Check if the string is palindrome

Question

The following code runs correctly:

#include <iostream>
#include <algorithm>
#include <string>
bool is_palindrom(const std::string& str)
{
 std::string temp(str);
 std::transform(temp.begin(), temp.end(), temp.begin(), ::tolower);
 return temp == std::string(temp.rbegin(), temp.rend());
}
int main()
{
 std::string str{ "KArak" };
 std::cout << is_palindrom(str) << std::endl;
 return 0;
}

Can I do this better? The constraint is to keep the declaration of the function the same and only change the implementation.

Answer 1

To the readers: (削除) I had to use ::tolower because the std:: version was causing compiler errors. I'm investigating this right now. (削除ここまで) The issue stems from std::tolower being overloaded. What is funny is that it is in header <locale>, which means that the standard library implementation includes it in some way. Rakete1111 proposed lambda version, which works great.

Yes.

Use more standard library:

The std::string provides reverse iterators, so there is no need to do any copy at all. In fact, it is possible to it with indices, but it won't look that pretty:

 return std::equal(str.cbegin(), str.cbegin() + str.size() / 2, str.crbegin(), 
 [](unsigned char lhs, unsigned char rhs) {
 return std::tolower(lhs) == std::tolower(rhs);
 });

The actual version was different, thanks to Rakete1111 for a new version. There is a note in the comments from Loki and Jerry: it seems like using functions in <cctype> are dangerous and will result in undefined behavior unless the input is cast into unsigned char first.

It is also possible to refactor the ugly + str.size() / 2 into reusable function:

#include <iterator>
//some namespace
template <typename Container>
constexpr auto mid(const Container& container)
{
 auto half_distance = std::distance(std::begin(container), 
 std::end(container)) / 2;
 return std::next(container.begin(), half_distance);
}

Then it transforms into:

bool is_palindrome(const std::string& str) {
 return std::equal(str.cbegin(), mid(str), str.crbegin(), 
 [](unsigned char lhs, unsigned char rhs) {
 return std::tolower(lhs) == std::tolower(rhs);
 });
}

Other things that could be done better:

• Avoid std::endl. It can cause big performance issues.

• There is a typo in the word palindrome.

• std::boolalpha could be used to provide more user friendly output.

• more unit tests.

• It seems like the code is missing <cctype> header as well.

• putting everything into a namespace. Absence of it will force users to do unqualified call, which is prone to ADL, which is sometimes very evil.

Put together:

#include <iomanip>
#include <iostream>
#include <algorithm>
#include <string>
#include <cctype>
#include <iterator>
//some namespace
template <typename Container>
constexpr auto mid(const Container& container)
{
 auto half_distance = std::distance(std::begin(container), 
 std::end(container)) / 2;
 return std::next(container.begin(), half_distance);
}
bool is_palindrome(const std::string& str) {
 return std::equal(str.cbegin(), mid(str), str.crbegin(), 
 [](unsigned char lhs, unsigned char rhs) {
 return std::tolower(lhs) == std::tolower(rhs);
 });
}
int main()
{
 std::string str{ "KArak" };
 std::cout << std::boolalpha;
 std::cout << "is \"KArak\" palindrome? " << is_palindrome(str) << '\n';
 std::cout << "is \"fuf\" palindrome? " << is_palindrome("fuf") << '\n';
 std::cout << "is empty string palindrome? " << is_palindrome("") << '\n';
 return 0;
}

Live demo.

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