Question

PROBLEM: Write a recursive method named SumEvens that takes an integer X and returns the sum of even digits in X. Please comment on every line of code.

EXISTING CODE:

int numTwos(int n)

{

int right = n % 10;

int remain = n / 10;

if(n==0)

{

return 0 ;

}

else if(right ==2)

{

//rightmost digit is 2

return 1 + numTwos(remain);

else

{

return 0 + numTwos(remain);

}

}

int SumDigits(int X) {
    if (X == 0) // base case
        return 0;
    else    // recursive case
        return X % 10 + SumDigits(X / 10); //make recursive call
}

int NumDigits(int X){
  if(X<10){
    return 1;
  }
  else{
    return 1+NumDigits(X/10);
  }
}

int LargestDigit(int X) {
    if (X == 0) // base case
        return 0;
    else    // recursive case
    {
        int d = LargestDigit(X / 10); //make recursive call
        if (X%10 > d) {
            d = X%10;
        }
        return d;
    }
}

int LogY(int X, int Y) {
    if (X <= 1) // base case
        return 0;
    else    // recursive case
        return 1 + LogY(X/Y, Y);    //make recursive call
}

int Log2(int X) {
    if (X <= 1) // base case
        return 0;
    else    // recursive case
        return 1 + Log2(X/2);    //make recursive call
}

int Division(int X, int Y) {
    if (X < Y) // base case
        return 0;
    else    // recursive case
        return 1 + Division(X-Y, Y);    //make recursive call
}

int Exponentiation(int X, int Y) {
    if (Y == 0) // base case
        return 1;
    else    // recursive case
        return X * Exponentiation(X, Y - 1);    //make recursive call
}

int Multiply(int X, int Y){
    if(Y == 0){
        return 0;
    }
    else{
        return X + Multiply(X, Y-1);
    }
}

int MultiplyRange(int X, int Y){
    if(Y == 0){
        return 0; 
    }
    else{
        return X + MultiplyRange(X, Y-1);
    }
}

int Subtraction(int X, int Y) {
    if (Y == 0) // base case
        return X;
    else    // recursive case
        return Subtraction(X, Y - 1) - 1;    //make recursive call
}

int Addition(int X, int Y) {
    if (Y == 0) // base case
        return X;
    else    // recursive case
        return 1 + Addition(X, Y - 1);    //make recursive call
}

int AddToN(int n) {
    if (n == 0) {   // base case
        return 0;
    } else { // recursive case
        return n + AddToN(n-1); //make recursive call
    }
}

Answer 1

#include <iostream>

using namespace std;

int SumEvens(int X) {
    if (X == 0) // base case
        return 0;
    else    // recursive case
    {
        int sum = SumEvens(X / 10); //make recursive call
        if ((X%10) % 2 == 0)
            sum += X%10;
        return sum;
    }
}

int main() {
    cout << SumEvens(81023) << endl;
    cout << SumEvens(1923) << endl;
    cout << SumEvens(247) << endl;
    return 0;
}