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WalterBright • yesterday at 10:15 PM • 4 replies • view on HN

NaNs are a very underappreciated feature of IEEE-754 floating point. In the D programming language, floats get default initialized to NaN, not to 0.0.

    double y = 0.0; // initialized to 0.0
    double x; // initialized to NaN
The discussion routinely comes up as "why not default initialize to 0.0?" The reason is a routine mistake in programming is forgetting to initialize a variable. With a floating point 0.0, one may never realize that the floating point calculation results are wrong. But with NaN, the result of a floating point computation will be NaN, which is unlikely to go unnoticed.

I don't know of any other programming language with this safety feature.

Also, the D `char` type is initialized to 0xFF, not 0, because Unicode says that 0xFF is an invalid character.

Replies

p1necone • yesterday at 10:58 PM

Just requiring explicit assignment before first use feels like the superior approach to automatic initialization, regardless of whether the automatic initialization is with 0 or with NaN.

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WalterBright • yesterday at 10:19 PM

Another crucial use of NaNs is if you have a sensor. If the sensor has failed, the sensed value should be transmitted as NaN, not 0, so the receiver knows the data is bad.

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anitil • yesterday at 11:07 PM

That's a very thoughtful decision, I always enjoy your updates on D

wpollock • yesterday at 11:02 PM

> ... Unicode says that 0xFF is an invalid character.

Not so. You may be thinking of UTF-8 encoding. 0xff is DEL in Unicode.

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