What is palindrome in programming?

Answer 1

A palindrome is any data sequence that reads exactly the same forwards as it does backwards.

To test a sequence to see if it is a palindrome or not, we begin by pointing to the elements at the beginning and end of the sequence. So long as the pointers do not meet or cross each other and both refer to the same value, we advance the pointers one element towards the middle of the sequence. If the pointers meet or cross each other, the sequence is a palindrome, otherwise it is not.

The following tests a vector of some type T:

template<typename T>

bool is_palindrome (const std::vector<T>& v) {

size_t x {0}, y {v.size()-1);

while (x<y && v[x]==v[y]) ++x, --y;

return x<=y;

}

Example usages:

std::vector<double> d {1.2, 3.4, 5.6, 3.4, 1.2}; // is a palindrome std::vector<int> i {1, 2, 3, 4, 5}; // is not a palindrome

std::vector<char> c {'m', 'a', 'd', 'a', 'm'}; // is a palindrome

assert (is_palindrome (d) == true);

assert (is_palindrome (i) == false);

assert (is_palindrome (c) == true);

When testing strings, we typically strip out all non-alpha-numeric characters and use a case-insensitive comparison. To achieve this, we simply convert the string to a lower-case character array and invoke the previous function:

bool is_palindrome (const std::string& str) {

std::vector<char> v {};

for (size_t i=0; i<str.size(); ++i)

if (str[i]>='0' && str[i]<='9') v.push_back (str[i]); // push digit characters

else if (str[i]>='a' && str[i]<='z') v.push_back (str[i]); // push lowercase characters

else if (str[i]>='A' && str[i]<='Z') v.push_back (str[i]+32); // push uppercase as lowercase characters

return is_palindrome (v);

}

Example usage:

std::string s {"Madam, I'm Adam!");

assert (is_palindrome (s) == true);

Answer 2

// returns true if num is a palindrome (e.g., 12321 is a palindrome)bool is_palindrome (unsigned num) {

unsigned rev = 0; // initialise the reverse of the number

while (num) { // repeat until num is zero

rev *= 10; // move all digits in the reverse number one position to the left

rev += num%10; // add the least significant digit of the number

num /= 10; // strip off the least-significant digit

}

return num == rev; // test for equality

}