[[Explain idioms of "computer arithmetic".]]

Sometimes, [Tcl]'s [expr]ession handling can be a little bit unexpected to newcomers:
 % expr 1/2
 0

This is because the above code asks for ''integer'' division (this is true in many other languages too) and that (conceptually) contains a round-down-to-int of the result. (The remainder operation, %, is often useful when you're working with integer division).
 % puts "1/2 is [expr 1/2] remainder [expr 1%2]"
 1/2 is 0 remainder 1

If you really want to end up with a half, force one of the arguments to the division operation to be a floating-point value:
 % expr double(1)/2
 0.5

See [expr problems with int] for more on integer division and how to avoid it.

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rdt : 2005-01-27 : This is a property of integers that can take on values of 0,1,2,... and therefore have no values between 0 and 1.  However:
 % expr 1.0 / 2
 0.5
The expected value.