Tirgul 2 Subjects of this tirgul: –Short discussion about standard output/error. –Short discussion about comparators. –Main subject: asymptotic analysis.

Default I/O Streams Class System has 3 default streams, available to every Java program: Input from the keyboard goes into the ‘standard input’. This is the data member System.in of type java.io.InputStream Output (usually to the terminal window) written through 2 streams: –‘Standard output’ – System.out of type java.io.PrintStream –‘Standard error’ – System.err of type java.io.PrintStream

The standard output and error are directed by the Operating System. By default - both to the terminal. The convention - standard error for error messages, standard output for regular output. In UNIX, the user can redirect to a file: –standard output by adding “ > my_out.txt ”. For example: java MyClass param1 > out.txt –both to the same file, by adding “ >& my_out.txt ” –You can’t redirect only the standard error, but redirecting to different files is possible (by outsmarting): ( java MyClass > out.txt) >& err.txt Default I/O Streams

Comparators You already know interface Comparable to compare objects. Problem: some classes may be compared by different criteria (participants may be compared by ID, or by name) Solution: create a Comparator for this specific class. –In the constructor specify the comparison criteria. –The method compare() receives two objects of this class and compares them (similar to CompareTo() ). Many Java API data structures can compare classes either by Comparable or by a Comparator.

Asymptotic Analysis Motivation: Suppose you want to evaluate two programs according to their run-time for inputs of size n. The first has run-time of: and the second has run-time of: Answer: For small inputs, it doesn’t matter, both programs will finish before you notice. What about (really) large inputs?

Big - O Definition : if there exists constants c>0 and n0 such that for all n>n0, In other words, g(n) bounds f(n) from above (for large n’s) up to a constant. Examples:

Big - Omega Definition : if there exists constants c>0 and n0 such that for all n>n0, In other words, g(n) bounds f(n) from below (for large n’s) up to a constant. Examples:

Big - Theta Definition : if: and In other words, g(n) is a tight estimate of f(n) (in asymptotic terms). Examples:

Example 1 (question 2-4-e. in Cormen) Claim: If (for n>n0) then Proof: Take. Thus for n>n0, Note: if there is no such for f(n) then the claim is not true: take for example f(n) = 1/n. For any constant c, take n>c and so: f(n) > (c/n)f(n) = c (f(n)) 2

Example 2 (question 2-4-d. in Cormen) Does imply Answer: No. For example, Then, so but, for any constant c: so:

Summations (from Cormen, ex , page 52) note how we “got rid” of the integer rounding The first term is n so the sum is also It’s an example how the largest item dominates the growth of the term in an exponential decrease/increase.

Summations (example 2) (Cormen, ex. 3.1-a., page 52) (r is a constant) : note that But it seems that we can do better by tightening our analysis a bit more...

Example 2 (Cont.) For an even n: For an odd n: Thus: so our upper bound was tight!

Recurrences What is the running time of the “Towers of Hanoi” ?? Reminder: We can express the running-time as a recurrence: h(n) = 2h(n-1) + 1 h(1) = 1 How do we solve this ? H(s,t,m,k) { if (k > 1) { H(s,m,t,k-1) moveDisk(s,t) H(m,t,s,k-1) } else { moveDisk(s,t) } }

Step 1: “guessing” the solution h(n) = 2h(n-1) + 1 = 2[2h(n-2)+1] + 1 = 4h(n-2) + 3 = 4[2h(n-3)+1] + 3 = 8h(n-3) + 7 When repeating k times we get: h(n)=2 k h(n-k) + (2 k - 1) Now take k=n-1. We’ll get: h(n) = 2 n-1 h(n-(n-1)) + 2 n = 2 n n-1 -1 =2 n - 1

Step 2: proving by induction If we guessed right, it will be easy to prove by induction that h(n)=2 n - 1 For n=1 : h(1)= 2-1=1 (and indeed h(1)=1) Suppose h(n-1) = 2 n Then, h(n) = 2h(n-1) + 1 = 2(2 n-1 - 1) + 1 = 2 n = 2 n -1 So we conclude that: h(n) = O(2 n )

Another Example And we get: T(n) = k T(n/k)+(k-1) For k=n we get T(n)= n T(1)+n-1=2n-1 Now proving by induction is very simple. Another way: “guess” right away T(n) =1). Conclusion: T(n) = O(n) T(n) = 2T(n/2) + 1 = 2 (2T(n/4) + 1) + 1 = 4T(n/4) + 3 = 4 (2T(n/8) + 1) + 3 = 8T(n/8) + 7 T(n) = 2 T(n/2) + 1 T(1) = 1

Common errors A precise guess is important for the induction to work: h(n) = 2 n - 1, and not just h(n)=O(2 n ) or h(n)=2 n. For example, the method from the previous slide: If we guess just T(n) c n (!!!) Notice that it is not correct to say “c n + 1 = O(n) so we proved T(n)=O(n)”. The induction step must be precise. If guessing T(n) <= cn then this is what you have to show.

Recursion Trees For each level we write the time added due to this level. In Hanoi, each recursive call adds one operation (plus the recursion). Thus the total is: The recursion tree for the “towers of Hanoi”: