如何初始化递归函数的长度

Question

int countChars(string str)
{
    int count = 0;
    if (str == "")
        return count;
    else
    {
        count++;// add a character to the count
        return count + countChars(str.substr(1));// function calls itself
    }
}

I need to take the above function and call in in the program below and I'm not sure how to initialize it properly. Below is what I tried and it doesn't work. I'm not allowed to use the .length() because otherwise the program would be done.

int main()
{
    char find = '\0';
    string str;
    int count = 0;
    int length = int(countChars);

    //ask the user for a sentence 
    cout << "Enter a sentence " << endl;
    getline(cin, str);
    //ask the user which letter they want the count of 
    cout << "Which letter would you like to find the number of appearances: " << endl;
    cin >> find;


    for (int i = 0; i < length; i++)
    {
        if (str[i] == find)
        {
            count++;
        }
    }

    cout << "the letter " << find << " appears " << length << " times " << endl;

    //waits for user to exit
    system("pause");
    cin.get();
}

Answer 1

It seems the function should count the number of appearances of a letter in a string. If so then it is declared and defined incorrectly. It has to have at least two parameters an object of type std::string and an object of type char .

Here is shown how such a recursive function can look

#include <iostream>
#include <string>

size_t countChars( const std::string &s, char c )
{
    return s.empty() ? 0 : ( s[0] == c ) + countChars( { s, 1 }, c );
}

int main() 
{
    std::cout << "Enter a sentence ";

    std::string s;

    std::getline( std::cin, s );

    std::cout << "Which letter would you like to find the number of appearances: ";

    char c = '\0';

    std::cin >> c;


    std::cout << "The letter " << c 
              << " appears " << countChars( s, c ) 
              << " times " << std::endl;

    return 0;
}

The program output might look like

Enter a sentence My name is Alycia
Which letter would you like to find the number of appearances: a
The letter a appears 2 times 

If you mean a function that just calculates the length of a string then it can look like

size_t countChars( const std::string &s )
{
    return s.empty() ? 0 : 1 + countChars( { s, 1 } );
}

and shall be called after the statement

getline(cin, str);