The eXpresser microworld

In eXpresser you can construct models made out of patterns of square tiles. The image below shows the main interface of eXpresser and introduces its key features and terminology (please read below and watch the tutorial video for more information).

In the image above, a flower-model has been constructed in the main area of eXpresser (construction area). This is where you build all your models. You can make models by creating different coloured patterns. For example, in this case the flower model comprises one pattern for the petals of the flower and another for the stem and leaf. You make patterns by repeating a building block of several tiles.

A key feature to note is the General Model window. This allows the user to 'keep an eye' on the generality of their model. If their model is constructed in a non-general way, then the General Model will look the same as their Model in the construction area. If their model is constructed in a general way, then the General Model will automatically adapt to different values for the number of repetitions of its underlying building block.

Underneath the General Model window on the left hand-side is the Activity Document which includes task descriptions and reflective questions.

Note that in the case of the eXpresser snapshot above the number of repetitions of the building block is "unlocked" (indicated by the pink frame surrounding it). Unlocked numbers act like variables. When a number is unlocked, it can be changed by editing it by hand (in the construction area). Within the General Model, the system picks random values for every unlocked number (see General Model in image below). Note that to make the model look right the same unlocked number has been used both in the petal and the stem-and-leaf pattern (hint: try to use different numbers and see the challenges this introduces).

In order for a pattern to be always coloured, you need to express a rule that gives the number of coloured tiles for any number of repetitions of the building block (since this number is unlocked and therefore can be changed, this rule should always give the correct number of coloured tiles needed).

In order to colour the pattern in the General Model (as in the image above) you need to construct the corresponding Model Rule for all the tiles in the Model (i.e. not just any pattern). Note that unlocked numbers can be named (in this case it has been named 'Model Number') and dropped onto the construction area for easy access.