Recursion of an index - c++

Question

Hello fellow programmers, I have a question about recursion that I do not understand, being new to C++ and all. So for this exercise I am completing, I need to: 1. Ask user for a string 2. Ask user for a string to search in the first string entered. 3. report and index of the string if it finds it. For instance the user enters the string "Search me", string to search for is "me", and the index would return "7". I am throwing my hands up at this point for some help on how to complete it, any help would be appreciated. Here is my code thus far. The stuff that is in the for loop isn't a complete thought, just FYI.
int index_of(string stringToSearchIn, string stringToSearchFor, int index)
{
if (stringToSearchIn.length() < stringToSearchFor.length())
{
cout << "Your string cannot be found." << endl;
system("PAUSE");
return OUTLIER;
}
else
{
bool found = true;
for (int i = ZERO; i < stringToSearchFor.length; i++)
{
found = found && stringToSearchIn(i) == stringToSearchFor(i);
}
return index;
}
return index_of(stringToSearchIn.substr(INCREASE), stringToSearchFor, index++);
}
int main()
{
//Initializing values
string userString;
string userSearch;
int userIndex = 0;
//Asking for user input for string
cout << "This program will find the occurence of one string inside of another." << endl;
cout << "Enter the string to be searched: " << userString;
//Getting the string
getline(cin, userString);
//Asking for user input for search input
cout << "Now enter the string you want to search for: " << userSearch;
//Getting the string
getline(cin, userSearch);
//Displaying results
cout << "The index of the substring is = " << index_of(userString, userSearch, userIndex);
//Keeping console window open until key press
system("PAUSE");
return ZERO;
}

Answer 1

Catch.:)
#include <iostream>
size_t index_of( const char *s, const char *t )
{
const char *p = s;
const char *q = t;
while ( *q && *p == *q ) ++p, ++q;
if ( *q == '\0' )
{
return 0;
}
else if ( *p == '\0' )
{
return -1;
}
else
{
size_t n = 1 + index_of( s + 1, t );
return n == 0 ? -1 : n;
}
}
int main()
{
const char *s = "Search me";
const char *t = "me";
size_t n = index_of( s, t );
if ( n != -1 )
{
std::cout << "string " << t << " is found at position " << n << std::endl;
}
else
{
std::cout << "string " << t << " is not found" << std::endl;
}
return 0;
}
The output is
string me is found at position 7
For other strings I did not test the function.:)
For objects of type std::string you can call it like
size_t n = index_of( s.c_str(), t.c_str() );
Otherwise if you want to write a similar recursive function for objects of type std::string then you need either to add a static local variable that to keep the current position inside the source string or add one more parameter for index or use member function substr.
Also my advice is do not use manifest constant ZERO for zero. In my opinion it is a bad style of programming. It only confuses readers because ZERO can be anything including some user-defined class.
For example (without testing)
#include <iostream>
#include <string>
std::string::size_type index_of( const std::string &s, const std::string &t )
{
static std::string::size_type pos;
if ( s.size() < t.size() ) return std::string::npos;
if ( s.compare( pos, t.size(), t ) == 0 ) return 0;
++pos;
std::string::size_type n = index_of( s, t );
--pos;
return n == std::string::npos ? std::string::npos : n + 1;
}
int main()
{
std::string s = "Search me";
std::string t = "me";
std::string::size_type n = index_of( s, t );
if ( n != std::string::npos )
{
std::cout << "string " << t << " is found at position " << n << std::endl;
}
else
{
std::cout << "string " << t << " is not found" << std::endl;
}
return 0;
}
As for your function realization then it can look for example the following way
#include <iostream>
#include <string>
std::string::size_type index_of( const std::string &stringToSearchIn,
const std::string &stringToSearchFor,
std::string::size_type index )
{
if ( stringToSearchIn.length() < stringToSearchFor.length() + index )
{
return std::string::npos;
}
else if ( stringToSearchIn.compare( index,
stringToSearchFor.length(),
stringToSearchFor ) == 0 )
{
return index;
}
else
{
std::string::size_type n =
index_of( stringToSearchIn, stringToSearchFor, ++index );
return n == std::string::npos ? std::string::npos : n;
}
}
int main()
{
std::string s = "Search me";
std::string t = "me";
std::string::size_type n = index_of( s, t, 0 );
if ( n != std::string::npos )
{
std::cout << "string " << t << " is found at position " << n << std::endl;
}
else
{
std::cout << "string " << t << " is not found" << std::endl;
}
return 0;
}