J - 7 3

>.-

When assigned to a function: take the maximum between the negative and the number, using a hook so: (f g) y = y f g y

f=:>.-
f _4 5 _1 0
   4 5 1 0

The older version:

-^:(<-) 

Takes the inverse (the inverse is bigger than the number itself) times.

Would loosely translate to:

if -num > num then
    num= -num
end

J - 7 3

Max of number and inverse (3)

>.-

When assigned to a function: take the maximum between the negative and the number, using a hook so: (f g) y = y f g y

f=:>.-
f _4 5 _1 0
   4 5 1 0

root of the square (5 or 4)

]&.*: _4 5 _1 0
    4 5 1 0
NB. or if an expression is good enough:
%:*: _4 5 _1 0
    4 5 1 0

Negate if number smaller than its negative (7)

-^:(<-) 

Takes the inverse (the inverse is bigger than the number itself) times.

Would loosely translate to:

if -num > num then
    num= -num
end