The Difference Between Calling and Returning a Recursive Function in C++ - c++

Question

Can someone please explain with a simple example what precisely is the difference between calling and returning a recursive function in c++?
Here is my code which seeks to find a character in a string recursively. It works fine when I just call find(); the function displays an integer value, But, when I code return find(letter,word), it gives the correct result as either a 1 or 0. Thanks
bool find(char f,string str)
{
static int len = str.length() - 1;
static int count = 1;
if (len<0)
{
return false;
}
else
{
if (str[len] == f)
{
return true;
}
else
{
len--;
return find(f, str);
}
}
}

Answer 1

You don't return the function you return the result of the call.
Here a basic example of recursion :
unsigned int factorial(unsigned int n)
{
if (n == 0)
return 1;
return n * factorial(n - 1);
}
If we call the function with n == 2 the program will do:
1) factorial (2) => return 2 * factorial(1); // It calls factorial with n == 1, do the multiplication and then return the result.
2) factorial(1) => return 1 * factorial(0); // same here with n == 0
3) factorial(0) => return 1; // from here the program will come back from the calls with the results
4) factorial(1) => return 1 * 1; => return 1;
5) factorial(2) => return 2 * 1; => return 2;
Few other things : be aware that your code will only work once as you use static int len, and count is a useless variable.

Answer 2

The function signature indicates a return value, so you need to embed the function call in a return statement for a value to be returned by the function, especially when neither of the if-conditionals are true.
Consider the following code (a variant of what the OP provided):
#include <iostream>
using namespace std;
int find(char f,string str) {
static int len = str.length() - 1;
static int count = 1;
int temp = 0;
if (len < 0) {
cout << count << "\n";
return -99;
}
else
if ( str[len] == f) {
return len;
}
len--;
count++;
temp = find( f, str );
cout << temp << "\n";
return temp;
}
int main() {
char ch = 'z';
int res = find(ch,"I");
if (res < 0) {
cout << "Letter '" << ch << "' was not found";
}
return 0;
}
See demo
Note that find() has a return value of -99 when the letter is not found. That value is captured by temp in find() and the function then returns the value of temp.
Now, consider main() -- its signature also indicates a return value, although it is discarded. If you attempt to execute the code without returning some kind of an integer in main(), the execution will be flawed.
So, whether you use a function recursively or not, if the function's signature indicates a return value then you need to return a value of the expected type. Since find() may return -99 or the position of the found letter, when it executes it will evaluate as one of those two values which will be returned by means of the return statement.