reversing a string

**swappo** · 06-13-2009

Hello,

I am trying to learn the C++ standard library and I am having trouble with this program.

Code:

//cin with strings
#include <iostream>
#include <string>
#include <vector>
#include <cstdio>
using namespace std;

vector<string> reverse(const string& s)
{
	vector<string> return_s;
	typedef string::size_type string_size;
	string_size i = 0;
	
	for(i = s.size() - 1; i > 0; i--)//Here is the problem
		return_s.push_back(s.substr(i, 1));
	
	return return_s;
}

int main ()
{
	string s;
	vector<string> v;
	cout << "Enter a string: " << endl;
	
	while(getline(cin, s))
		v = reverse(s);
		
	cout << "Your string in reverse: " << endl;		
	for(vector<string>::size_type i = 0; i < v.size(); i++)
		cout << v[i];
	cout << endl;
		
	return 0;
}

In the indicated for loop, if I change the middle part to i >= 0, then I get an error for being out of range. If i doesn't go to 0 then it doesn't take the last character. Why do I get an out of range error for going to 0? Any suggestions?

**Sebastiani** · 06-13-2009

string::size_type is an unsigned integer, so decrementing past zero yields a positive number.

**CodeMonkey** · 06-13-2009

You're using a vector of strings to hold an array of chars. string is an array of chars. In the spirit of "learning the standard library," you can avoid the loops altogether.

Code:

std::string reverse(const std::string & s)
{
   std::string ret( s.size(), char() );
   std::reverse_copy( s.begin(), s.end(), ret.begin() );
   return ret;
}
 //Then just use strings in main()

**swappo** · 06-13-2009

Sebastiani, I changed string::size_type to int and the program now runs as I expect it. CodeMonkey, can you explain what the first two lines after the first brace are doing? I'm not sure I understand them. Thanks for the help.

**CodeMonkey** · 06-13-2009

Sure. The first line creates a temporary string to hold the value you wish to return. The initializer says to make it the same size as s, and full of char(). It doesn't matter what it's filled with.
The second line copies the range s.begin() to s.end() to the range beginning at ret.begin(). But it copies them backwards. So now ret has all of the elements of s, but in reverse order. Then you just return ret.

*edit*
One of the beauties of using this method is that is is standard and generic. You can change all of the std::strings to std::vector<long double>s, and the code will work the same way.
Even more generally, functions in the family of reverse_copy() (STL algorithms), can operate with any object that acts like an "iterator" (i.e. that is an iterator). So, instead of iterating through predefined iterator types, like in string and vector, you could use a low-level pointer-to-array. It's really neat!

**hk_mp5kpdw** · 06-13-2009

FWIW, such a function could be reduced even further if desired:

Code:

std::string reverse(const std::string& s)
{
    return std::string(s.rbegin(),s.rend());
}

**CodeMonkey** · 06-14-2009

That's much better. Hardly need the function, then, except for clarity.

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