3.1 Selection

Selection is an activity that identifies a portion of an expression to be transformed. The selected portion is called the subject of a transformation.

To explore the activity of selection, begin by entering the expression (x-1)⋅(x+1). (Remember, any expression displayed in the book reader can be copied to the algebra display by touching it.)

If the first x is touched, it is selected. A selected operand is indicated by highlighting^[1] like this: (.{x}-1)⋅(x+1).

With the first x selected, touching it again expands the selection to include the subtraction operator and its other operand: (.{x-1})⋅(x+1).

Touching anywhere outside an expression deselects the selection.

With nothing selected, touch the addition operator; the selection will include the addition operator and its two operands: (x-1)⋅(.{x+1}).

Again with nothing selected, touch the multiplication operator. The selection will include both its operands and in turn will include the operands of the subtraction and addition operators: .{(x-1)⋅(x+1)}.

To summarize, touching a simple operand selects it; touching an operator selects it along with all its operands; touching a selection expands it to include the operator of which the selection is an operand along with all the operands of the operator.

Notice that when a subexpression is selected, the highlighting is drawn in red. And notice also that after a short time, the red highlighting changes to black. The two colours indicate two levels of sensitivity. When touching a red selection, the selection is expanded. But when touching a black selection, the selection remains the same, although the highlighting changes to red. The two sensitivity levels accommodate reselection and swipe-initiated commutation.

Probe can be used to make selection of small expression details easier.