Find the largest palindrome made from the product of two 3-digit numbers C#

I tried to solve projecteuler 4th project with C# but I don't receive the correct answer, I get 90909. Can someone spot my mistake?

The problem goes like this:

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.

namespace Largest_palindrome_product{



class Program

{

    static void Main(string args)

    {

        string Reverse(string s)

        {

            char charArray = s.ToCharArray();

            Array.Reverse(charArray);                

            return new string(charArray);

        }





        int result = 0;

        string rev= "hello";

        string palindrome = "hello";

        string bingo = "hello";

        int j = 1;

        for (int i = 1; i< 1000; i++)

        {

            for (int y = 1; y< 1000; y++)

            {

                result = i * y;

                bingo = result.ToString();

                rev = Reverse(bingo);

                j = int.Parse(bingo);

            }



            if (rev == bingo)

            {

                palindrome = bingo;

            }

        }

        Console.Write(palindrome);

        Console.Read();

    }

}

}

asked Nov 21 '18 at 15:51

George

132

2

You're checking rev and bingo outside the inner loop.

– John
Nov 21 '18 at 15:56

1

I take it you're turning it into a string to easily reverse it? I'd keep it as an int and just compare the ints.

– Matt
Nov 21 '18 at 15:56

2

Why don't you try starting you loops at the MAX value and substract 1 each iteration. This way the first coincidence will be the maximum palindrome, not the first one. Also the if should be inside the inner loop as stated by @John.

– r1verside
Nov 21 '18 at 16:00

1

Ops, yep my mistake.

– r1verside
Nov 21 '18 at 16:02

add a comment |

I tried to solve projecteuler 4th project with C# but I don't receive the correct answer, I get 90909. Can someone spot my mistake?

The problem goes like this:

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.

namespace Largest_palindrome_product{



class Program

{

    static void Main(string args)

    {

        string Reverse(string s)

        {

            char charArray = s.ToCharArray();

            Array.Reverse(charArray);                

            return new string(charArray);

        }





        int result = 0;

        string rev= "hello";

        string palindrome = "hello";

        string bingo = "hello";

        int j = 1;

        for (int i = 1; i< 1000; i++)

        {

            for (int y = 1; y< 1000; y++)

            {

                result = i * y;

                bingo = result.ToString();

                rev = Reverse(bingo);

                j = int.Parse(bingo);

            }



            if (rev == bingo)

            {

                palindrome = bingo;

            }

        }

        Console.Write(palindrome);

        Console.Read();

    }

}

}

asked Nov 21 '18 at 15:51

George

132

2

You're checking rev and bingo outside the inner loop.

– John
Nov 21 '18 at 15:56

1

I take it you're turning it into a string to easily reverse it? I'd keep it as an int and just compare the ints.

– Matt
Nov 21 '18 at 15:56

2

Why don't you try starting you loops at the MAX value and substract 1 each iteration. This way the first coincidence will be the maximum palindrome, not the first one. Also the if should be inside the inner loop as stated by @John.

– r1verside
Nov 21 '18 at 16:00

1

Ops, yep my mistake.

– r1verside
Nov 21 '18 at 16:02

add a comment |

I tried to solve projecteuler 4th project with C# but I don't receive the correct answer, I get 90909. Can someone spot my mistake?

The problem goes like this:

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.

namespace Largest_palindrome_product{



class Program

{

    static void Main(string args)

    {

        string Reverse(string s)

        {

            char charArray = s.ToCharArray();

            Array.Reverse(charArray);                

            return new string(charArray);

        }





        int result = 0;

        string rev= "hello";

        string palindrome = "hello";

        string bingo = "hello";

        int j = 1;

        for (int i = 1; i< 1000; i++)

        {

            for (int y = 1; y< 1000; y++)

            {

                result = i * y;

                bingo = result.ToString();

                rev = Reverse(bingo);

                j = int.Parse(bingo);

            }



            if (rev == bingo)

            {

                palindrome = bingo;

            }

        }

        Console.Write(palindrome);

        Console.Read();

    }

}

}

asked Nov 21 '18 at 15:51

George

132

I tried to solve projecteuler 4th project with C# but I don't receive the correct answer, I get 90909. Can someone spot my mistake?

The problem goes like this:

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.

Find the largest palindrome made from the product of two 3-digit numbers.

namespace Largest_palindrome_product{



class Program

{

    static void Main(string args)

    {

        string Reverse(string s)

        {

            char charArray = s.ToCharArray();

            Array.Reverse(charArray);                

            return new string(charArray);

        }





        int result = 0;

        string rev= "hello";

        string palindrome = "hello";

        string bingo = "hello";

        int j = 1;

        for (int i = 1; i< 1000; i++)

        {

            for (int y = 1; y< 1000; y++)

            {

                result = i * y;

                bingo = result.ToString();

                rev = Reverse(bingo);

                j = int.Parse(bingo);

            }



            if (rev == bingo)

            {

                palindrome = bingo;

            }

        }

        Console.Write(palindrome);

        Console.Read();

    }

}

}

asked Nov 21 '18 at 15:51

George

132

asked Nov 21 '18 at 15:51

George

132

asked Nov 21 '18 at 15:51

George

132

asked Nov 21 '18 at 15:51

George

132

asked Nov 21 '18 at 15:51

George

132

2

You're checking rev and bingo outside the inner loop.

– John
Nov 21 '18 at 15:56

1

I take it you're turning it into a string to easily reverse it? I'd keep it as an int and just compare the ints.

– Matt
Nov 21 '18 at 15:56

2

Why don't you try starting you loops at the MAX value and substract 1 each iteration. This way the first coincidence will be the maximum palindrome, not the first one. Also the if should be inside the inner loop as stated by @John.

– r1verside
Nov 21 '18 at 16:00

1

Ops, yep my mistake.

– r1verside
Nov 21 '18 at 16:02

add a comment |

2

You're checking rev and bingo outside the inner loop.

– John
Nov 21 '18 at 15:56

1

I take it you're turning it into a string to easily reverse it? I'd keep it as an int and just compare the ints.

– Matt
Nov 21 '18 at 15:56

2

Why don't you try starting you loops at the MAX value and substract 1 each iteration. This way the first coincidence will be the maximum palindrome, not the first one. Also the if should be inside the inner loop as stated by @John.

– r1verside
Nov 21 '18 at 16:00

1

Ops, yep my mistake.

– r1verside
Nov 21 '18 at 16:02

You're checking rev and bingo outside the inner loop.

– John
Nov 21 '18 at 15:56

I take it you're turning it into a string to easily reverse it? I'd keep it as an int and just compare the ints.

– Matt
Nov 21 '18 at 15:56

Why don't you try starting you loops at the MAX value and substract 1 each iteration. This way the first coincidence will be the maximum palindrome, not the first one. Also the if should be inside the inner loop as stated by @John.

– r1verside
Nov 21 '18 at 16:00

Ops, yep my mistake.

– r1verside
Nov 21 '18 at 16:02

add a comment |

1 Answer
1

active

oldest

votes

I think what has caused so much confusion is the use of String this just complicates thing having to convert them back and forth.

Your program works fine (if the if is moved as per John's comment) if only you'd checked the new number was larger!

here is my take on it:

// stolen from https://www.geeksforgeeks.org/reverse-digits-integer-overflow-handled/

int Reverse(int num)

{

    int rev_num = 0;

    while (num > 0)

    {

        rev_num = rev_num * 10 + num % 10;

        num = num / 10;

    }

    return rev_num;

}





int result = 0;

int palindrome = 0;

int j = 1;

for (int i = 999; i > 0; i--)

{

    for (int y = 999; y > 0; y--)

    {

        result = i * y;

        if (result == Reverse(result))

        {

            if (result > palindrome)

            {

                palindrome = result;

            }

        }  

    }

}

Console.Write(palindrome);

Console.Read();

edited Nov 21 '18 at 17:09

answered Nov 21 '18 at 16:13

Matt

476210

2

Before the second for-loop, you could add the following statement: if (i*999 <= palindrome) break; You cannot find a greater palindrome in that case, so you can stop immediately.

– Stefan Illner
Nov 21 '18 at 16:29

Nice call creating a decrescent loop

– Kevin Kouketsu
Nov 21 '18 at 17:17

@KevinKouketsu it was r1verside's idea! Also think you meant decrement not decrescent ?

– Matt
Nov 21 '18 at 17:18

1

@Matt I'm sorry, i'm Brazilian so you know... i`m always trying to write better. Thanks for the advice

– Kevin Kouketsu
Nov 21 '18 at 17:40

add a comment |

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1 Answer
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oldest

votes

I think what has caused so much confusion is the use of String this just complicates thing having to convert them back and forth.

Your program works fine (if the if is moved as per John's comment) if only you'd checked the new number was larger!

here is my take on it:

// stolen from https://www.geeksforgeeks.org/reverse-digits-integer-overflow-handled/

int Reverse(int num)

{

    int rev_num = 0;

    while (num > 0)

    {

        rev_num = rev_num * 10 + num % 10;

        num = num / 10;

    }

    return rev_num;

}





int result = 0;

int palindrome = 0;

int j = 1;

for (int i = 999; i > 0; i--)

{

    for (int y = 999; y > 0; y--)

    {

        result = i * y;

        if (result == Reverse(result))

        {

            if (result > palindrome)

            {

                palindrome = result;

            }

        }  

    }

}

Console.Write(palindrome);

Console.Read();

edited Nov 21 '18 at 17:09

answered Nov 21 '18 at 16:13

Matt

476210

2

Before the second for-loop, you could add the following statement: if (i*999 <= palindrome) break; You cannot find a greater palindrome in that case, so you can stop immediately.

– Stefan Illner
Nov 21 '18 at 16:29

Nice call creating a decrescent loop

– Kevin Kouketsu
Nov 21 '18 at 17:17

@KevinKouketsu it was r1verside's idea! Also think you meant decrement not decrescent ?

– Matt
Nov 21 '18 at 17:18

1

@Matt I'm sorry, i'm Brazilian so you know... i`m always trying to write better. Thanks for the advice

– Kevin Kouketsu
Nov 21 '18 at 17:40

add a comment |

I think what has caused so much confusion is the use of String this just complicates thing having to convert them back and forth.

Your program works fine (if the if is moved as per John's comment) if only you'd checked the new number was larger!

here is my take on it:

// stolen from https://www.geeksforgeeks.org/reverse-digits-integer-overflow-handled/

int Reverse(int num)

{

    int rev_num = 0;

    while (num > 0)

    {

        rev_num = rev_num * 10 + num % 10;

        num = num / 10;

    }

    return rev_num;

}





int result = 0;

int palindrome = 0;

int j = 1;

for (int i = 999; i > 0; i--)

{

    for (int y = 999; y > 0; y--)

    {

        result = i * y;

        if (result == Reverse(result))

        {

            if (result > palindrome)

            {

                palindrome = result;

            }

        }  

    }

}

Console.Write(palindrome);

Console.Read();

edited Nov 21 '18 at 17:09

answered Nov 21 '18 at 16:13

Matt

476210

2

Before the second for-loop, you could add the following statement: if (i*999 <= palindrome) break; You cannot find a greater palindrome in that case, so you can stop immediately.

– Stefan Illner
Nov 21 '18 at 16:29

Nice call creating a decrescent loop

– Kevin Kouketsu
Nov 21 '18 at 17:17

@KevinKouketsu it was r1verside's idea! Also think you meant decrement not decrescent ?

– Matt
Nov 21 '18 at 17:18

1

@Matt I'm sorry, i'm Brazilian so you know... i`m always trying to write better. Thanks for the advice

– Kevin Kouketsu
Nov 21 '18 at 17:40

add a comment |

I think what has caused so much confusion is the use of String this just complicates thing having to convert them back and forth.

Your program works fine (if the if is moved as per John's comment) if only you'd checked the new number was larger!

here is my take on it:

// stolen from https://www.geeksforgeeks.org/reverse-digits-integer-overflow-handled/

int Reverse(int num)

{

    int rev_num = 0;

    while (num > 0)

    {

        rev_num = rev_num * 10 + num % 10;

        num = num / 10;

    }

    return rev_num;

}





int result = 0;

int palindrome = 0;

int j = 1;

for (int i = 999; i > 0; i--)

{

    for (int y = 999; y > 0; y--)

    {

        result = i * y;

        if (result == Reverse(result))

        {

            if (result > palindrome)

            {

                palindrome = result;

            }

        }  

    }

}

Console.Write(palindrome);

Console.Read();

edited Nov 21 '18 at 17:09

answered Nov 21 '18 at 16:13

Matt

476210

I think what has caused so much confusion is the use of String this just complicates thing having to convert them back and forth.

Your program works fine (if the if is moved as per John's comment) if only you'd checked the new number was larger!

here is my take on it:

// stolen from https://www.geeksforgeeks.org/reverse-digits-integer-overflow-handled/

int Reverse(int num)

{

    int rev_num = 0;

    while (num > 0)

    {

        rev_num = rev_num * 10 + num % 10;

        num = num / 10;

    }

    return rev_num;

}





int result = 0;

int palindrome = 0;

int j = 1;

for (int i = 999; i > 0; i--)

{

    for (int y = 999; y > 0; y--)

    {

        result = i * y;

        if (result == Reverse(result))

        {

            if (result > palindrome)

            {

                palindrome = result;

            }

        }  

    }

}

Console.Write(palindrome);

Console.Read();

edited Nov 21 '18 at 17:09

answered Nov 21 '18 at 16:13

Matt

476210

edited Nov 21 '18 at 17:09

answered Nov 21 '18 at 16:13

Matt

476210

answered Nov 21 '18 at 16:13

Matt

476210

answered Nov 21 '18 at 16:13

Matt

476210

2

Before the second for-loop, you could add the following statement: if (i*999 <= palindrome) break; You cannot find a greater palindrome in that case, so you can stop immediately.

– Stefan Illner
Nov 21 '18 at 16:29

Nice call creating a decrescent loop

– Kevin Kouketsu
Nov 21 '18 at 17:17

@KevinKouketsu it was r1verside's idea! Also think you meant decrement not decrescent ?

– Matt
Nov 21 '18 at 17:18

1

@Matt I'm sorry, i'm Brazilian so you know... i`m always trying to write better. Thanks for the advice

– Kevin Kouketsu
Nov 21 '18 at 17:40

add a comment |

2

Before the second for-loop, you could add the following statement: if (i*999 <= palindrome) break; You cannot find a greater palindrome in that case, so you can stop immediately.

– Stefan Illner
Nov 21 '18 at 16:29

Nice call creating a decrescent loop

– Kevin Kouketsu
Nov 21 '18 at 17:17

@KevinKouketsu it was r1verside's idea! Also think you meant decrement not decrescent ?

– Matt
Nov 21 '18 at 17:18

1

@Matt I'm sorry, i'm Brazilian so you know... i`m always trying to write better. Thanks for the advice

– Kevin Kouketsu
Nov 21 '18 at 17:40

Before the second for-loop, you could add the following statement: if (i*999 <= palindrome) break; You cannot find a greater palindrome in that case, so you can stop immediately.

– Stefan Illner
Nov 21 '18 at 16:29

Nice call creating a decrescent loop

– Kevin Kouketsu
Nov 21 '18 at 17:17

@KevinKouketsu it was r1verside's idea! Also think you meant decrement not decrescent ?

– Matt
Nov 21 '18 at 17:18

@Matt I'm sorry, i'm Brazilian so you know... i`m always trying to write better. Thanks for the advice

– Kevin Kouketsu
Nov 21 '18 at 17:40

add a comment |

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