CODEWITHSUNDEEP
javaarraysrecursion
user3242607
user3242607

Reputation: 219

How to find out the odd integers in an array using a recursive method?

I am trying to write a method that finds how many odd numbers are between first position and last position. The method accepts an array, and then two ints for the low and high position. This method needs to be made recursively. Here is what I have so far. Here is the method call and the int array. I'm getting an output of 1 but the answer should be 2.

int array [] = {5, 2, 5, 6, 3, 1};
int n = countOddsInRange(array, 1, 4)

public static int countOddsInRange(int [] a, int first, int last)
{
    int count = 0;
    if(first <= last)
    {
        countOddsInRange(a, a[first + 1], a[last]);
        if(a[first] % 2 == 0)
        {
            count++;
        }
    }
    return count;   
}

Upvotes: 0

Views: 5736

Answers (4)

java_geek
java_geek

Reputation: 18035

public class CountOddsInRange {
  public static void main(String[] args) {
    int array[] = {5, 2, 5, 6, 3, 1};
    int n = countOddsInRange(array, 0, array.length - 1, 0);
    System.out.println("No of odds is " + n);
}

public static int countOddsInRange(int[] a, int first, int last, int count) {
  if (first <= last) {
    if (a[first] % 2 != 0) {
      count++;
    }
    return countOddsInRange(a, first + 1, last, count);
  }
  return count;
}

}

Upvotes: 0

Eran
Eran

Reputation: 393856

You have some bugs in your code :

  1. You are counting even numbers, not odd. Change your condition to if(a[first] % 2 != 0)
  2. The recursive call should get indices of the array, not the values in those locations.
  3. You should add the result of the recursive call to the total : count+=countOddsInRange(a, first + 1, last)

To summarize :

public static int countOddsInRange(int [] a, int first, int last)
{
    int count = 0;
    if(first <= last)
    {
        count+=countOddsInRange(a, first + 1, last);
        if(a[first] % 2 != 0)
        {
            count++;
        }
    }
    return count;   
}

Upvotes: 5

java_geek
java_geek

Reputation: 18035

public class CountOddsInRange {
    public static void main(String[] args) {
        int array[] = {5, 2, 5, 6, 3, 1};
        int n = countOddsInRange(array, 0, array.length - 1);
        System.out.println("No of odds is " + n);
    }

    public static int countOddsInRange(int [] a, int first, int last)
    {
        int count = 0;
        if(first <= last)
        {
            count+=countOddsInRange(a, first + 1, last);
            if(a[first] % 2 != 0)
            {
                count++;
            }
        }
        return count;   
   }
}

Upvotes: 0

Arthur Va&#239;sse
Arthur Va&#239;sse

Reputation: 1571

Your function should looks like :

public static int countOddNumber(int [] a, int first, int last){
    return countOddNumber(a, first, last, 0);
}

public static int countOddNumber(int [] a, int first, int last, int count){
    //in recursive function start with termination test.
    if (first == last){
        return count + isOdd(a[first]);
    }
    else{
        return countOddNumber(a, first+1, last, count) + isOdd(a[first]);
    }
}

public static int isOdd(int number){
    if(number%2 == 0){return 1;}
    else{return 0}
}

You should also add a test to check if first is lesser than last to avoid infinite loop ;)

PS : filter element whose mod(element,2) = 0 and then get the size of the collection. That method use functional style too, and use new Java8 feature. And is probably much more faster.

Upvotes: 0

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