C++ Return Vowels in String using subString

Question

I'm new to c++... I'm trying to make a function that takes in a string and returns only the vowels in the string. I need to use substrings to perform this. If I use a word with only 1 vowel the function runs correctly... however I've tested a few other strings and it does not work. Do any of you have some suggestions? Thanks so much!

#include <iostream>
using namespace std;
string vowel_search(string str)
{
    string test = str;
    string result = "";
    for (int i = 0; i < test.length(); i++)
    {
        if (str[i] == 'a' || str[i] == 'A'||
            str[i] == 'e' || str[i] == 'E' ||
            str[i]== 'i'|| str[i] == 'I' ||
            str[i] == 'o' || str[i] == 'O' ||
            str[i] == 'u' || str[i] == 'U')
        {
          result += test.substr(i, i);
        }
     }
     return result;
}
int main() {
string strWord = "Aaron";
string str1 = vowel_search(strWord);
cout << str1 << endl; // prints out: aon

string strWord1 = "Harry";
string str2 = vowel_search(strWord1);
cout << str2 << endl; //prints out: a -- Correct

string strSentence = "Harry is Funny";
string str3 = vowel_search(strSentence);
cout << str3 << endl; //prints out: ais Fununny

return 0;
}

Answer 1

result += test.substr(i, 1);

replace the second i with a 1, 1 is the number of chars, or length you want returned