Palindrome

[fahmankhan75]

My instructor was showing us palindrome numbers today and instead if using string size method she used some division method where last number got extracted. Can anyone give me a hint in how to find if a certain number is palindrome? I thought about dividing the number by ten to extract the last digit, but I can clearly figure out how to compare without using string size.

[giblit]

Are the numbers stored as integers or strings?

Here's one method to get the first and last digit of an integer:

#include <iostream>

long long pow( int x , int n )
{
	return( n > 1 ? x * pow( x , n - 1 ) : 1 );
}

int main()
{
	int number = 12345;
	int power_ten = 0;
	int first_digit = 0;
	int last_digit = number; //using last_digit as a temporary variable right now

	while( last_digit > 0 )
	{
    	last_digit /= 10;
    	++power_ten;
	}
	
	//get first digit
	first_digit = number / pow( 10 , power_ten );
	//get last digit
	last_digit = number  % 10;

	
	std::cout << "first digit: " << first_digit << std::endl;
	std::cout << "last digit: " << last_digit << std::endl;
}

This is probably how your teacher did it if she used the divide method.

[fahmankhan75]

	return( n > 1 ? x * pow( x , n - 1 ) : 1 );

Giblit, can you explain to me what this code does here. I know it's similar to if and else, but the expression I can't understand.

[giblit]

Its a c++ operator called the ternary operator.

Basically the pseudo code is like this


(return statement or a variable you are assigning a value to ) (condition to test) ? ( if true return this value ) : (otherwise return this value )

so lets look at my example


The thing I am using the value with is the return statement.
the condition I am using n > 1 //should of been n > 0 I had a typo because x^0 == 1
if n is greater than 1 I am using recursion ( return the value of x * pow( x , n - 1 )
if n is less than or equal to 1 then return 1 //again should have been less than 1 aka 0

Basically it can be rewritten as either of these two:

long long pow( int x , int n )
{
    if( n > 0 )
        return( x * pow( x , n - 1 );
    return( 1 ); //if x is not greater than 1
}

//or

long long pow( int x , int n  )
{
    int increment = x;
    if( n == 0 )
        return( 1 ); 
    for( int i = 1; i < n; ++i )
        x *= increment;
    return( x );
}

Then it should have been first_digit = number / pow( 10 , power_ten - 1 ) since 1000 = 10^3 not 10^4 I had a brain fart.

Here is somethign to read up on:
http://www.cplusplus.com/articles/1AUq5Di1/
http://www.learncpp.com/cpp-tutorial/34-sizeof-comma-and-arithmetic-if-operators/

*edit also you may use the pow function in the math library if you want. I just didn't feel like it.

**edit
The power_ten in my code is really the amount of digits in the number.