• Overview
• Recommended Problems
  • C level
  • B level

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 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. Naturally, project 2 is going on right now, so you probably don’t have the spare capacity to be thinking too deeply, but make sure to work through the problems in lecture and below once you have room to breathe again.

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 == 0){
        values[0] = 0;
        return
      }
      if(n == 1){
        values[1] = 1;
        return 1;
      }
      else{
        int val = values[n];
        if(n == 0){
          val = modified_fib(n-1) + modified_fib(n-2);
          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){
        for(int i  = 0; i < N; i++){
          System.out.println("Gul-great")
        }
        grigobreath(N * 1/2);
        grigobreath(N * 1/4);
        grigobreath(N * 1/4);
    }