Recursion and functions - function

Question

Thanks for helping everyone. I will continue looking at it so I can better understand! I am still struggling with recursion but I will study it more. Thanks again for all your time and effort for trying to help me
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int countEven(int arr[i]){
//I'm not sure what to do here... how to fix it...
int evens = 0;
if(arr[i] <= 0) return 0; //base case
while(arr[i] > 0){
int digit = arr[i]%10; //get the last digit
if(digit%2 == 0){
evens = evens+1;
}
arr[i] = arr[i]/10;
}
cout << evens;
}
}
}
int main(){
cout << "Part A:\n";
int arr[3] = { 5050155, 5, 707070 };
for (int i = 0; i < 3; i++){
cout << "countEven(" << arr[i] << ") = " << countEven(arr[i]) << endl;
cout << "removeEven(" << arr[i] << ") = " << removeEven(arr[i]) << endl;
cout << "hasEven(" << arr[i] << ") = ";
if (hasEven(arr[i])) cout << "Yes" << endl;
else cout << "No" << endl;
printStarDigit(arr[i]);
cout << endl << endl;
}
cout << "Part B:\n";
int a[4] = { 7, 2, 8, 3 };
int b[5] = { 3, 4, 5, 6, 7 };
cout << "The range of array a is " << range(a, 4) << endl;
cout << "The range of array b is " << range(b, 5) << endl;
reverse(a, 4);
reverse(b, 5);
cout << "Array a reversed: ";
for (int i = 0; i < 4; ++i)
cout << a[i] << " ";
cout << endl;
cout << "Array b reversed: ";
for (int i = 0; i < 5; ++i)
cout << b[i] << " ";
cout << endl;
return 0;
}

Answer 1

int countEven(int arr[i]){
Parameters must have simple names, and do not need to be identical to the expressions passed in. arr[i] is not a valid name. A simple name to use here is n.
//I'm not sure what to do here... how to fix it...
int evens = 0;
if(arr[i] <= 0) return 0; //base case
This base case is wrong for two reasons. Firstly, you treat all negative integers as the base case, but -202020 has six even digits. Secondly, you return the wrong value: 0 has one even digit, but you return zero.
A possible base case could be n > -10 && n < 10 (single digit number). I'll let you figure out the expression to return for that base case.
while(arr[i] > 0){
If your task is to write a recursive function, then you shouldn't use a loop here. Instead, see below.
int digit = arr[i]%10; //get the last digit
...
arr[i] = arr[i]/10;
This is a correct way of obtaining the last digit, and everything other than the last digit.
if(digit%2 == 0)
This is a correct way of determining whether the last digit is even.
Now, you need to combine what you have, by observing that the count of even digits is equal to "1 if the last digit is even, else 0", plus the count of even non-last digits. The goal of the exercise is to get you to write "the count of even non-last digits" as countEven(n / 10).

Answer 2

See these code snippets.
int countEven(int n, bool first_time = true){
static int total ;
int num =n;
if (first_time)
{
total = 0;
}
if (num <= 9) {
return num % 2 == 0 ? 1 : 0;
}
int temp = num - (num / 10) * 10;
num = num / 10;
total += temp % 2 == 0 ? 1 : 0;
countEven(num,false);
return total;
}
int removeEven(int n){
int result = 0;
/*TODO*/
return result;
}
bool hasEven(int n){
return countEven(n);
}
void printStarDigit(int* arr){
/*TODO*/
}
int range(int* arr, int n){
int result = *arr;
for (int i = 0; i < n; i++) {
if (result < *(arr + i)) result = *(arr + i);
}
return result;
}
void reverse(int* arr, int n){
int* temp = new int[n];
for (int i = 0; i < n; i++) {
*(temp + i) = *(arr + i);
}
for (int i = 0; i < n; i++) {
*(arr + i) = *(temp + n - 1 - i);
}
delete[] temp;
}
Main implementation :
int main() {
cout << "Part A:\n";
int arr[3] = { 5050155, 5, 707070 };
for (int i = 0; i < 3; i++) {
cout << "countEven(" << arr[i] << ") = " << countEven(arr[i]) << endl;
cout << "removeEven(" << arr[i] << ") = " << removeEven(arr[i]) << endl;
cout << "hasEven(" << arr[i] << ") = ";
if (hasEven(arr[i])) cout << "Yes" << endl;
else cout << "No" << endl;
printStarDigit(arr + i);
cout << endl << endl;
}
cout << "Part B:\n";
int a[4] = { 7, 2, 8, 3 };
int b[5] = { 3, 4, 5, 6, 7 };
cout << "The range of array a is " << range(a, 4) << endl;
cout << "The range of array b is " << range(b, 5) << endl;
reverse(a, 4);
reverse(b, 5);
cout << "Array a reversed: ";
for (int i = 0; i < 4; ++i)
cout << a[i] << " ";
cout << endl;
cout << "Array b reversed: ";
for (int i = 0; i < 5; ++i)
cout << b[i] << " ";
cout << endl;
return 0;
}
Output: