
    {
        if (!infoFound)
        {
            // log it (ONCE)(important)
            infoFound = true;
        }
        // use info (every time)
    }
    else if (infoFound || firstTime)
    {
        // log it (ONCE)(important)
        infoFound = false;
    }
    status = WaitForStatusUpdate();
} while (STATUS_QUIT != status);

(I’m tagging this as c++ since that’s what I’m using, but this really could apply to any language with similar constructs)

Answer 1

Wouldn’t an enum be clearer?

enum State { Unknown, Pass, Fail };
State state = Unknown;
...
State newState = getInfo() ? Pass : Fail;
if (newState != state) { log();  state = newState; }