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Typo in accelerometer code?

I'm new to Arduino programming and didn't find this syntax on the Arduino page: http://arduino.cc/en/Reference/Boolean

Is the double !! a typo here or is there some value to using the bitwise 'not' twice or is this unique to Arduino?

tapped = !!(tilt & 0x20);

shaked = !!(tilt & 0x80);

Answer this question I have this problem too

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Short answer

It will return a 1 if the tapped register is set or a 0 if not.

Long answer

! will perform a boolean negation. If a value is not 0, turn it to a 0. If a value is a 0, turn it to a 1.

This code first does a bitwise comparison, like so:

Bxxxxxxxxxxxxxxxx (tilt) &

B00000001000000000 (0x20)

This filters out everything but the 10th bit. The result is 0 or 0x20 (512 in decimal).

Then we not it twice:

!512->0, !0->1 or

!0->1, !1->0.

So, the double negation (!!) code converts any result to a 1 or 0.

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This is actually an infrequently used C idiom that forces the result to be true (1) or false (0). It has nothing to do with Arduino. So in the case where (tilt & 0x20) is 0x20, the result is 1. It is a lot easier to read than say..

(tilt & 0x20) >> 5

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Kevin Hoyt will be eternally grateful.
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