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Create a class SingaporeNumbers.
 Part 1
 A typical vehicle registration number comes in the format xxx #### y: 
• x – prefixes
• #### – Numerical series (from 1 to 9999, without leading zeroes)
• y – Checksum

 • The checksum letter is calculated by converting the letters into numbers, i.e., where A=1 and Z=26, potentially giving seven individual numbers from each registration plate. However, only two letters of the prefix are used in the checksum. For a three-letter prefix, only the last two letters are used; for a two-letter prefix, both letters are used; for a single letter prefix, the single letter corresponds to the second position, with the first position as 0. For numerals less than four digits, additional zeroes are added in front as placeholders, for example "1" is "0001". SBS 3229 would therefore give 2, 19, 3, 2, 2 and 9 (note that "S" is discarded); E 12 would give 0, 5, 0, 0, 1 and 2. SS 108 would be given as 19, 19, 0, 1, 0, 8. 

• Each individual number is then multiplied by 6 fixed numbers (9, 4, 5, 4, 3, 2). These are added up, then divided by 19. The remainder corresponds to one of the 19 letters used (A, Z, Y, X, U, T, S, R, P, M, L, K, J, H, G, E, D, C, B), with "A" corresponding to a remainder of 0, "Z" corresponding to 1, "Y" corresponding to 2 and so on. In the case of SBS 3229, the final letter should be a P; for E 23, the final letter should be a H. SS 11 back letter should be a T. The letters F, I, N, O, Q, V and W are not used as checksum letters. Write a static method car_plate_checksum that returns the checksum from a given string. You should use the try and except blocks to find out is a character in a string is an integer or not. The input string may contain 1-3 letters for prefixes while there can be 1 to 4 digits for the numerical series that follows.


What I have tried:

Python
 This answers is not correct

def car_plate_checksum(registration_number):
        # Mappings of the remainder corresponding to characters as given
        remainder_mappings = {0: "A", 1: "Z", 2: "Y", 3: "X", 4: "U", 5: "T", 6: "S", 7: "R",
                              8: "P", 9: "M", 10: "L", 11: "K", 12: "J", 13: "H",
                              14: "G", 15: "E", 16: "D", 17: "C", 18: "B"}
        # Weights as given
        weights = [9, 4, 5, 4, 3, 2]

        numericals = list()
        prefix = ""
        # Using try and catch we first separate out the numericals and the alphabets from the input
        for character in registration_number:
            try:
                numericals.append(int(character))
            except ValueError:
                prefix += character

        # Remove whitespace character at the end
        prefix = prefix.strip()

        # According to the conditions given, we determine the effective prefix depending on the number
        # of alphabets in the prefix
        if len(prefix) == 3:
            effective_prefix = prefix[1:]
        elif len(prefix) == 2:
            effective_prefix = prefix
        elif len(prefix) == 1:
            effective_prefix = "0" + prefix

        # We convert all the alphabets to their numeric representation as given and store it in a list
        prefix_numericals = []
        for prefix_char in effective_prefix:
            if prefix_char == "0":
                prefix_numericals.append(0)
            else:
                prefix_numericals.append(ord(prefix_char) - 64)

        # Numericals is going to be the list which will hold all the digits needed for final checksum calculation
        # We add 0s if needed (when number of given digits is less than 4) to the list.
        numericals = [0] * (4 - len(numericals)) + numericals
        numericals = prefix_numericals + numericals

        # We multiply each of the numbers to their given corresponding weights and sum them up
        elementwise_product = [a*b for a, b in zip(numericals, weights)]
        sum_elements = sum(elementwise_product)

        # We determine the remainder after diving the sum of products by 19
        remainder = sum_elements % 19

        # We derive the required checksum from the remainder_mappings dict
        checksum = remainder_mappings[remainder]
        return checksum
Posted
Updated 12-Nov-20 21:07pm
v2
Comments
Richard MacCutchan 13-Nov-20 4:21am    
What do you mean "is not correct", what is wrong with it?

Just because it runs does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
Python
def Double(value):
   return value * value;

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
See here: pdb — The Python Debugger — Python 3.9.0 documentation[^]
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
 
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Comments
Maciej Los 13-Nov-20 2:06am    
Hmmm... You say, writing an email... Agree!
The solution is up to you. As for the 'easy solution', "there is no Royal Road to Geometry".
The requirements are clear, the task is straightforward but laborious. Good luck and check carefully each step.
 
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Quote:
This answers is not correct

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

27.3. pdb — The Python Debugger — Python 3.6.1 documentation[^]
Debugging in Python | Python Conquers The Universe[^]
pdb – Interactive Debugger - Python Module of the Week[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.
 
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