VBA error: A value used in the formula is the wrong data type? - vba

Question

I'm trying to create a function which generates a secret encoded message. It takes in three things: a string, for example, "testingtestingonetwothree", as well as the desired number of characters, for example 5, and the desired number of words, for example 5. It generates the message by starting at the first character, and extracting every fifth character through the string, putting these characters into a codeword, then starting at the second character and extracting every fifth character through the string, putting these into a second codeword, and so on. It just outputs a string, with the codewords separated by a space. So for this example it would produce: "tntnt egieh stntr tegwe isooe".
I'm okay at coding but new to VBA. I've made what I think is a valid function, but when it's used in the spreadsheet I get a #VALUE! error: "A value used in the formula is the wrong data type". This is the user defined function I made:
Function encode(strng, numchars, numwords)
Dim word As Integer
Dim step As Integer
Dim temp As String
Dim output As String
For word = 1 To numchars
step = word
temp = ""
Do While step <= Len(strng)
temp = temp & Mid(strng, 1, step)
step = step + numchars
Loop
If word = 1 Then output = temp Else output = output & " " & temp
Next word
encode = output
End Function
And when it's used in the spreadsheet I just call it, as in
=encode(A16,A7,A10)
Where A16 contains testingtestingonetwothree, A7 contains 5 and A10 contains 5.
Does my function seem okay? And is there anything you guys can see which could be giving the value error? Any help would be greatly appreciated, thanks a lot for reading.
EDIT: This now outputs a value, but the wrong value. It outputs: "ttestintestingtesttestingtestingontestingtestingonetwot tetestingtestingtestitestingtestingonetestingtestingonetwoth ", when it should output: "tntnt egieh stntr tegwe isooe". Is there anything you guys can see that my function is doing wrong?
EDIT2: After fixing the Mid function, to
temp = temp & Mid(strng, step, 1)
as per vacip's answer, the function now produces the correct answer.

Answer 1

Ok, everyone says it works, but for me, it doesn't produce the desired output. What the...???
Anyway, I think your Mid function is in the wrong order, try it like this:
temp = temp & Mid(strng, step, 1)
Also, make sure to properly declare your variables, like this:
Function encode(strng As String, numchars As Integer, numwords As Integer) As String
I have also rewritten your IF statement, that one-line thing is strange for me...
If word = 1 Then
output = temp
Else
output = output & " " & temp
End If
This way it worked for me.

Answer 2

Other people have addressed the type problem. Here is a different suggestion. The cipher that you are describing is a simple transposition cipher, specifically a columnar transposition ( https://en.wikipedia.org/wiki/Transposition_cipher#Columnar_transposition )
The way people did this pre-computer was to write the characters into a grid row by row then read them off column by column. In fact -- this is probably still the easiest way to implement it even with computers. Declare a variant which can be redimensioned to be an array with e.g. 5 columns (where 5 is the skip between letters) and the number of rows is chosen to be large enough so that the grid can hold the string. After you load up the characters row by row, read them off column by column using nested for loops.
Once you get a basic example working, you can try to implement a version which uses a key to determine the order that you read off the columns for added security.
Coding classical cryptography/cryptanalysis as an excellent way to learn a programming language. Almost the first thing I do when I try to learn a new language is to implement a Vigenere cipher in it. Even though it is long out of print and can be somewhat tricky to translate to modern dialects of Basic the book "Cryptanalysis for Microcomputers" by Caxton Foster is great fun and can be purchased for just a few dollars from online used bookstores.

Answer 3

You need to define your Function's type. So in this case I believe you would want
Function encode(strng, numchars, numwords) As String

Answer 4

I tested your code exactly as it is, and it worked fine.
So, your problem may be:
A certain argument of your function is not the right type. (I bet the len method is the problem in there).
Check if A16 is really a string. If not, consider converting it to a string before if you want to pass numbers too:
Function encode(strng as variant, numchars as integer, numwords as integer) as string
strng = str(strng)
Check also if A7 and A10 are really integers.