Our novel calculator corrects all user errors: everything is always correct. Here, we discuss what happens when you block user errors, and how this helps reduce accidents and other unfortunate consequences of incorrect data.

A novel calculator, designed primarily for interactive whiteboards and pen-based devices, provides a better task fit than conventional approaches. The calculator provides a natural, dynamic method of doing calculations by handwriting using conventional notation. This paper discusses the calculator's underlying design principles, which collectively create a coherent and innovative 'look and feel.' The principle set could be used to help improve user interfaces for other domains.

A novel calculator, ideal for interactive whiteboards and pen-based devices, is introduced and evaluated. The calculator provides a natural, dynamic method of entering conventional expressions by handwriting and provides continual feedback showing the calculation and results. The user interface adjusts and copes with partial expressions, morphing the expressions to correct position and syntax. Gestures are also used to edit and manipulate calculations. The user interface is declarative, in that all displays, even with partial user input, are of correct calculations.

The new calculator is faster for more complex expressions and importantly, gives users more confidence in its results. The majority of users said that they would prefer to use this calculator rather than their conventional calculator.

In the two decades hand-held calculators have been readily available there has been ample time to develop a usable design and to educate the consumer public into choosing quality devices. This article reviews a representative calculator that is 'state of the art' and shows it has an execrable design. The design is shown to be confusing and essentially non-mathematical. Substantial evidence is presented that illustrates the inadequate documentation, bad implementation, feature interaction, and feature incoherence. These problems are shown to be typical of calculators generally. Despite the domain (arithmetic) being well-defined, the design problems are profound, widespread, confusing - and needless. Worrying questions are begged: about design quality control, about consumer behaviour, and about the role of education - both at school level (training children to acquiesce to bad design) and at university level (training professionals to design unusable products). The article concludes with recommendations.

Existing calculators are unreliable and difficult to use, and they are marketed misleadingly. A new calculator is described that is simple, reliable and very powerful.

Conventional calculators are badly designed: they suffer from bad computer science - they are unnecessarily difficult to use and buggy. I describe a solution, avoiding the problems caused by conventional calculators, one that is more powerful and arguably much easier to use. The solution has been implemented, and design issues are discussed.

This paper shows an interactive system that is declarative, with the advantages of clarity and power that entails. It frees people from working out how a calculation should be expressed to concentrating on what they want solved. An important contribution is to demonstrate the very serious problems users face when using conventional calculators, and hence what a freedom a declarative design brings.