def f(n):
    i = 1
    while i < n:
        i *= 2
        print(i)

2: c_1
3, 4, 5: c_2

Line 2:  run 1 time
Lines 3, 4, 5:  Run lg(n) times, since i doubles each time.

c_1 + c_2 lg(n)

Theta( log(n) )