Trouble using Array.Reverse

Question

I am writing code to check for symmetry within a string. I convert to a Char array and then I would like to use the Array.Reverse method. I keep getting a cannot convert type void to Char[]. Any insight would be greatly appreciated my code is as follows.

namespace ConsoleApp1
{
  class Program
  {
    static void Main(string[] args)
    {
        checkPalindrome("aabaa");
    }
   public static bool checkPalindrome(string inputString)
    { 
        char[] charArray = inputString.ToCharArray();
        char[] reverseArray = Array.Reverse(charArray);
        Array.Reverse(charArray);

    //Console.Write(charArray);
    //Console.Write(reverseArray);

    if (charArray = Array.Reverse(charArray))
    {
        return true;
    }
    else
    {
        return false;
    }
  }

  }
}

@huck_cussler Array.Reverse returns void, so neither the assignment nor the comparison will work. — user47589
– user47589, Commented May 8, 2018 at 15:16
@Amy Correct. I will edit my comment to note that it is not relevant to OP's actual question. Thanks. — Tony Tuttle
– Tony Tuttle, Commented May 8, 2018 at 15:17
Array.Reverse is an in place reverse, meaning it mutates the array. — juharr
– juharr, Commented May 8, 2018 at 15:17
@huck_cussler Even it if did return an array comparing them would be a reference equality check, not a check that each item in both arrays is equal. — juharr
– juharr, Commented May 8, 2018 at 15:19

Ehsan Sajjad · Accepted Answer · 2018-05-08 16:21:29Z

8

Your problem is here:

char[] reverseArray = Array.Reverse(charArray);

Array.Reverse changes the contents of the array that you pass it. It doesn't return any value.

You can use LINQ to do something like this instead:

char[] reverseArray = charArray.Reverse().ToArray();

Then, when you're checking for equality, you need to do more than the default object-equals type of equality check:

if (charArray.SequenceEqual(reverseArray))

Also, .ToArray() is only necessary if you need an array, and you don't need one for SequenceEqual(). And the if(something) {return true;} else {return false;} pattern can be simplified to return something;

So the entire method can actually be written in a single line:

public static bool checkPalindrome(string inputString)
{ 
    return inputString.Reverse().SequenceEqual(inputString);
}

edited May 8, 2018 at 16:21

Ehsan Sajjad

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answered May 8, 2018 at 15:15

StriplingWarrior

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2 Comments

Ginkobaloba Over a year ago

I truly appreciate your solution to my entire algorithim I will use your code in my final product. I am still confused as to why Array.Reverse(genericArray) cannot be applied to a simple if statement I still encounter the same char [] cannot be compared to void.

StriplingWarrior Over a year ago

@AndrewMattick: An if statement has to take a boolean expression, and your boolean expression is a "binary expression" (meaning it has two parts) around the == operator. The left-hand side is a char[], but the right-hand side is a call to a method that doesn't return anything (i.e. it's a "void" method). Calling Array.Reverse causes a side-effect on the parameter you give it, but since no value is returned, the method call can't be used in a context that needs to have some kind of value.

juharr · Accepted Answer · 2018-05-08 15:22:18Z

0

Here's a better way to determine symetry in a string.

public bool IsSymetric(string str)
{
    if(str == null) return true; // or false or throw an exception

    for(int i = 0; i < str.Length/2; i++)
    {
        if(str[i] != str[str.Length - 1 - i])
        {
            return false;
        }
    }

    return true;
}

answered May 8, 2018 at 15:22

juharr

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Comments

armenm · Accepted Answer · 2018-05-08 15:34:50Z

0

A better way - no array cloning/array extraction:

    static bool checkPalindrome(string inputString)
    {
        var length = inputString.Length;
        var half = length / 2 + 1;
        return length <= 1 || inputString.Take(half).SequenceEqual(inputString.Reverse().Take(half));
    }

edited May 8, 2018 at 15:34

answered May 8, 2018 at 15:21

armenm

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2 Comments

adjan Over a year ago

why do you call the extension method in such a weird way

armenm Over a year ago

To kinda emphasize the symmetry or the compared objects, but you're right - I changed it now. And optimized a little to check only the half.

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