# On Teaching Software Engineering

## Proving why ''those who can't, teach.''

When I was a Computer Science undergraduate, at the turn of the millennium, by far the least engaging subjects that I studied were those on Software Engineering. Few know how to make Gantt charts exciting, and discussions of software development process models have never been my cup of tea, even if I did develop a real appreciation for empirical software engineering in the meantime. I deeply dig the irony, therefore, that I chose to be a Software Engineering teacher in 2016. (More on that choice some other time.)

In this post I want to share a snippet of code I came across in some course material from an unnamed university, while I was researching ideas for 2016’s instalment of SWEN90006 - Software Testing and Reliability. It reinforced my view that software engineering teaching needs to be rooted in a deep appreciation for programming. Absent such understanding, it is all too easy to find yourself modelling worst practices to your students.

# What’s Wrong With This Code?

 1        int fibonacci( int N ) {
2            int i, j, Sum;
3            int *C;
4
5            C = malloc( sizeof( double ) * ( N + 1 ) );
6            if( C == NULL )
7                FatalError( "Out of space!!!" );
8
9            C[ 0 ] = 1; C = 1;
10            Sum = 2;
11
12            for( i = 2; i <= N; i++ ) {
13                C[i] = C[i-1] + C[i-2];
14                Sum += C[i];
15            }
16
17            free( C );
18            return Sum;
19        }

It’s been over 10 years since I did any serious C programming; these days I spend far more time writing proofs than I do programs. But even to my eye this code is obviously poor, and objectively exhibits a number of poor engineering choices (outlined below) with real negative effects (i.e. it’s buggy as hell). That it (purportedly) implements such a trivial function only makes things worse: trivial code ought to be nothing but. While I’m sure it was included with the best of intentions, its presence in a syllabus on how software should be engineered, of all things, is disappointing. By putting this snippet in your teaching materials, you’re proving the adage that ‘’those who can’t [do], teach.’'

So let’s see why. (For the curious, at the end of this post I’ve included my own model solution to show how this code could have been written to avoid the issues I outline below.)

## Specification

The accompanying material says that function’s spec is to:

compute the sum of the first N fibonacci numbers: 1, 1, 2, 3, 5 …

This brings us to the first problem with the code: it doesn’t meet the spec.

Even assuming a perfect world, where undefined behaviour was absent and C’s designers had heeded the principles of safe defaults and least surprise1, the spec that this code implements is, at best, to:

compute the sum of the first N+1 fibonacci numbers or the result 2, whichever is larger.

There is a straightforward off-by-one error. However, even ignoring that, fibonacci(0) is still liable2 to return 2.

## Design

Spec aside, let’s assume that the second specification was the one intended. This then brings us to the function’s design, i.e. how it does what the revised spec says it ought to. Roughly speaking, the design that this code implements is:

to compute the sum of the first N+1 fibonacci numbers, dynamically allocate an array of sufficient length to hold them all, populate the array and, while doing so, sum its elements.

The use of dynamic memory allocation is totally unnecessary: computing the kth fibonacci number (for k >= 2) requires only the preceding two fibonacci numbers, not all of the preceding ones.3

By unnecessarily using dynamic memory allocation, its design breaks the fundamental principle of simplicity (returning yet again to Saltzer and Schroeder, it violates the principle of economy of mechanism). This poor choice has real negative implications.

Firstly, its space complexity is now O(N). But secondly, this one design decision then forces an impossible choice onto the programmer: what to do when malloc() fails? There is no sensible way for fibonacci() to indicate this failure to its caller. The programmer in this case has resolved this impossible situation by having fibonacci() abort execution entirely when malloc() fails.

That consequent choice, while borne of an impossible situation, is disastrous. Any function that calls fibonacci() now runs the risk of having the entire program’s execution aborted. Thus fibonacci() has lost all pretensions to modularity: it can now be used only in the most restrictive of situations.

Returning to the specification above, it might be (a little) more correct to say, then, that the effects of calling fibonacci(N) are to:

compute the sum of the first N+1 fibonacci numbers or the result 2, whichever is larger, or nondeterministically abort all execution.

## Implementation

As with the spec, let us assume that the design is legitimate, and turn our attention to how this function’s design has been rendered in code, i.e. its implementation, in this case in C.

As I said above, I’m no C programmer; however, the bug density here is awesome.

• line 1: choice of inappropriate argument type (namely signed integer) to represent a non-negative integer value;
• line 5: signed integer overflow on addition or multiplication will cause undefined behaviour;
• line 9: possibility of out-of-bounds array access;
• lines 13 and 14: signed integer overflow on addition, as above, plus potential array access out-of-bounds.

The choice of the signed type int for the argument N (and then also the local variables) is a fatal first step that is then compounded by the potential for overflow when computing the amount of memory to allocate for the array C, leading to C being allocated with a size far too short to accommodate the subsequent array accesses.

Other odd or poor implementation choices include:

• line 1: lack of common variable naming scheme (specifically, inconsistent use of upper vs. lower case);
• line 2: unused variable j declared;
• line 5: use of double in the sizeof expression, for an array of ints.
• line 6: the condition C == NULL: one typo would render that condition C = NULL, which would be fatal for the memory-safety of this program.4

## Testing and Maintenance

Rounding out the traditional software development lifecycle, the poor design and implementation choices above mean that this function will be both difficult to test and difficult to maintain. Let me never be a software tester or quality assurance engineer who has to pay the price for somebody else’s poor design and implementation choices.

# Model Solution

So how could this code have been better designed and written? Please chime in with your own solution in the comments. However, I would have written it like so:

 1        unsigned int fibonacci(unsigned int n) {
2            unsigned int a = 0, b = 1;
3            unsigned int sum = 0;
4            while (n > 0) {
5                unsigned int t = b;
6                b += a;
7                a = t;
8                n -= 1;
9                sum += a;
10            }
11            return sum;
12        }

This code:

• Correctly5 implements the original specification (literally modulo the effects of fixed-width machine word arithmetic) to compute the sum of the first n fibonacci numbers: 1, 1, 2, 3, 5 …

• Avoids the O(N) space complexity of the original; its space complexity O(1).

• Avoids dynamic memory allocation.

• Won’t cause unwanted effects (like halting all execution), so is far more modular (a virtue of being a pure function).

• Is free of undefined behaviour and other implementation bugs.

• Is far easier to test and maintain than the original.

• At about 60% of the size of the original, is also more concise while being no less readable.

# Lessons

What lessons should we take away as teachers?

1. Take the time to properly understand the programming language in which you cast your teaching examples.

2. Model best practices in your example material.

3. Consider using type- and memory-safe languages for your examples, to limit the number of effects you need to worry about, unless what you are teaching is relevant only to unsafe languages like C.

4. Consider having your software engineering materials vetted by an experienced software engineer.

5. Spend some time programming. I pity any software engineering teacher who has never contributed to an open source project or otherwise spent time working with (and learning from) great programmers.

I’d love to hear your thoughts.

 I’m indebted here to @alexmurray who pointed out the parallel between C’s choice that int should denote signed integer, making signed the default interpretation, and Saltzer and Schroeder’s principles.

 I say ‘’liable’’ because I can’t tell from reading the C standard whether fibonacci(0) might exhibit undefined behaviour (e.g. whether an array of length sizeof(double) is guaranteed to accommodate two ints), thereby absolving the compiler of having it return any specific answer.

 This follows trivially from the mathematical definition of the fibonacci sequence, which may be defined as follows. Here we’ll write fib k to denote the kth fibonacci number indexed from zero (i.e. fib 0 is the first fibonacci number.)

fib 0       = 1
fib 1       = 1
fib (n + 2) = fib n + fib (n + 1)  -- for n >= 0

        if( C = NULL )
FatalError( "Out of space!!!" );
then it would always assign NULL to C but likely never take the then-branch (i.e. never invoke FatalError()) as the value that the statement c = NULL evaluates to is the right-hand-side of the assignment, i.e. NULL, which is falsy.

@alexmurray informs me that Clang will warn about such mistakes, while gcc does not unless -Wparentheses or stronger is given.

 How do I know this code is correct? It is pretty easy to see that it is memory-safe and free of undefined behaviour, and so forth. But everyone knows that it is all too easy to make a simple off-by-one error, for instance, as the original code does. I believe this code has no such errors because I can prove it is correct.

While this blog post is not the place for a tutorial on Hoare logic, the way to prove that a loop is correct is by defining a loop invariant. This is a logical statement that:

• is true right before the loop,
• is true after each loop iteration, and
• tells us enough information to conclude that the loop is correct.

If we let N denote the initial value of the parameter n that is passed to this function, then a suitable loop invariant for this program is the following, which I’ll admit is a little cumbersome:

(N = n and b = 1 and a = 0 and sum = 0)

or

(n < N and b = fib (N - n) and a = fib (N - n - 1) and sum = fib 0 + ... + fib (N - n - 1))

(Edit: Credit goes to Paul Vergeev who noticed that I’d originally written down the invariant incorrectly, writing sum = fib 0 + ... + fib (N - n) instead of sum = fib 0 + ... + fib (N - n - 1) as it is fixed above. While I’d proved the invariant in Isabelle using AutoCorres, I’d transcribed it incorrectly into the post. Thanks, Paul!)

This invariant is phrased in two parts as a case distinction. The first part (the top line, i.e. everything before “or”) specifies the initial conditions and is true before the loop is entered, which means the entire invariant holds before we enter the loop. The second part specifies what is true after each loop iteration: given that the invariant holds after the kth iteration of the loop, this second part is always guaranteed to hold after the k + 1th iteration, meaning that the invariant will hold after each loop iteration. Note here that we are using the fib function, defined in note 3 above, as a formal specification for the fibonacci sequence, which we’ll interpret as operating over the same word type as the C fibonacci() function that we’re proving correct.

Once the program reaches line 11 (either because the loop terminated or because its body was never executed), we know that the invariant is guaranteed to be true: it is true before we entered the loop, so if the loop body never executed it will certainly be true, and it is always maintained by the loop body so by induction it must be true after the loop finishes. We also know that n = 0 once we hit line 11, because we only get here when the loop guard is false. From these two facts, we know therefore that:

(N = 0 and b = 1 and a = 0 and sum = 0)

or

(N > 0 and b = fib N and a = fib (N - 1) and sum = fib 0 + ... + fib N - 1)

The top part tells us that the function is correct when called with argument 0: fibonacci(0) will return 0, since in this case sum = 0. The bottom part tells us that the function is correct in all other cases: fibonacci(N) will return fib 0 + ... + fib N - 1 when N > 0, since in this case sum = fib 0 + ... + fib N - 1.

You can stare at the code and the loop invariant and try to convince yourself whether my reasoning is correct. Alternatively, this kind of reasoning is automated or assisted by program verification tools like Dafny or AutoCorres.