如何比较两个字符串是否相同？

Question

So I'm doing this years Advent of Code and I'm stuck on the second day, part 2.

You are given inputs which look like this:

 "1-3 c: caaasa"

You have to check how many passwords are valid due to the policy like, in above example:

letter c has to be in position 1 OR 3 in the string caaasa . If yes, the password is valid.

I've broken down that string to different sections, and now I try to compare a string "znak" which contains that given letter to a letter on position zakresmin and zakresmax in string "passdiv"

Yet, everytime it returns False , so it doesn't add up to the count of passwords.

I tried using Equals() and CompareTo() , but they don't seem to work.

How can I modify my code so it returns proper values?

var iloschasel = 0;
using (StreamReader sr = new StreamReader(@"C:\Users\Wurf\Desktop\text.txt"))
{
   string line;
   while ((line = sr.ReadLine()) != null)
   {
      string[] linia = line.Split(" ");
      string zakres = linia[0];
      string[] zakresy = zakres.Split("-");
      int zakresmin = Convert.ToInt32(zakresy[0]);
      int zakresmax = Convert.ToInt32(zakresy[1]);
      string znak = (linia[1].Replace(":", "")).Trim();
      var suma = Regex.Matches(linia[2], znak);
      string passdiv = linia[2];
      if(passdiv[zakresmin].Equals(znak) || passdiv[zakresmax - 1].Equals(znak))
      {
         iloschasel += 1;
      }

   }
}
Console.WriteLine(iloschasel);

Answer 1

As mentioned, when you call Equals on two different types you are playing a game of chance with how the actual types are implemented. In this case you lose. Strings and chars will never have an equivalence or the same reference.

_{I believe the compiler or resharper would give you a warning alerting you that neither type derive from string and char}

However, I was bored enough to give an alternate solution

public static bool IsValid(string input)
{
   var match = Regex.Match(input, @"(\d)-(\d) (\S): (.*)");
   if(!match.Success)
      throw new ArgumentException( $"Invalid format : {input}",nameof(input));

   var first = int.Parse(match.Groups[1].Value);
   var second = int.Parse(match.Groups[2].Value);
   var c = char.Parse(match.Groups[3].Value);
   var password = match.Groups[4].Value;

   return password[first-1] == c && password[second-1] == c;
}

Test

Console.WriteLine($"Is Valid = {IsValid("1-3 c: caaasa")}");
Console.WriteLine($"Is Valid = {IsValid("1-3 c: cacaasa")}");

Output

Is Valid = False
Is Valid = True

_{Note : this is not meant to be a complete bullet-proof solution. Just a novel elegant way to solve your problem}

Answer 2

Your problem is that you are comparing a string to a char

var match = "c" == 'c'; 

Will give a compile error because they are different data types

var match = "c".Equals('c');

will let you compile, but will always return false because a char will never equal a string. You have to turn the char into a string or visa versa for the check to work

var match = "c"[0] == 'c'; 

So in your if statement, if you fix the check to compare strings with strings or chars with chars you should get some positive results. And also fix your indexing issue to decide if you want a 0 based index or a 1 based index with zakresmin and max

Also as a side note, it can be helpful to step through your code line by line in debug mode, to find out which line isn't behaving like you expect it to. In your case debugging would have helped you zero in on the if statement as a starting point to fixing things.

Answer 3

So it turns out (if I understand that correctly) that a compared element of the string passdiv was a char which I tried to compare to znak which was a string . I added ToString() to my code and it works well. Also fixed the range of zakresmin by subtracting 1 so it works properly.

if((passdiv[zakresmin - 1].ToString() == znak && passdiv[zakresmax - 1].ToString() != znak) || (passdiv[zakresmin - 1].ToString() != znak && passdiv[zakresmax - 1].ToString() == znak))
                    {
                        iloschasel += 1;
                    }