Programming Forums > Application Development > C++

Jessehk · May 2nd, 2006, 8:12 PM

I wrote a simple program to test for a palindrome, and for some reason, a simple comparison of strings does not return good results. I have a feeling this has to do with comparing references, though I don't really know. As usual, I'm sure it is something stupid:

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#include <string>
#include <iostream>

std::string reverse(const std::string &str);
bool palindrome(const std::string &str);

int main() {
    const std::string msg = "abba";
    if(palindrome(msg))
        std::cout << "Msg is a palindrome" << std::endl;
	
	return 0;
}

std::string reverse(const std::string &str) {
	std::string result;

	for(int x = str.size(); x > -1; x--)
		result += str[x];

	return result;
}

bool palindrome(const std::string &str) {
	return reverse(str) == str;
}

I am 99.99% sure that my reverse() function works correctly, but I still don't get good output. Any tips would be great. Thanks.

OpenLoop · May 2nd, 2006, 8:28 PM

In your reverse function, start the for loop from std.size() - 1 like this:

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for(int x = str.size()-1; x > -1; x--)
		result += str[x];

size() returns the actual size of the string, not the subscript of the last character.

DaWei · May 2nd, 2006, 8:30 PM

Reverse returns a string. You've defined palindrome to return a bool, but are attempting to return a string. The existence of the string will cause a true to be returned. You should have mentioned HOW it was failing, not just that it was. A debugger or some debug output before and after each return would tell you a lot.

Jason Isom · May 2nd, 2006, 8:36 PM

Quote:

Originally Posted by DaWei

You've defined palindrome to return a bool, but are attempting to return a string.

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return reverse(str) == str;

What?

Twilight · May 2nd, 2006, 8:37 PM

But wouldn't palindrome return the return value of his comparison?

Jimbo · May 2nd, 2006, 8:47 PM

Why not just use something like:

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for(int i = 0, j = str.length - 1; i < j; i++, j--)
   if(str[i] != str[j])
      return false;
return true;

That way you don't create a new string, and you only iterate over 1/2 of the string instead of the whole thing to reverse it...

Jessehk · May 2nd, 2006, 8:58 PM

Jimbo: Because I hadn't thought of that :p

OpenLoop: Yup, that fixed it. Thanks

DaWei: You did misread the code, but I suppose it is partly my fault for not explaining the exact errors I was recieving (in this case, everything ran, but I didn't get the output I wanted.)

DaWei · May 2nd, 2006, 9:23 PM

My apologies, I did misread the code.

titaniumdecoy · May 3rd, 2006, 1:27 AM

I just had to post this because I saw it in a C++ book I was reading. It tests for a palindrome in a single line.

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bool is_palindrome(const string& s)
{
    return equal(s.begin(), s.end(), s.rbegin());
}

The Dark · May 3rd, 2006, 2:55 AM

Its nice and short, but a simple addition makes it take about half as long to run:

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bool is_palindrome(const string& s)
{
    return equal(s.begin(), s.begin() + s.length() / 2, s.rbegin());
}

May 2nd, 2006, 8:12 PM	#1
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 648 Rep Power: 4	Testing for a palindrome using std::string I wrote a simple program to test for a palindrome, and for some reason, a simple comparison of strings does not return good results. I have a feeling this has to do with comparing references, though I don't really know. As usual, I'm sure it is something stupid: (Toggle Plain Text) #include <string> #include <iostream> std::string reverse(const std::string &str); bool palindrome(const std::string &str); int main() { const std::string msg = "abba"; if(palindrome(msg)) std::cout << "Msg is a palindrome" << std::endl; return 0; } std::string reverse(const std::string &str) { std::string result; for(int x = str.size(); x > -1; x--) result += str[x]; return result; } bool palindrome(const std::string &str) { return reverse(str) == str; } #include <string> #include <iostream> std::string reverse(const std::string &str); bool palindrome(const std::string &str); int main() { const std::string msg = "abba"; if(palindrome(msg)) std::cout << "Msg is a palindrome" << std::endl; return 0; } std::string reverse(const std::string &str) { std::string result; for(int x = str.size(); x > -1; x--) result += str[x]; return result; } bool palindrome(const std::string &str) { return reverse(str) == str; } I am 99.99% sure that my reverse() function works correctly, but I still don't get good output. Any tips would be great. Thanks. __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

May 2nd, 2006, 8:28 PM	#2
OpenLoop Expert Programmer Join Date: May 2005 Location: East Lansing, MI Posts: 663 Rep Power: 4	In your reverse function, start the for loop from std.size() - 1 like this: (Toggle Plain Text) for(int x = str.size()-1; x > -1; x--) result += str[x]; for(int x = str.size()-1; x > -1; x--) result += str[x]; size() returns the actual size of the string, not the subscript of the last character.

May 2nd, 2006, 8:30 PM	#3
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Reverse returns a string. You've defined palindrome to return a bool, but are attempting to return a string. The existence of the string will cause a true to be returned. You should have mentioned HOW it was failing, not just that it was. A debugger or some debug output before and after each return would tell you a lot. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

May 2nd, 2006, 8:47 PM	#6
Jimbo Battle Programmer Join Date: Feb 2006 Location: Bellevue, WA, USA Posts: 770 Rep Power: 3	Why not just use something like: (Toggle Plain Text) for(int i = 0, j = str.length - 1; i < j; i++, j--) if(str[i] != str[j]) return false; return true; for(int i = 0, j = str.length - 1; i < j; i++, j--) if(str[i] != str[j]) return false; return true; That way you don't create a new string, and you only iterate over 1/2 of the string instead of the whole thing to reverse it...

May 2nd, 2006, 8:58 PM	#7
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 648 Rep Power: 4	Jimbo: Because I hadn't thought of that :p OpenLoop: Yup, that fixed it. Thanks DaWei: You did misread the code, but I suppose it is partly my fault for not explaining the exact errors I was recieving (in this case, everything ran, but I didn't get the output I wanted.) __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

May 2nd, 2006, 8:37 PM	#5
Twilight Programmer Join Date: Apr 2006 Location: Calgary, Alberta Posts: 67 Rep Power: 3	But wouldn't palindrome return the return value of his comparison?

May 2nd, 2006, 9:23 PM	#8
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	My apologies, I did misread the code. __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

May 3rd, 2006, 1:27 AM	#9
titaniumdecoy Expert Programmer Join Date: Nov 2005 Posts: 931 Rep Power: 4	I just had to post this because I saw it in a C++ book I was reading. It tests for a palindrome in a single line. (Toggle Plain Text) bool is_palindrome(const string& s) { return equal(s.begin(), s.end(), s.rbegin()); } bool is_palindrome(const string& s) { return equal(s.begin(), s.end(), s.rbegin()); }

May 3rd, 2006, 2:55 AM	#10
The Dark Expert Programmer Join Date: Jun 2005 Posts: 893 Rep Power: 4	Its nice and short, but a simple addition makes it take about half as long to run: (Toggle Plain Text) bool is_palindrome(const string& s) { return equal(s.begin(), s.begin() + s.length() / 2, s.rbegin()); } bool is_palindrome(const string& s) { return equal(s.begin(), s.begin() + s.length() / 2, s.rbegin()); }

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Programming Forums > Application Development > C++

Testing for a palindrome using std::string