kindly explain.When n<=2 ,the answer is 5 ,but when n<=1,answer is 3. While n=n%5 so 9%5= 4. While 4 is bigger than both 1 and 2 | Sololearn: Learn to code for FREE!

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kindly explain.When n<=2 ,the answer is 5 ,but when n<=1,answer is 3. While n=n%5 so 9%5= 4. While 4 is bigger than both 1 and 2

def rec(n): n%=5 if n<=1: return n else: return rec(n-1)+rec(n-2) print(rec(9)) While 4 is bigger than both 1 and 2 Answer is 3 when it should be 5 in both cases.

6/25/2022 11:42:12 AM

Tafseer Ahmad

7 Answers

New Answer

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Just put n<=2 , instead of n<=1 and output is 5.

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This kind of number sequences is almost nonpredictable. Reminds me to Collatz sequence.

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When n<=2 the rec(n) function returns 1 Because 1%5 = 1 and 2%5 is 2 It all depends on the value of n%5, results will cycle from 0,1,1,2,3. You can never get 5 from the output of this function

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what you mean n <= 2 For input 5, you get 0 For input 4 , you get 3 according to your code..! What is your doubt now? What is purpose of program actually..?

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You are skipping rec(2) or below rec(4) calls rec(3) (*1) + rec(2) (*2) then (*1) => rec(3) calls rec(2) +rec(1) =>2+1 and (*2) => rec(2) returns 2 So finally (*1) + (*2) => 3+2=5 If it is n<=1 then recursion continues for rec(2) => rec(1) +rec(0) => 1+0=1 So rec(3) => rec(2)+rec(1)=>1+1=2 (*1) + (*2) => 2+1=3 only... .

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Oh I understand what you mean now. Its because the critera to exit the recusion has now changed Again it is dependant on the n%5 value Now when we exit recusion n can be either 1 or 2 Best thing to do here is write down each iteration for n. So lets do that: rec(2) n = 2 As n <= 2 we return n which is 2 So you can see if you changed the if statement to n<=2 The rec(2) => 2 whereas when you had n<=1 it would go to rec(2-1) + rec(2-2) so you’re output if n<=1 for rec(2) is 1 You can then do this with rec(9) and compare the two When you have if n<=1 Rec(9) n = 4 Rec(3) + rec(2) As we did earlier rec(2) is 1 Rec(3) n= 3 Rec(2) + rec(1) Rec(1) => 1 Rec(2)=> 1 So rec(9) => 1+1+1 which is 3 Now we change to if n<=2 rec(9) n=4 rec(3) + rec(2) Rec(2) => 2 Rec(3) n= 3 Rec(2) + rec(1) rec(2) => 2 Rec(1) => 1 So rec(9) => 1 +2+2 which is 5 This time you’ll find the output values cycle from 0,1,2,3,5

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I haven't been through recursions. Beginner so... I actually got this question in challenge.