Double negation is used for example within wiring_shift.c for shiftOut as below: void shiftOut(uint8_t dataPin, uint8_t clockPin, uint8_t bitOrder, uint8_t val) { uint8_t i; for (i = 0; i < 8; i++) { if (bitOrder == LSBFIRST) digitalWrite(dataPin, !!(val & (1 << i))); else digitalWrite(dataP…

why is double negation useful in shiftOut for example?

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