天美传媒

By Greg Michaelson, School of Mathematical and Computer Sciences, Heriot- Watt 天美传媒

Contact: G.Michaelson@hw.ac.uk

Keywords: computational thinking; informational thinking; programming; teaching.

1.  Background

Computers are the dominant technology of the early 21st century: pretty well all aspects of economic, social and personal life are now unthinkable without them. In turn, computer hardware is controlled by software, that is, codes written in programming languages. Programming, the construction of software, is thus a fundamental activity, in which millions of people are engaged worldwide, and the teaching of programming is long established in international secondary and higher education. Yet, going on 70 years after the first computers were built, there is no well-established pedagogy for teaching programming.

There has certainly been no shortage of approaches. However, these have often been driven by fashion, an enthusiastic amateurism or a wish to follow best industrial practice, which, while appropriate for mature professionals, is poorly suited to novice programmers. Much of the difficulty lies in the very close relationship between problem solving and programming. Once a problem is well characterised it is relatively straightforward to realise a solution in software. However, teaching problem solving is, if anything, less well understood than teaching programming.

Problem solving seems to be a creative, holistic, dialectical, multi-dimensional, iterative process. While there are well established techniques for analysing problems, arbitrary problems cannot be solved by rote, by mechanically applying techniques in some prescribed linear order. Furthermore, historically, approaches to teaching programming have failed to account for this complexity in problem solving, focusing strongly on programming itself and, if at all, only partially and superficially exploring problem solving.

Recently, an integrated approach to problem solving and programming called Computational Thinking (CT) (Wing, 2006) has gained considerable currency. CT has the enormous advantage over prior approaches of strongly emphasising problem solving and of making explicit core techniques. Nonetheless, there is still a tendency to view CT as prescriptive rather than creative, engendering scholastic arguments about the nature and status of CT techniques. Programming at heart is concerned with processing information but many accounts of CT emphasise processing over information rather than seeing then as intimately related.

In this paper, while acknowledging and building on the strengths of CT, I argue that understanding the form and structure of information should be primary in any pedagogy of programming.

2.  Computational Thinking, Computer Science and Computing

2.1.  Overview

Computing involves making models of the world, to understand it and change it in predictable ways. We can then write programs that map the abstractions of a conceptual model onto the concrete behaviours of a physical computer so the behaviour of the program on the computer tells us about how the world works.

We make models by solving real world problems. And we realise models as programs by programming real computers. So should we separate out problem solving, model making and programming, or are they inextricably linked? We also need languages and notations to solve problems, make models and write programs. Should these be the same or different? And isn't this all just Computing Science?

I think that it's important to distinguish Computing from Computing Science. Computing Science is an academic discipline which underpins all ICT, especially model making and tool making. Computing Science is concerned with the theory and practice of computations, which involves making models of reality from information structures and algorithms, and then animating the models on computers. That is, programming bridges models and computers.

It has been argued that everyone needs to learn how to program and that somehow programming is the 'new Latin' (e.g. Naughton, 2012). Shein (2014) offers a useful summary. I think we need to teach everyone how to think, in particular how to characterise a problem before realising a solution in some given hardware and software technology.

Still, the hardest part of problem solving is characterising the problem.

2.2.  Computational thinking and information

In the same period, there were a number of techniques that augmented and extended modular structured programming, making strong use of diagrams, in particular Yourdon and Constantine's structured design with structure charts (Yourdon, 1979) and Constantine's data flow diagrams(Stevens, 1974). Where structure charts reflect a modular hierarchy of algorithmic components, data flow diagrams focus on the movement of information between algorithmic components. Once again, no method is offered for identifying the components in modular hierarchies or the information that flows between them.

In contrast, Jackson's (1975) structured programming (JSP) linked the structure of the data stream to the structure of the program that processed it. JSP was widely used as the methodology for teaching commercial data processing techniques in COBOL. However, JSP was only applicable to a highly constrained class of programs and gave no support for identifying intermediate information structures.

2.3.  Problem solving techniques

2.3.1.  Introduction

Now, it's easy to write down these stages but harder to see how they apply in practical problem solving for programming. It's really not clear where to begin.

Programming isn't hard when you know how to solve the problem. It then becomes a matter of battling with the vagaries of language-specific syntax, semantics and tools. For people new to programming, this language specific detail can become overwhelming, leading to a plethora of tiny, low level concerns at the expense of understanding how to solve an original problem.

And there is curiously little material on problem solving and programming.  For example, Wienberg's (1971) classic study of the psychology of programming assumes that programming is an activity based on a specification that is elaborated from analysis, but says nothing about analysis itself. This echoes the then prevalent waterfall model of software development with distinct stages which are never revisited.

One of the few books that ostensibly focuses on problem solving and programming, Dromey's How to solve it by computer (Dromey, 1982), draws explicitly on Polya's 1950 foundational study How to solve it (Polya, 1990) and on later work by Wickelgren (1974).

Polya (pp5 & 6) characterises problem solving as a four stage process of:

• understanding the problem;
• linking unknowns to data to make a plan;
• carrying out the plan;
• looking back and reviewing the solution.

He offers a long list of problem solving heuristics, many of which correspond to different aspects of CT (e.g. analogy, auxiliary problem, decomposing and recombining, do you know a related problem, specialisation) but doubts any systematic way of deploying them:

'Rule of discovery. The first rule of discovery is to have brains and good luck. The second rule of discovery is to sit tight and wait till you get a bright idea. ... To find unfailing rules applicable to all sorts of problems is an old philosophical dream; but this dream will never be more than a dream.' (p172)

Nonetheless, Wickelgren attempts to provide a methodical approach to solving what he terms formal problems, that is those couched in some formal notation, typically logical or mathematical. Wickelgren sees a  problem as being specified  as a starting state, a  set of allowable operations over states, and a goal state. Thus, a solution is found by a sequence of state to state transitions leading from the start state to the goal state. Much of the book focuses on techniques for pruning the space of transitions, in particular reasoning backwards from the goal, but there is little on problem formulation.

Dromey (1982) is a proponent of top down design and stepwise refinement, linking decomposition to algorithm, which we will consider briefly below. He also uses logical statements to capture properties of program stages, typically loop invariants. While he acknowledges the central role of the choice of data structures in programming, he largely focuses on algorithm design, suggesting that structures are somehow chosen from a menu of

2.3.2.  Programming language oriented

The oldest approach to programming is to just write down code, that is somehow using a programming language as some language of thought. Arguably, this programming language oriented approach is what the examples in Lovelace's 1842 account of Babbage's Analytic Engine represent (Morrison, 1961). Today we call this extremely widespread and long lived approach hacking. From a mathematician's perspective, using a programming language to construct algorithms is directly akin to using any mathematical notation to solve problems.

However, ad-hoc language oriented techniques suitable for solving small, well defined problems scale poorly for even moderately sized problems. Furthermore, effective hacking requires a deep facility with the programming language.

While language choice for teaching programming remains controversial, it is widely agreed that high level languages are more suitable for beginners than low level forms like assembly language and machine code. Nonetheless, programming languages in themselves do not offer any approach to problem solving beyond a means of formulating algorithms.

Some favour the use of an industrial strength programming language, for example FORTRAN, COBOL, C/C++, Ada and Java. Skill in an industrial language is seen as attractive to employers but such languages may be less well suited for conveying key principles to beginners. A more nuanced approach is to start with a teaching subset of a full strength language, leaving full detail to more advanced courses.

Others prefer to use a language originally intended for educational purposes, which often have sounder pedagogic bases than industrial languages. Some, for example Pascal, BASIC and the ALGOLs have crossed over into industrial use.

It is easy to overlook the use of scripting languages in teaching programming. Such languages, especially HTML, are widely used in early teaching of ICT skills for web design but, as they are used to layout and instantiate interface components, this may not be conceived of as programming.

Another alternative is to deploy a language-neutral pseudocode. This avoids the often observed problem of an excess focus on specific language syntax, and of first language bias, which both make it hard to generalise programming concepts for realisation in new languages (Cutts, 2014). Pseudocode works particularly well  for  presenting  algorithms. For  example, Knuth's (1968) classic text on fundamental algorithms uses both structured English and the MIX pseudo-assembly language. However, it is much harder to come up with a language- neutral notation for data structures, where extant languages show considerably more variation. For one approach, see Horowitz and Sahni's (Horowitz, 1976) structured pseudocode. Nonetheless, overall, I think that a pseudocode offers a valuable bridge from fundamental concepts to multiple languages.

This approach also has a long history. For example, Wilkes (1951) describes the predominantly mathematical program library for the late 1940's EDSAC computer. Enormous libraries are available for contemporary programming languages, but, as in Wilkes' day, intimate understanding of library contents is crucial for effective component use. Indeed, an original problem decomposition may have to be modified, and additional algorithmic glue contrived, to make the problem specific components fit the library components.

Of considerable contemporary interest for initial teaching of programming are component based languages where learning is based on composing problem domain specific components. Thus, Logo (Papert, 1980) provides turtle graphics operations. More recent visual languages, like Alice, Scratch, BYOB, Snap and AppInventor, offer richer component sets, augmenting graphics with sound and animation.  These are increasingly used in very early learning of programming, say at primary level. A disadvantage is that such languages take up more space than their textual equivalents, markedly limiting what can be presented. Once again, there is a tendency to revert to language oriented teaching.

2.3.3.  Component oriented

Component based programming is based on using a programming language to compose pre- given  elements,  typically  assembled  in  libraries.  The  components  themselves  represent abstractions by domain experts and their use requires problem decomposition to identify appropriate components.

2.3.4.  Flowcharts

Diagrams have long been used in designing software and teaching programming. The use of flowcharts to design software is almost as old as digital computers, originating in von Neumann and Goldstine's reports from 1947 and 1948 (Haigh, 2014), and is still surprisingly wide today. At simplest, flowcharts are based on boxes for decisions and commands, linked by arcs indicating flow of control. As such, they combine the decomposition and algorithm CT stages. Flowcharting has long been an adjunct to learning a programming language. However, flowcharts are greedy of space, there is no clear discipline for their use, and, at worst, there may be one box for each command.

2.3.5.  Structured programming

Early programming languages relied heavily on constructs oriented to the underlying platform, in particular the ability to branch to different instructions in memory, abstracted as GOTOs and labels in high level languages. Following Dijkstra's (1968) highly influential critique of branching, the use of structured programming (Dijkstra, 1969) was widely advocated, based on the use of the basic constructs of sequence, choice and repetition. Like flowcharting, this combines the decomposition and algorithm CT stages.

Structured programming was complemented by modular programming (Parnas, 1972), an approach where programs are composed from identified components. Parnas suggested that techniques for identifying modules are to decompose 'major steps', starting from flowcharts, and 'information hiding', where a module abstracts away from underlying detail, typically formed from a data structure and associated operations, prefiguring object-orientation. However, no methodology is offered for so doing.

Wirth's (1971) stepwise refinement, a systematisation of top down design for structured programming, was widely used as a teaching approach from the late 1970s. Here, a broad top level specification is refined to lower and lower levels, systematically identifying modules and structured programming constructs. However, refinement decisions tend to be driven by algorithmic considerations and based largely on criteria of efficiency and aesthetics. Subsequently, Wirth (1973) further emphasised the need to identify and deploy both standard and novel data structures and algorithms, but again offering no clear advice on how these may be selected.

2.3.6.  Declarative programming

Following the announcement of the Japanese 5th Generation Computer Systems Project in 1982 (McCorduck,1983), there was renewed pedagogic interest in the use of declarative languages for initial teaching of programming. Broadly, such languages are rooted in mathematical logic formalisms, with a distinction made between functional languages, based on lambda calculus and recursive function theory (Michaelson, 1989), for example Standard ML and Haskell, and logic languages based on predicate calculus, for example Prolog (Clocksin, 1981). Unlike other approaches, declarative programming strongly emphasises the CT techniques of abstraction, and pattern identification and generalisation, enabling a close link between algorithm and data structure. And polymorphic functional programming encouraged the complementary use of types and  higher order functions for abstraction  in  structuring programs. However, the core declarative trope of recursion makes progress beyond elementary programming challenging, especially for those already used to procedural/structured approaches. Furthermore, declarative programming represented a reversion to a language oriented approach.

2.3.7.  Object oriented programming

The leading contemporary approach to initial teaching of programming, at least in Higher Education, is object orientated (OO) where components (classes) encapsulate both data structures and algorithms (methods) to manipulate them. While OO languages have a long pedigree (e.g. Simula, Smalltalk), today Java predominates for teaching, followed by C++ and Python.

One popular contemporary approach to OO teaching is the components based objects first (Barnes,2012), where learners are given libraries of classes and are taught how to instantiate and compose them. This is reminiscent of Logo and visual languages mentioned above.

OO programming is complemented by the Unified Modelling Language (UML) (Stevens, 2000), a rich, multi-faceted diagrammatic approach, drawing on earlier structured approaches. UML is well suited to problem decomposition and identification of composable components for realisation as objects. However, it has little to say about information structures.

Design patterns (Gamma, 1995) are abstractions from common computations and offer standard templates that may be instantiated with problem specific components, providing a bridge from decomposition to algorithm. Design patterns are widely used for OO. Once again, considerable facility with the patterns is required for their effective use.

2.3.8.  Parallel programming approaches

One domain where systematic problem solving is vital for efficient use of computational resources, and where problem taxonomies are widely used, is parallel programming. We will next briefly consider three methodologies for constructing parallel programs which  have strong resonances with CT and which explicitly consider information decomposition.

Foster's approach (Foster, 1995) has partitioning and communication stages concerned with identifying tasks for subsequent optimal placement on processors in agglomeration and mapping stages. The partitioning phase involves information centric domain decomposition and computation centric functional decomposition, both to atomic components. Subsequently, these components are grouped to optimise allocation of tasks to available processors and to minimise inter-processor information communication.

Mattson et al's (2005) parallel design patterns strongly complement Foster's approach. As with Foster, the first stage, finding concurrency, involves task and information decomposition to atomic components. Subsequently, components are grouped using standard patterns for parallelism with known coordination and processing properties.

Finally, Cole's algorithmic skeletons (Cole, 1989) offer abstract constructs with well characterised properties for  coordinating multi-processor  computations.  Skeletons may be provided in libraries and design patterns used to select them.

Furthermore, PBL must be fuelled by at least some ground knowledge. For example, Ma et al's (2005) approach combines pair programming with PBL. In what they call the initial 'explorephase', students are already equipped with 'basic concepts, library classes and relevant algorithms'. Similarly, while O'Kelly et  al's (2004) PBL-based approach uses stepwise refinement for problem decomposition, it builds on basic programming concepts such as 'looping, recursion, conditional statements' for solving 'problems of increasing difficulty'. Once again, in both cases there is some emphasis on the later stages of programming.

2.3.9.  Problem based programming and problem based learning

It would be misleading to present the above approaches as somehow abstracted from the process of solving real problems. Indeed, the teaching of programming is perhaps unique in being so thoroughly grounded in, and  driven  by, seemingly endless open-ended  concrete exemplars, often chosen to be motivational.

In such problem based programming, learners are typically encouraged to understand, modify and extend extant solutions to problems or sub-problems. Here, the emphasis is strongly on comprehension of the specific solution in context, not on either how the solution was derived or how it might be generalised to other solutions. The latter would require some taxonomy of problems.

It is interesting to contrast problem based programming with the application of problem based learning (PBL) to programming. PBL originated in medical education and is widely deployed in professional training. Nuutila et al (2005), who have applied PBL to teaching programming, cite Schmidt (Schmidt, 1983) in observing that:

'PBL refers to learning that is stimulated by descriptions of real world problems. Students do not necessarily try to solve a problem but rather define learning goals to better understand it.'(p125)

They distinguish PBL from problem-oriented learning, where 'learning takes place as a side effect of the solution process' and project-based learning, where 'the size of the problem becomes larger'. For example, Sierra et al (2013) explicitly conflate problem and project based learning in teaching programming. Here, the project is very precisely specified and sculptured to guide the learner to well defined outcomes.

I suspect that, as Nuutila et al suggest for two studies they cite, much teaching of programming that is claimed to be based on PBL is actually problem-oriented or project-based learning.

2.3.10.      Summary

We have seen that each of the approaches corresponds to a subset of CT stages, with decomposition and algorithms predominating. The emphasis on algorithms is reflected in Curzon et al's (2014) contemporary account of CT.

Thus, in terms of which programming oriented approach to choose, really, all we can say is that fashions change. There are too many possibilities and none work easily beyond simple cases. More to the point, they all have a strong focus on the final program.

Nonetheless, in the following attempt to formulate an information-driven approach to problem solving through CT, the reader will spot the direct influence and opportunistic use of diverse aspects of the approaches discussed above.

(Stevens, 1974) W. Stevens, G. Myers and L. Constantine, Structured Design, IBM Systems Journal, Vol. 13, No. 2, May , 1974, pp115-39, in E. N. Yourdon (Ed), Classics in Software Engineering, Yourdon Press, 1979.

(Stevens, 2000) P. Stevens with R. Pooley, Using UML: Software Engineering with Objects and Components, Addison- Wesley, 2000.

(Wickelgren,1974). W.A. Wickelgren, How to Solve Problems, Freeman, 1974.

(Yourdon, 1979) E. Yourdon and L. L. Constantine, Structured Design: Fundamentals of a Discipline of Computer Program and System Design, Prentice-Hall, 1979.

(Weinberg, 1971) G.  M. Weinberg, The Psychology of Computer Programming, Van Nostrand Reinhold, 1971. (Wilkes, 1951) M. Wilkes, The preparation of programs for an electronic digital computer, Addison-Wesley, 1951.

(Wing, 2006) J.M. Wing, Computational Thinking, CACM Viewpoint, March 2006, pp. 33-35.

(Wirth, 1971) N. Wirth Program Development by Stepwise Refinement, Communications of the ACM, Vol. 14, No. 4, April 1971, pp. 221-227 

(Wirth,1973) N. Wirth, Algorithms + Data Structures = Programs, Prentice Hall, 1973.

3.  Information driven computational thinking

3.1.  Information structures

Our familiar approaches are certainly useful at the end of problem solving when we come to implement a solution, but at the start it's vital to focus on the problem without regard for the implementation. I think that the key is to work out how to characterise the information and computations. And it's best to start with the information, not the computation.

We can characterise information as: information = base elements + structure

Note that we are not concerned with type or class, which are programming concepts to do with representing and implementing problem information.

Here, the base elements are the names of real-world things. At simplest, these are represented as sequences of characters to form meaningful unitary entities like words and numbers. Our things may also be built out of sub-elements, that is, things themselves may be structured.

Thus, we find:

• sequences: things before and after each other, either unordered or ordered on some property;
• tables: things arranged in rows of columns;
• arrays: things accessed by indices;
• records: things accessed by field name;
• lists: things arranged in chains, accessed by heads and tails;
• trees: things arranged in branching structures, accessed by branches;
• graphs: things arranged as nodes linked by arcs.

Note again that we are not concerned with data structures which are programming concepts to do with representing and implementing problem information structures.

3.2.  Finding information structures

Alas, it's still no clearer where we should begin. It is tempting to go back to good old fashioned Computer Science. But there's no need. Rather, we should think about how real-world things are organised, and look at lots of concrete examples. Once we adopt an informational thinking approach, we can see information structures everywhere, often in how physical things are arranged but more conventionally in how data is organised.

For example, try thinking about how we interact with:

• parked cars;
• numbered houses;
• supermarket queues;
• English v Scottish bank queues;
• shopping lists;
• receipts;
• bills;
• account statements;
• itineraries;
• diaries;
• calendars;
• invitation lists;
• address books;
• seating diagrams;
• shop catalogues;
• library catalogues;
• family trees;
• cladistic trees;
• decision trees;
• underground maps;
• road maps;
• lottery tickets;
• betting slips;
• sports league tables;
• mobile contacts;
• browser favourites;
• social media friends;
• digital photo albums.

To characterise the information structure we need to interrogate the information. We need to ask how the information is organised, that is if it is:

• simple or composite;
• linear or grid or branching or cyclical;
• unordered or ordered;
• fixed or changeable in content or size or shape.

3.3.  Generalising information structures

We now have some vague inkling about how to identify a concrete information structure that fits some specific real-world problem scenario. Applying computational thinking, we can now try to generalise by comparing apparently disparate information structures. Thus we can ask what stays the same and what changes. We can also look for similarities in concrete detail and in gross structure, ignoring the elements. In particular, are there commonalities in how to access, update, add and delete elements. Ultimately, we can use these comparisons to draw out the abstract structures we identified above.

For example, what do a car park and tables in a caf茅, a bank statement and a utility bill, an allocated seat in an aircraft and in a cinema, a road map and an underground map, have in common? What are their differences? What common abstract information structure is suitable for both of them?

Finally, we can start to formalise the idea of an information structure as an abstract data type, with ways to:

• make a new structure;
• add information;
• check if information is present;
• find information;
• change information;
• remove information.

Not all information structures need have all these capabilities. For example, in many real world information structures, the information cannot easily be changed.

3.4.  From information to computation

We can view these operations on abstract data types as being like the verbs of a language. We then make sentences by apply the verbs to nouns. That is, computation structures operations on information structures into algorithms. The key here is to make the structure of the computation follow the structure of the information.  Note that we're making  algorithms; we're not yet programming.

Once again, we should be driven by the problem scenario. We know that a computation solves a problem by turning old information into new information. So in the problem scenario we need to explore the sequences of information change.

Typically, we want to traverse the information structure, often visiting each element once, and stopping when some condition is met. On the way, we might do something to each element and accumulate some intermediate information.

3.5.  Notation

I have quite deliberately talked about problem solving in very general terms, without using any notation. But we do need to describe all these aspects of solving a specific problem in a concrete, consistent manner, so why don't we just use a programming language? After all, programming and seeing things working at an early stage are both highly motivating.

One problem is that the choice of language affects what can be described. Furthermore, the fine detail of a specific language can get in the way of understanding fundamental concepts. Finally, as noted above, the language-centric techniques used to knock up quick hit, small programs usually don't scale well to larger problems and this can prove frustrating and demotivating.

I think that, for problem solving, we should use a neutral notation like a pseudo code, for example the Haggis pseudo code (Michaelson, 2104).

4.  Conclusions

To conclude, in trying to elaborate a pedagogy of programming, I think that we should be driven by problem solving. We've seen that:

computational thinking =

decomposition+ abstraction + patterns + algorithms

Now, CT is a framework not a recipe. In CT, the components overlap and interact. It just isn't possible to separate out definite stages of decomposition, abstraction, patterns  and algorithms. Finding these is itself a creative, iterative activity.

I've argued that that classic CT overemphasises computation over information. For me: solution = information + computation

I think that we should let the information structure the computation, and we should start with concrete instances of our problem scenario. We should then use CT to ask good questions, to tease out well known information structures and to guide computation design.

Finally, I think that, as with all education, we should view our students as active subjects and try to ground our practice in solving substantial problems drawn from their real world experiences, and expressed in terms they are likely to find familiar and engaging.

Acknowledgements

I'd like to thank:

• Quintin Cutts of the 天美传媒 of Glasgow, for prompting me to think about this peculiar stuff in the first place;
• Ian King of Kelso High School and his Scottish Borders teacher colleagues, with whom I first explored this material;
• participants  of  my   workshop  at  the  2013  Computing  At  School  Conference  in Birmingham;
• Elaine Kao of Google Education, Simon Humphreys of Computing at School and Jeremy Scott of George Heriot's School, Edinburgh for discussion about the origins and definitions of CT;
• David Mathew of the  JPD for his encouragement and support, and the anonymous referees for their irritating yet thought provoking comments;
• my long suffering students and colleagues.

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