Get previous character eg: S to R

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Hello,
what is the best way to get previous character in A-Z, i am having some APIs that return character say 'E' & am thinking to write a function that will return me 'D' ( previous charater)

is there any direct function in c# that returns previous charater in 26 alphabets

Thanks

What I have tried:

C#

function ( string "S")
(
return "R"
)

Posted 18-Apr-24 21:27pm

Member 14224038

Updated 19-Apr-24 5:45am

PIEBALDconsult

v2

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PIEBALDconsult 19-Apr-24 11:48am

Personally, I would probably set up a map of characters in an array.
But I would need more detail. Are you actually supposed to be implementing a Caesar Cipher?

Caesar cipher - Wikipedia[^]

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OriginalGriff · Answer 1 · 2024-04-18T21:42:00

Solution 1

Yes: subtraction ...

C#

using System;
					
public class Program
    {
	public static void Main()
    	{
		string str = "Hello World";
		foreach (char c in str)
    		{
			Console.Write((char)(c - 1));
    		}
		Console.WriteLine();
    	}
    }

Obviously, you need to check for punctuation and decide what to do when presented with an "a" or "A", but that's pretty trivial.

Posted 18-Apr-24 21:42pm

OriginalGriff

Updated 18-Apr-24 21:43pm

v3

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Member 14224038 19-Apr-24 3:49am

OK so i should add string "ABCDEFGHIJKLMNOPQRSTUVWXYZ" & will do c-1 thanks

OriginalGriff 19-Apr-24 3:58am

As I said, "you need to check for punctuation and decide what to do when presented with an "a" or "A""

Richard Deeming · Answer 2 · 2024-04-18T21:48:00

Given a char, you can subtract 1 to get the previous code-point:

C#

char input = 'E';
char output = (char)(input - 1); // Contains 'D'

You just need to decide what range of characters you want to support, and validate that the input is within that range.

For example, if input is 'A', then output will be '@'.

For 'a', you will get '`'.

Look through the list of Unicode characters[^] to see the supported characters.

Pete O'Hanlon · Answer 3 · 2024-04-19T01:40:00

To add to the answers given above. You should never limit your code to just consider the latin alphabet; there are so many cases where you need to consider international users. Fortunately, this is very simple to cater for. In the following snippet, I'm using a French character (E with a Grave), and I detect whether or not it's a letter. If it is, I then go on to get the previous character. Again, I check to see if it's a letter.

C#

char character = 'È';
System.Diagnostics.Debug.WriteLine($"Is a character {char.IsLetter(character)}");

if (char.IsLetter(character))
{
  char previousCharacter = (char)(character - 1);
  if (char.IsLetter(previousCharacter))
  {
    System.Diagnostics.Debug.WriteLine(previousCharacter);
  }
}

It really is that simple.

For sheets and giggles, I changed my input character to 字, and this returns 孖 as the previous character.

Get previous character eg: S to R

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