Thread: C++ algorithm help

If you're looking for speed, try this:

Code:

bool checkPalindrome(int testSubject)
{
	int tempValue = 0;
	int origValue = testSubject;
	int pValue;

	while (testSubject != 0)
	{
		pValue = testSubject;
		pValue %= 10;
		tempValue *= 10;
		tempValue += pValue;
		testSubject /= 10;
	}

	if(origValue == tempValue)
	{
		return true;
	} else {
		return false;
	}
}

>Any suggestions about how to go about solving this would be helpful.
There are a number of ways to go about this. Here is my solution this time, the value is reversed into a std::string and tested for a palindrome. The string is reversed because you wanted the indexing function to work from the least signifigant digit to the most signifigant. This way was easier.

Code:

#include <iostream>
#include <string>

class Palin
{
  int value; // Original value
  std::string buffer; // Reversed value

  // Check the value for a palindrome
  bool check_val();
public:
  Palin ( int val )
    : value ( val )
  {}

  // Print the results of check_val()
  void check ( std::ostream& out ); 
  // Get the digit of value located at index
  // in the least signifigant order.
  //   Ex. 1234, get_digit ( 2 ) returns 3.
  int get_digit ( const int index );
};

bool Palin::check_val()
{
  int low, high, count = 0;
  int copy = value;

  // Copy the integer to the buffer in reverse
  for ( count = 0; copy != 0; count++ ) {
    buffer += ( copy % 10 ) + '0';
    copy /= 10;
  }

  // Check for a palindrome
  for ( low = 0, high = count - 1; low < high; low++, high-- ) {
    if ( buffer[low] != buffer[high] )
      return false;
  }

  return true;
}

void Palin::check ( std::ostream& out)
{
  if ( check_val() )
    out<< value <<" is a palindrome.\n";
  else
    out<< value <<" is a not palindrome.\n";
}

int Palin::get_digit ( const int index )
{
  return buffer[index - 1] - '0';
}

int main()
{
  Palin my_pal ( 1221 );

  my_pal.check ( std::cout );
  std::cout<<"The second digit is "<< my_pal.get_digit ( 2 ) <<'\n';

  return 0;
}

-Prelude

>how do I flip a string around?
You can either copy it to another buffer in reverse, or swap elements from the outside in until you get to the middle:

Code:

copy to a buffer:

step 1: 1234 5
step 2: 123  54
step 3: 12   543
step 4: 1    5432
step 5:      54321

swap elements:

step 1: 12345
step 2: 52341
step 3: 54321

-Prelude

>if it's not for school you could use strrev();
And if your compiler supports it, strrev is not ISO standard.

-Prelude