Find the absolute value of a number without built-in functions
J, 2 bytes

**


Usage:

   f =: **
   f 9.3
9.3
   f -9.3
9.3


Explanation:

This uses the * verb in both its monadic and dyadic forms. The monadic form returns -1 if it's given a negative number, 0 if it's given 0 and 1 if it's given a positive number. The dyadic form is just plain old multiplication. Putting them in a function literal turns them into a hook which gets evaluated like this: y * (*y).