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jotapezm

Wed Jan 13, 2016 6:11 pm


Turing encoding program

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I have created a program which takes a text file, encodes it, prints it into another text file. After that you run the decoding program to input the encoded text file and it decodes back to the original text.


My decoding procedure is not working, it  is not successfully decoding the text file.



Encoding program :
% Encryption
var stream : int

var sWord : string
const dictionary := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
const numbers := "1234567890"
var key : string := "123"
var FileName : string

function encrypt (sWord, key : string) : string
    var s : string
    var news := ""
    s := sWord
    for i : 1 .. length (s)
        if index (dictionary, s (i)) > 0 then
            news += chr (65 +(((ord (s (i)) + 65) + (ord (key (i mod length (key) + 1)) - 65)) mod 26))
        elsif index (numbers, s (i)) > 0 then
            news += chr (49 +(((ord (s (i)) + 49) + (ord (key (i mod length (key) + 1)) - 49)) mod 10))
        else
            news += s (i)
        end if
    end for
    result news
end encrypt



procedure DisplayCharss (FileName : string)
    var stream2 : int
    var stream : int
    open : stream, FileName, get
    open : stream2, "encoded.txt", put
    if stream > 0 then
        var sWord : string

        loop

            %input from file
            get : stream, skip
            exit when eof (stream)
            get : stream, sWord : *

            sWord := encrypt (sWord, key)

            put : stream2, sWord
            put sWord


        end loop

        close : stream
        close : stream2
    else

        put "Error could not open file"

    end if

end DisplayCharss
put " input file name"
get FileName
DisplayCharss (FileName)



Decoding program :

const dictionary := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuwxyz"
const numbers := "1234567890"
var stream : int

var key : string := "123"
var FileName : string
var sWord : string
function decrypt (sWord, key : string) : string
    var news := ""
    var s := sWord
    for i : 1 .. length (s) 
        if index (dictionary, s (i)) > 0 then 
            news += chr (65 + (((ord (s (i)) +65) -(ord (key (i mod length (key) + 1)) +65)) mod 26)) 
        elsif index (numbers, s (i)) > 0 then 
            news += chr (49 + (((ord (s (i)) + 49)-(ord (key (i mod length (key) + 1)) + 49)) mod 10)) 
        else 
            news += s (i) 
        end if 
    end for
    result news
end decrypt
procedure DisplayCharss (FileName : string)
    var stream2 : int
    var stream : int
    open : stream, FileName, get
    open : stream2, "decoded.txt", put
    if stream > 0 then
        var sWord : string

        loop
            %input from file


            get : stream,skip
            exit when eof (stream)
            get : stream, sWord : *
            sWord := decrypt (sWord, key)
            put  : stream2, sWord
            put sWord

        end loop

        close : stream
        close : stream2

    else

        put "Error could not open file"

    end if
end DisplayCharss
put " input file name"
get  FileName
DisplayCharss (FileName)


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Dreadnought

Fri Jan 15, 2016 9:24 pm


Re: Turing encoding program

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In the future, you should wrap your code like this

Your code here
news += chr (65 + (((ord (s (i)) +65) + (ord (key (i mod length (key) + 1)) -65)) mod 26))

Think carefully about the logic here (hint: one or more +'s should be -'s or vice versa). This should fix most of your decoding. 

To fix the rest, the ASCII standard (the standard that defines which letters correspond to which numbers) characters 48 to 57 are "0123456789" and characters 65 to 122 are "ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz". The important things to notice are that 0 appears before 1 and that there are non-alphabetic characters between Z and a. 

I also noticed that you like to use values like 49 and 65 in your program. You might want to consider writing these using [tdoc]ord[/tdoc] such as ord('A') for 65. This will make your code more readable, and make it harder to enter the wrong number.