isbn9789526046266

Figure 13.9: A visual program simulation exercise in UUhistle. The user is simulating the execution of
a small recursive program, and has just dragged a parameter value into the topmost frame
in order to create a new variable there. He is just about to name the variable using the
context menu that has popped up.
• To invoke a routine, click on the appropriate element (operator or function/method call) in the
evaluation area.
• To create a new element in memory (a frame, a function definition, etc.), click on the appropriate
memory panel. This brings up a context menu in which you get to select what to create. To bring
up the menu, you can click anywhere within the frame background or on the little plus in the upper
right-hand corner, which is there only to hint that the frame is clickable. Exception: variables are
created by dragging values, as described above.
• To move to a new line, click the line or its number. (This is not always required on default settings;
see below).
More examples of simulation steps
I will now briefly describe how to perform certain common simulation steps in UUhistle.
To deal with if and while, first evaluate the condition expression. Once you have True or False
in the evaluation area, click the line that execution proceeds to. On default settings, UUhistle’s VPS
exercises mostly do not require the user to manually control the ‘instruction pointer’. With the exception
of branching instructions such as if, UUhistle changes lines automatically as soon as the user is done
with the current line. This is one of many default behaviors that can be affected by an associated setting.
To evaluate a * b / 2 + (c + 1) * d, first drag and drop the value of a into the topmost expression
evaluation area, then do the same for the * operator and the value of b. Evaluate the subexpression a * b
by clicking on the operator; the product appears in the evaluation area, replacing the subexpression. Add
the / operator, then the literal 2 from the heap. Apply the operator. Add the +. Add the value of c.
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