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Thread: problem with understanding this recursion

  1. #1
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    Default problem with understanding this recursion

    public static boolean goo(int n){
    If(n<10){
    Return n;
    }
    Int k=goo((n/100)*10+(n%10));
    Return (k*10 +(n%100)/10);
     
    }
    I am still a beginner to rec concept.
    We need to check for n=529.
    Why k is 95?


  2. #2
    Junior Member fireredlink5000's Avatar
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    Default Re: problem with understanding this recursion

    Hey eyalfish,

    I'm guessing the code should look like this:

    	public static int goo(int n){
    		if(n < 10){
    			return n;
    		}
    		int k = goo((n/100)*10+(n%10));
    		return (k*10 +(n%100)/10);
    	}

    When I run the code, where n = 529, I get the value 952.

    So, when you are doing these recursion problems or any problems, it nice sometimes to add print statements. It's not that efficient, but it will do the job.

    If we change the code to this:

    	public static int goo(int n){
    		System.out.println("n == " + n);
    		// Base Case
    		if(n < 10){
    			System.out.print("Returning: ");
    			System.out.println("n == " + n);
    			return n;
    		}
    		//Inductive Case
    		int k = goo((n/100)*10+(n%10));
    		System.out.println("k == " + k);
    		int returnValue = (k*10 +(n%100)/10);
    		System.out.println("returnValue == "+ returnValue);
    		return returnValue;
    	}

    This is our output:
    n == 529
    n == 59
    n == 9
    Returning: n == 9
    k == 9
    returnValue == 95
    k == 95
    returnValue == 952
    952

    We can see here that the first output is n == 529. Next we can see the second output is "n == 59". Since the second output is "n == 59", it must be the case that the line:

    int k = goo((n/100)*10+(n%10));

    must have ran. You should be able to figure out the rest by reading the output. Just go through the code and the output one line at a time.

  3. The Following User Says Thank You to fireredlink5000 For This Useful Post:

    GregBrannon (May 22nd, 2014)

  4. #3
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    Default Re: problem with understanding this recursion

    Thanks! Btw i didnt write the code i ordered to analayzed it.the value is indeed 952

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    Default Re: problem with understanding this recursion

    i didnt write the code i ordered to analayzed it
    Now fireredlink5000 has shown you a way to do the analysis on your on. If you want to learn to program, play with it until it makes sense as demonstrated above.

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