Check a variable within a range with a binary variable I have a value, a, it can be any value from 0 to 1. In an integer linear program, how can I formulate a constraint that uses a binary variable, y, to determine whether a is within a range of 0 and 1 or not. 

Thanks
If I understand corectly, you want to formulate a system of linear constraints on a real variable $a$ and a $\{0,1\}$-variable $y$ 
such that the solutions are given by
$$[0,1]\times\{1\} \bigcup [1,\infty)\times \{0\}.$$

This is impossible.  No linear constraint can put an upper bound on $a$ for some values of $y$ without putting an upper bound on $a$ for all values of $y$.

You can, however, just split the problem into two pieces and solve each one separately.