HackerRank Caesar Cipher, not using map, filter, reduce, regex

Enumerating UTF-8 characters: Your code uses two nested loops to encrypt letters (based on the UTF-8 code):

var  eMessage = ""
let arr = messageToCipher.characters.map { String($0) }
for ch in arr {
    for code in String(ch).utf8 {
        if (65<=code && code<=90) || (97<=code && code<=122) {
            // ... translate `code` and append to `eMessage` ...
        }
        else{
            eMessage = eMessage + ch
        }
    }
}

This is unnecessary complicated and has an unwanted side effect if the message contains non-ASCII characters (which are encoded as 2 or more UTF-8 code units):

print(cipher(messageToCipher: "ä € 😀 🇧🇩", k: 2) )
// ää €€€ 😀😀😀😀 🇧🇩🇧🇩🇧🇩🇧🇩🇧🇩🇧🇩🇧🇩🇧🇩

This could be solved by adding a break in the else-case. The better solution is to enumerate the UTF-8 code units directly:

var encoded: [UInt8] = []
for code in messageToCipher.utf8 {
    if (65<=code && code<=90) || (97<=code && code<=122) {
        // ... translate `code` and append to `encoded` ...
    }
    else{
        encoded.append(code)
    }
}
return String(bytes: encoded, encoding: .utf8)!

Code duplication: There is some duplicate code in

            if k > 26{
                let value =  k % 26 == 0 ? k / 26 : k % 26
                var pCode  = ((Int(code))  + (value))
                if (pCode  > 122 && (97<=code && code<=122)) || (pCode  > 90 && (65<=code && code<=90)) {
                    pCode = pCode - 26
                }
                let s = String(UnicodeScalar(UInt8(pCode)))
                eMessage = eMessage + s
            }else
            {
                var pCode  = Int(code) + k
                if (pCode  > 122 && (97<=code && code<=122)) || (pCode  > 90 && (65<=code && code<=90)) {
                    pCode = pCode - 26
                }
                let s = String(UnicodeScalar(UInt8(pCode)))
                eMessage = eMessage + s
            }

which is not necessary. The if-part works for the case k <= 26 as well.

Exceptional key values: The special handling of the case k % 26 == 0 causes unexpected results:

print(cipher(messageToCipher: "abc xyz ABC XYZ", k: 26) ) // abc xyz ABC XYZ
print(cipher(messageToCipher: "abc xyz ABC XYZ", k: 52) ) // cde zab CDE ZAB

and is not needed. Negative key values are not handled at all:

print(cipher(messageToCipher: "abc xyz ABC XYZ", k: -2) )   // _`a vwx ?@A VWX
print(cipher(messageToCipher: "abc xyz ABC XYZ", k: -200) )
// Fatal error: Negative value is not representable

Both issues can be solved by computing the remainder of k modulo 26, as a number in the range 0...25:

var value = k % 26
if value < 0 { value += 26 }

In addition, this computation can be done once, before entering the loop.

Other issues:

• I would use some different argument/variable names, e.g. message instead of messageToCipher (it is clear from the context and the function name that this string should be encrypted), or key instead of k.

• There are unnecessary parentheses, e.g. in

  var pCode  = ((Int(code))  + (value))
  

• The usage of white space is not consistent, e.g. in

  if (pCode  > 122 && (97<=code && code<=122)) || (pCode  > 90 && (65<=code && code<=90)) {
  

Summarizing the suggestions so far, we have

func cipher(message: String, key: Int) -> String {

    var offset = key % 26
    if offset < 0 { offset += 26 }

    var encoded: [UInt8] = []
    for code in message.utf8 {
        if (65 <= code && code <= 90) || (97 <= code && code <= 122) {
                var pCode  = Int(code) + offset
                if (pCode > 122 && (97 <= code && code <= 122)) || (pCode > 90 && (65 <= code && code <= 90)) {
                    pCode -= 26
                }
                encoded.append(UInt8(pCode))
        } else {
            encoded.append(code)
        }
    }
    return String(bytes: encoded, encoding: .utf8)!
}

Make it functional: If the encryption of a single character is moved to a separate function then one can apply this to the UTF-8 view with map():

func cipher(message: String, key: Int) -> String {

    var offset = key % 26
    if offset < 0 { offset += 26 }

    func utf8encrypt(code: UInt8) -> UInt8 {
        // ... compute and return encrypted character ...
    }

    return String(bytes: message.utf8.map(utf8encrypt), encoding: .utf8)!
}

Further suggestions:

• Use a switch-statement with range patterns instead of the if-conditions:

  switch code {
  case 65...90:
      // ...
  case 97...122:
      // ...
  default:
      // ...
  }
  

• Use constants instead of the literal numbers, e.g.

  let letter_A = 65
  let letter_Z = 90
  let letter_a = 97
  let letter_z = 122
  

so that a future reader of your code does not have to guess what the numbers stand for.