abs(x)

Computes the absolute value of a number.

x: the number

x: if x is greater than or equal to 0.

-x: if x is less than 0.

Because of the way the abs() function is implemented, avoid using other functions inside the brackets, it may lead to incorrect results.

abs(a++);   // avoid this - yields incorrect results
 
a++;          // use this instead -
abs(a);       // keep other math outside the function

Source: arduino.cc