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    <blockquote> <p>when a variable is assigned a number entered with one decimal place, such as 4.2, the float or double used in the comparison actually may have a value such as 4.1999998092651367</p> </blockquote> <p>Not <em>may</em>. <strong>will</strong>. To be specific:</p> <pre><code>float f = 4.2; // f is exactly 4.19999980926513671875 double d = 4.2; // d is exactly 4.20000000000000017763568394002504646778106689453125 </code></pre> <p>The problem comes when you write something like:</p> <pre><code>float f = 4.2; if (f &gt;= 4.2) { // this block of code is not executed. } </code></pre> <p><code>f</code> is exactly <code>4.19999980926513671875</code>, but you're comparing it to the <strong>double-precision literal</strong> "4.2", which has the value <code>4.20000000000000017763568394002504646778106689453125</code>, so the comparison fails. If instead you compare against the single precision literal "4.2f":</p> <pre><code>float f = 4.2; if (f &gt;= 4.2f) { // this block of code is exectued. } </code></pre> <p>the comparison succeeds, because the values are exactly equal. Floating-point is complicated, but it is entirely deterministic; one of the simplest things you can do to make it more intuitive is to not mix precisions. If you're working with <code>float</code>, make sure all of your literals are suffixed with <code>f</code> to make them single precision, too.</p> <p>(This can also improve performance, but that's not the reason to do it; the reason to do it is because it will make your code more correct).</p>
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    1. USStephen Canon
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    1. COThanks for setting me straight. Now the code works as expected with comparisons using the "f" suffix on integers. As a follow-on, is this sort of issue with floating-point arithmetic a "feature" or an unavoidable problem? Seems like something that easily could introduce computational errors from code that seems logical and compiles without a problem.
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    2. CO@user183804: I wouldn't call it a feature or an unavoidable problem. More of a "that's the nature of binary floating point, learn about it or be doomed to trip over this sort of thing". It's not unique to floating-point by any stretch of the imagination; people are thrown off by integer overflow and other aspects of integer arithmetic all the time. Arithmetic on the computer is not the same as the arithmetic we learn in school, and that basically just a fact that we as programmers need to learn to work with.
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    3. COThanks again. I'll be going over all my code with this new knowledge in mind. You've been extremely helpful, since my coming iPhone app deals with medical calculations, so accuracy is paramount.
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