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void setup() {
  Serial.begin(9600);
  while (!Serial) {
    ;  // wait for serial port to connect. Needed for native USB port only
  }
  int x = 42;
  Serial.print("The absolute value of ");
  Serial.print(x);
  Serial.print(" is ");
  Serial.println(abs(x));
  x = -42;
  Serial.print("The absolute value of ");
  Serial.print(x);
  Serial.print(" is ");
  Serial.println(abs(x));
}

void loop() {
}

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abs(a++); // avoid this - yields incorrect results

// use this instead:
abs(a);
a++;  // keep other math outside the function