Palindromes using dynamic array

[DeathLeap]

Hello C++ nerds,

I am trying to do a palindrome using a dynamic array but before I spend 6 hours on this, is it possible to do it this way?

As you can see the user will input how many characters he wants in the array, then proceed to enter the chars one by one.

I have several questions regarding my code:

1) I learned how to declare a dynamic array from the tutorials, yet, I don't understand the use of (notthrow) in my code. Line 10. Can someone explain what it means?

2) Am I doing the right thing when I equalize p[0] = left and p[n] = right? my initial thought was that I would set those values first then I will decrease p[n] - 1 and increase p[0] + 1 and compare them, if they're equal then it's a palindrome. The problem with this approach is when I reach the middle, I don't know what to do.

3) There is some conditions that need to be addressed, what is the best way to test this program? what are some common ways that, if I follow, will instantly increase my testing ability?

This is one of the conditions that returns true although it's not a palindrome:

How many letters would you like to type? 4
Enter letter:a
Enter letter:s
Enter letter:a
Enter letter:a
true
Press any key to continue . . .

#include <iostream>
using namespace std;

void isPalidrome()
{
	int i, n;   // i = number of elements, n = global variable in the function.
	char left, right;
	char * p;
	cout << "How many letters would you like to type? ";
	cin >> i;
	p = new (nothrow) char[i];
	if (p == nullptr)
	{
		cout << "Error: memory could not be allocated";
	}
	else
	{
		for (n = 0; n < i; n++)
		{
			cout << "Enter letter:";
			cin >> p[n];
		}
	}

	left = 0, right = n - 1;

	// testing if isPalindrome
	if (left == right)
	{
		while (left < right)
		{
			if (p[left++] != p[right--])
			{
				cout << "not a palindrome";
			}
		}
	}
	else
	{
		cout << "true" << endl;
	}
	
}

int main()
{
	isPalidrome();

	system("PAUSE");
	return 0;
}

[Bdanielz]

The problem with this approach is when I reach the middle, I don't know what to do.

So you have two cases. Odd or a Even number of inputs.

You could write a separate function to handle both cases, but you do not have to, if you keep track of how large your arrays are.


in pseudo.

read the input.
copy the input to a second variable. 
reverse the second variable (or read it backwards)
and compare nth index char to each other. 
if you find a difference then no palindrome

see my response here
http://www.cplusplus.com/forum/beginner/182218/

to see this implemented with strings.

[Chervil]

when I reach the middle, I don't know what to do

When you get to the middle, you stop comparing characters.

You could use a bool variable to store the result. Initialise it to true.

Use a pair of pointers (or subscripts) a and b to access the first and last characters of the string. Loop while a < b.
If a pair of characters are different, set result to false (and end the compare loop). Add 1 to a, subtract 1 from b.

When the loop is finished, you have the result.