def eval_U(formula, U):
    for f in formula:
        f.replace('U', str(U))
    if formula.size == 2:
        return [eval(formula[1]), eval(formula[2])]
    else:
        return eval(formula)

def calc_I(U):
    rng = {0:"0.5 * U", 2:"0", 4:"U - 2", 8:"4"}
    formula = []
    for i in rng.keys:
        if U < i:
            formula.push_back(rng.pop(i))
            return formula
        if U == i:
            formula.push_back(rng.pop(i))
    return "0"

t = []
s = []
for i in arange(-8.0, 8.0, 0.1):
    U = eval_U(calc_I(i))
    if U.size > 1:
        t.push_back(i)
        t.push_back(i)
        s.push_back(U[0])
        s.push_back(U[1])
    else:
        t.push_back(i)
        s.push_back(U[0])