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PODetect first pass through a do-while
 

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  1. PODetect first pass through a do-while
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    <p>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 <code>true</code>, 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).</p> <p>This seems like it would be a good place for a <a href="http://en.wikipedia.org/wiki/Nullable" rel="nofollow">nullable</a> boolean with i.c.=Null, but in languages where those aren't present, what's one to do?</p> <p>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 <code>int status = STATUS_QUIT</code>, and it certainly requires more explanatory comments than <code>bool firstTime = true</code>. A third option would be to use an enum instead of a bool and have {firstTime, true, false} or something.</p> <p>Are there other reasons for using one over the other, or are there better ways of doing this?</p> <p>Code example with two options I came up with:</p> <p><strong>Using <code>bool firsttime</code>:</strong></p> <pre><code>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()); </code></pre> <p><strong>Use the <code>while</code> loop 'break-out condition' as the initial condition for a check variable:</strong><br> (<code>status</code> is updated at the end of the do-while, so the <code>do</code> section will not be executed ever again if <code>status == breakOutCondition</code>; we <em>can</em> use this to our advantage here and set <code>status = breakOutContition</code> initially - the first time through it will be <code>breakOutCondition</code> but any subsequent loop <em>will be</em> something else... Still not sure I like this as it's kind of a hack...</p> <pre><code>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); </code></pre> <p>(I'm tagging this as c++ since that's what I'm using, but this really could apply to any language with similar constructs)</p>
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    1. USjohnny
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    1. COIf you are using C++, you should be using `bool` and `true/false` rather than `BOOL` and `TRUE/FALSE`.
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      1. USDavid Rodríguez - dribeas
    2. CO@DavidRodríguez-dribeas: yeah, we're using windef.h which has `#define FALSE 0`, but I'll switch to `false` here so others can compile and such without adding that... good catch :)
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    3. COstrangely enough (or perhaps not?), it seems windef also defines `BOOL` as `int`, so I could have just used an integer (or enum) with equivalent results... but again, want to stay away from windows-specific for this question
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