• Live QA
• Check-in Exercise
• Overview
• Recommended Problems
  • C level
  • B level

Live QA

Linked here.

Check-in Exercise

Linked here.

Overview

Runtime Analysis. Understanding the runtime of code involves deep thought. It amounts to asking: “How long does it take to do stuff?”, where stuff can be any conceivable computational process whatsoever. It simply cannot be done mechanically, at least for non-trivial problems. As an example, a pair of nested for loops does NOT mean $\Theta(N^2)$ runtime as we saw in lecture.

Cost Model. As an anchor for your thinking, recall the idea of a “cost model” from last lecture. Pick an operation and count them. You want the one whose count has the highest order of growth as a function of the input size.

Important Sums. This is not a math class so we’ll be a bit sloppy, but the two key sums that you should know are that:

• $1 + 2 + 3 + … + N \in \Theta(N^2)$
• $1 + 2 + 4 + 8 + … + N \in \Theta(N)$

Practice. The only way to learn this is through plenty of practice. Make sure to work through the problems in lecture and below when you have some time.

Recommended Problems

C level

1. Prove that $O(N + \frac{N}{2} + \frac{N}{4} +…. 2 + 1)= O(N)$ (hand wavy proof is okay as long as you gain the intuition)

2. What would the runtime of modified_fib be. Assume that values is an array of size n. If a value in an int array is not initialized to a number, it is automatically set to 0.

    public void modified_fib(int n, int[] values){
      if(n <= 1){
        values[n] = n;
        return n;
      }
      else{
        int val = values[n];
        if(val == 0){
          val = modified_fib(n-1, values) + modified_fib(n-2, values);
          values[n] = val;
        }
        return val;
      }
    }  
   

3. Prove to yourself that $\Theta(log_2(n)) = \Theta(log_3(n)) $

B level

1. Find the runtime of running print_fib with for arbitrary large n.

    public void print_fib(int n){
      for(int i = 0; i < n; i++i){
          System.out.println(fib(i));
      }
    }
   
    public int fib(int n){
      if(n <= 0){
        return 0;
      }
      elif(n == 1){
        return 1;
      }
      else{
        return fib(n-1) + fib(n-2);
      }
    }
   

2. Do problem 5 again, but change the body of the for loop in print_fib to be

    System.out.println(fib(n));
   

3. Find the runtime of this function

    public void melo(int N){
      for(int i = 0; i < N*N; i++){
        System.out.println("Gelo is fruit pudding");
      }
      for(int i = 0; i < N*N*N; i++){
        System.out.println("Zo Two the Warriors");
      }
    }
   

4. Find the runtime of this function

    public void grigobreath(int N){
        if(N==0){
          return;
        }
        for(int i  = 0; i < N; i++){
          System.out.println("Gul-great")
        }
        grigobreath(N * 1/2);
        grigobreath(N * 1/4);
        grigobreath(N * 1/4);
    }
   

5. Problem 8 from Spring 2018 midterm #2

6. Problem 4 from Spring 2017 midterm #2