# E-assessment of coding theory in Numbas

Christian Lawson-Perfect, Newcastle University

## Context

• Stage 3 coding theory module.
• Lecturer had 5 written homeworks, very procedural, sick of marking.
“CP, can you help?”

## The maths

A code is a set of codewords that are each strings over a finite alphabet ($$\mathbb{Z}_p^n$$ for our purposes).

A linear code is closed under addition.

Lots of linear algebra involved: matrices to generate codes, check codewords, etc.

Example:

$[4,2]\text{-repetition} = \{ \mathtt{0000}, \mathtt{0101}, \mathtt{1010}, \mathtt{1111} \}$

## Recipe

1. Receive homework sheets, solutions, and course notes from lecturer
2. Identify commonly-used routines, bundle them in an extension
3. Transcribe written questions into Numbas, without randomisation
4. Add in randomisation
5. Lecturer tests everything works
6. Delivered to students!

## In practice

Implementing the algorithms was fun.

But YEESH avoiding bugs was hard.

Having a "codeword" as a data type in Numbas made writing questions very easy.

## In practice

All but one of the written questions could be marked automatically.

We improved some questions, and added some more formative questions on top.

A couple of bugs found by students - equal mix of my fault and faulty algorithm from the lecturer.

## Codewords extension

Adds code and codeword data types,
and functions to work with them.

Example:


A = repeat(
codeword(repeat(random(0..field_size-1),word_length-dimension),field_size),
dimension
)

basis = identity_left(A)

C = code(set_generated_by(parity_check_matrix(basis)))

min_distance = minimum_distance(C)


Also provides scripts to mark lists of codewords.

## Permutations extension

I also used an extension I'd written for a group theory course, which adds routines for dealing with groups of permutations (elements of $$S_n$$).

github.com/numbas/numbas-extension-permutations

It helped with things like randomising generating matrices.

## Results

codewords extension available to everyone.

20 questions, free to reuse on mathcentre.