!

expr boolean negation operator

returns truth (1) if the argument is a falsehood, and falsity (0) of the argument is true

For bitwise "not" use the ~ operator.

AMG: To convert a value to its canonical boolean representation (i.e. 0 or 1), you can use [expr {!!$val}].

DGP: The bool() function is the recommended way these days.

AMG: Ah, didn't know about that one. Well, !! can still be used in C and critcl. :^)