without using reverse.string()

Question

1.00/5 (2 votes)

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C#

string s,revs="";
            Console.WriteLine(" Enter string");
            s = Console.ReadLine();
            for (int i = s.Length-1; i >=0; i--) //String Reverse
            {
                revs += s[i].ToString();
            }
            if (revs == s) // Checking whether string is palindrome or not
            {

Posted 4-Dec-15 1:15am

Member 12185899

Updated 4-Dec-15 1:29am

v3

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Comments

Patrice T 4-Dec-15 7:25am

What is the question ?

Member 12185899 4-Dec-15 7:30am

we should not use reverse.string()

Patrice T 4-Dec-15 7:38am

This is not a question.

Member 12185899 4-Dec-15 8:05am

check whether a string is paliandrome ot not
reverse the string is same as actual
EXAMPLE:madam
note:we should not use string.reverse()

3 solutions

Solution 3

Further to solution 1 ...

The phrase "A dog! A panic in a pagoda!" is deemed to be a palindrome (according to this list[^]) but solution 1 will not agree because of the whitespace, punctuation marks and capitalisation

So an alternative solution is to remove all whitespace and non-character or digit characters from the source text and ensure it's all the same case e.g.

C#

private static bool IsPalindrome(string sourceText)
{
    var testChars = new string((from c in sourceText.ToLower()
                                where char.IsLetterOrDigit(c)
                                select c).ToArray());

    for (var i = 0; i <= (testChars.Length % 2); i++)
        if (testChars[i] != testChars[testChars.Length - i - 1]) return false;

    return true;
}

Posted 4-Dec-15 2:00am

CHill60

Comments

Matt T Heffron 4-Dec-15 12:32pm

+5But a challenge:
Can it be done without preprocessing the string? ;-)

CHill60 8-Dec-15 9:43am

Interesting - the case problem can obviously be moved to the if-statement. I'll have a little think about the IsLetterOrDigit ... I'd have to have anther counter of course for the back end. Might have a play with that if I get time

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Andy Lanng · Accepted Answer · 2015-12-04T01:35:00

Solution 1

well I would do it like this:

C#

string s = "racecar";
bool b = s == s.Reverse().ToString();

but as you said not to use reverse:

C#

string s = "racecar";
for (int j = 0; j < (s.Length / 2) - s.Length % 2; j++)
{
  if (s[j] != s[s.Length - 1 - j])
    return false;
}
return true;

Posted 4-Dec-15 1:35am

Andy Lanng

Comments

Sergey Alexandrovich Kryukov 4-Dec-15 10:54am

5ed, see my comment to Solution 2.
—SA

CPallini · Accepted Answer · 2015-12-04T01:36:00

Solution 2

Instead hand-crafting a reverse method, you could iterate over the characters to find out if the string is palindrome or not.

Posted 4-Dec-15 1:36am

CPallini

Comments

Sergey Alexandrovich Kryukov 4-Dec-15 10:54am

It's already done in solution 1, but the time is the same, so a 5. :-)
The wise point here is: this iteration and checkup is much cheaper (no matter if it is important factor or not) then even calling existing string.Reverse. It would create a new string object, which we don't need.
—SA

Andy Lanng 4-Dec-15 12:11pm

agreed - I would still use reverse because I'm just lazy like that. Also, performance isn't often an issue I need to worry about.

PS: I also think solution 3 made a good point regarding special chars (although the execution looks a little broken)

Sergey Alexandrovich Kryukov 4-Dec-15 12:56pm

Not at all — it was you who demonstrated how to work without creating a new string. Special char? — there is no such thing. It depends on what sense of the words "palindrome" is required to be applied, but you solution would work on letters after a minimal modification.
—SA

CPallini 4-Dec-15 15:21pm

Thank you.