Tutorial 5, Q1.

1. Tutorial 5, Q1. Briefly describe the purpose and principles of program slicing. Program slicing analyses the definitions and uses of variables in a program to identify those parts of the program which affect the computation at some point of interest, the slicing criterion. This has applications in debugging, testing, maintenance as it enables attention to be focused on those parts of the program relevant to the slicing criterion. The program is converted into a control flow graph, which allows data and control dependencies to be computed. A data flow dependence is one which allows a value calculated in an assignment statement to pass to a use of the variable assigned to. A control dependence is one where a predicate node determines whether some other (controlled) node gets executed or is possibly avoided. Using these two dependencies a program dependence graph is constructed. Computing the slice from eth dependence graph is merely a matter of following the control and data dependencies in the graph.

2. Tutorial 5, Q2 • Consider the program below. a. By inspection give the dataflow slice wrt (10,{sqs}) b. What is the smallest possible slice of the program? What criterion would give this slice? • count := 0; • sum := 0; • sqs := 0; • read(x); • while( x >= 0) do • count := count + 1; • sum := sum + x; • sqs := sqs + (x*x); • read(x); • write(count,sum,sqs)

3. Tutorial 5, Q2 Solution(a) • count := 0; • sum := 0; • sqs := 0; • read(x); • while( x >= 0) do • count := count + 1; • sum := sum + x; • sqs := sqs + (x*x); • read(x); • write(count,sum,sqs) • sqs := 0; • read(x); • while( x >= 0) do • sqs := sqs + (x*x); • read(x); • write(sqs)

4. Tutorial 5, Q2 Solution (b) The smallest slice of a program is always the empty program. A slicing criterion which produces it is the (degenerate) criterion (n,{}) for any program point n and empty set of variables.