How to reverse a number and compare it with its original value

Question

I'm trying to multiply two numbers and then reverse the answer and then compare the original value with the reversed value to see if they are the same numbers. After I reverse the value how do I set it back to an int or a string so I can compare them?

int i = 0;

var x = Enumerable.Range(100,999);
var y = Enumerable.Range(100,999);

foreach (var xValue in x)
{
    foreach (var yValue in y)
    {
        i = (xValue * yValue);

        var t = i.ToString();
        var tempt = t.Reverse();

        var temp = new string(tempt);

        if (i.ToString() == temp)
        {

        }
    }
}

This looks very familiar... http://projecteuler.net/problem=4 — Dennis_E, May 22 '14 at 12:52

score 0 · Answer 1 · answered May 22 '14 at 12:50

0

int n; //given number
int left = n; 
int rev = 0;
while(left>0)
{
r = left % 10; //take the last number  
rev = rev * 10 + r; //add it to a new number to flip it
left = left / 10; //left = Math.floor(left / 10);  
}

answered May 22 '14 at 12:50

Robert P

137
2
10

While this may solve the problem OP is trying to solve, this is not an answer to the question. Would be good to add some textual explanation too – Tim May 22 '14 at 12:51

score 0 · Accepted Answer · answered May 22 '14 at 12:57

Like this perhaps? (One shouldn't really give away answers to projecteuler problems, but this is only problem 4, so what the heck)

    public int SolveProblem004()
    {
        int result = 0;
        for (int a = 999; a >= 100; --a) {
            for (int b = 999; b >= a; --b) {
                int product = a * b;
                if (product <= result) break;
                if (IsPalindromic(product)) { result = product; break; }
            }
        }
        return result;
    }

    public static bool IsPalindromic(int i)
    {
        return i == Reverse(i);
    }

    public static int Reverse(int number)
    {
        if (number < 0) return -Reverse(-number);
        if (number < 10) return number;
        int reverse = 0;
        while (number > 0) {
            reverse = reverse * 10 + number % 10;
            number /= 10;
        }
        return reverse;
    }

You can google out all the answers anyway. I just don't get what's the point of doing that.. =/ — msmolcic, May 22 '14 at 13:26
@msmolcic92 I totally agree, but just having the answer doesn't teach anyone anything. The fun part is to figure out how. — Dennis_E, May 22 '14 at 13:30
Wasn't really interested in projecteuler, I was more interested in how I could do this in C#. Thanks — NNassar, Jan 04 '15 at 18:28

msmolcic · Answer 3 · 2014-05-22T13:20:42.950

You can do it this way:

int i = 0;

var x = Enumerable.Range(100, 999);
var y = Enumerable.Range(100, 999);

foreach (var xValue in x)
{
    foreach (var yValue in y)
    {
        i = (xValue * yValue);

        char[] number = i.ToString().ToCharArray();
        char[] reversedNumber = i.ToString().ToCharArray();
        Array.Reverse(reversedNumber);

        if (new string(number).Equals(new string(reversedNumber)))
        {

        }
    }
}

It will enter inside if loop if number is the same as reversed number and will just pass by in oposite case.

How to reverse a number and compare it with its original value

3 Answers3