Asymptotes: Why?

1. Asymptotes: Why? How to describe an algorithm’s running time? (or space, …) How does the running time depend on the input? T(x) = running time for instance x Problem: Impractical to use, e.g., “15 steps to sort [3 9 1 7], 13 steps to sort [1 2 0 3 9], …” Need to abstract away from the individual instances.

2. Asymptotes: Why? Standard solution: Abstract based on size of input. How does the running time depend on the input? T(n) = running time for instances of size n Problem: Time also depends on other factors. E.g., on sortedness of array.

3. Asymptotes: Why? Most common. Default. Determining the input probability distribution can be difficult. Solution: Provide a bound over these instances. Worst case T(n) = max{T(x) | x is an instance of size n} Best case T(n) = min{T(x) | x is an instance of size n} Average case T(n) = |x|=n Pr{x}  T(x)

4. Asymptotes: Why? What’s confusing about this notation? Worst case T(n) = max{T(x) | x is an instance of size n} Best case T(n) = min{T(x) | x is an instance of size n} Average case T(n) = |x|=n Pr{x}  T(x) Two different kinds of functions: T(instance) T(size of instance) Won’t use T(instance) notation again, so can ignore.

5. Asymptotes: Why? Problem: T(n) = 3n2 + 14n + 27 Too much detail: constants may reflect implementation details & lower terms are insignificant. Solution: Ignore the constants & low-order terms.(Omitted details still important pragmatically.) 3n2 > 14n+17  “large enough” n

6. Upper Bounds Creating an algorithm proves we can solve the problem within a given bound. But another algorithm might be faster. E.g., sorting an array. Insertion sort  O(n2) What are example algorithms for O(1), O(log n), O(n), O(n log n), O(n2), O(n3), O(2n)?

7. Lower Bounds Sometimes can prove that we cannot compute something without a sufficient amount of time. That doesn't necessarily mean we know how to compute it in this lower bound. E.g., sorting an array. # comparisons needed in worst case  (n log n) Shown in COMP 482.

8. Definitions: O,  T(n)  O(g(n))   constants C,k > 0 such that  nk, T(n)  Cg(n) T(n) (g’(n))   constants C’,k’ > 0 such that  nk’, T(n)  C’g’(n) Cg(n) T(n) C’g’(n) k k’

9. Examples: O,  2n+13  O( ? ) Also, O(n2), O(5n), … Can always weaken the bound. O(n) 2n+13  ( ? ) (n), also (log n), (1), … 2n O(n) ? (n) ? Given a C, 2n  Cn, for all but small n. (n), not O(n). nlog n O(n5) ? No. Given a C, log n  C5, for all large enough n. Thus, (n5).

10. Definitions:  T(n)  (g(n))  T(n)  O(g(n)) and T(n)  (g(n)) Ideally, find algorithms that are asymptotically as good as possible.

11. Notation O(), (), () are sets of functions. But common to abuse notation, writing T(n) = O(…) instead of T(n)  O(…) as well as T(n) = f(n) + O(…)