November 2nd, 2018, 10:00 AM  #1 
Newbie Joined: Oct 2017 From: Rumba Posts: 29 Thanks: 0  Algorithms question https://prnt.sc/ldn10r Suppose there are two algorithms that solve the same problem: $\ \ \ \ \ $  $\ $ one algorithm performs $4n^2 + n + 300$ basic operations; $\ \ \ \ \ $  $\ $ another algorithm performs $101n  100$ basic operations, where $n$ is the input size, $n \ge 1$. (a) Which one would you prefer, assuming all other factors are equal? Hint: your choice may depend on the value of $n$. (b) Explain how the two algorithms can be combined so that the resulting algorithm works fast for all input sizes? How would I answer part (b)? Last edited by skipjack; November 2nd, 2018 at 10:33 AM. 
November 2nd, 2018, 10:50 AM  #2  
Senior Member Joined: May 2016 From: USA Posts: 1,192 Thanks: 489  Quote:
Do you see how that piece of information may help you to solve (b)?  
November 2nd, 2018, 10:58 AM  #3  
Newbie Joined: Oct 2017 From: Rumba Posts: 29 Thanks: 0  Quote:
Whereas (Red) second algorithm 101n  100 = O(n) time. Hence second algorithm is faster. In 0 to 100 and from 121 and beyond. So in order to get faster results in all kinds of input, we can combine these algorithms. T(n) = T1(n) or T2(n) , where T1(n) if n = 100 to 121 and T2(n) if n = 0 to 100 or if n > 121 Where T1(n) = 4n² + n + 100 and T2(n) = 101n  100. I'm not sure how to write as pseudocode with if else statements with inequality for (b) for e.g. if n < _ use algo 1 else use algo 2 Last edited by skipjack; November 2nd, 2018 at 07:29 PM.  
November 2nd, 2018, 11:56 AM  #4  
Senior Member Joined: May 2016 From: USA Posts: 1,192 Thanks: 489  Quote:
$\displaystyle \frac{100 \pm \sqrt{3600}}{8} = \frac{100 \pm 60}{8} = 5 \text { or } 20.$ Say n = 3. $4(3)^2 + 3 + 300 = 339. \text { BUT } 101(3)  100 = 203.$ So if n < 6, the red algorithm is as fast or faster than the blue algorithm because addend 300 is the dominant term. Say n = 10. $4(10)^2 + 10 + 300 = 710. \text { BUT } 101(10)  100 = 910.$ So if 5 < n < 21, the blue algorithm is as fast or faster than the red algorithm because the 101n is the dominant term. Say n = 21. $4(21)^2 + 21 + 300 = 2085. \text { BUT } 101(21)  100 = 2021.$ So if 19 < n, the red algorithm is as fast or faster than the blue algorithm because the 4n^2 is the dominant term. If n < 6, go to red. If n > 20, go to red. blue code. Go to print. red code. print. Last edited by skipjack; November 2nd, 2018 at 07:35 PM.  
November 2nd, 2018, 11:58 AM  #5 
Member Joined: Jan 2016 From: Athens, OH Posts: 91 Thanks: 47 
Actually, this illustrates a problem with completely relying on the order of an algorithm. For your given problem you want to know for what n is $$101n100\leq 4n^2+n+300$$ Simple algebra shows that the above inequality is true precisely for $$1\leq n\leq5\text{ or } 20\leq n$$ 
November 2nd, 2018, 12:38 PM  #6  
Newbie Joined: Oct 2017 From: Rumba Posts: 29 Thanks: 0  Quote:
Appreciate it Last edited by skipjack; November 2nd, 2018 at 07:36 PM.  
November 2nd, 2018, 07:57 PM  #7 
Global Moderator Joined: Dec 2006 Posts: 19,878 Thanks: 1834 
4n² + n + 300  (101n  100) = 4(n  5)(n  20), which exceeds 0 only for n < 5 or n > 20.


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