CSE 127: Lecture 8

The topics covered in this lecture are software testing (in the context of sorting), and proofs of correctness (continued in next [few] lecture[s]).

Software Testing

We are going to use the following sort procedure (in C) as the code which we will look for test cases. We will look at how to select test inputs for normal execution, boundary cases, and illegal inputs. (In the case of a simple subroutine, illegal inputs are sometimes covered by preconditions.)
void swap(void		*array,
	  int		i,
	  int		j,
	  int		s)
	char	*lhs = (char *) (array + i * s);
	char	*rhs = (char *) (array + j * s);
	int	t;

	while (s > 0) {
		t = *lhs; *lhs = *rhs; *rhs = t;
		lhs++; rhs++; s--;

void sort(void          *array,
          unsigned int  nelt,
          unsigned int  eltsize,
          int           (*fn)(void *, void *))
	int us, i;

         * us is the index of the first unsorted element and divides
         * the array into a sorted and a unsorted region.
	for (us = 1; us <= nelt; us++) {
		for (i = nelt-2; i >= 0; i--) {
			if ((*fn)(array + i * eltsize, array + (i+1) * eltsize) > 0) {


Preconditions are what we assume to be true when the code is called. They are part of the contract between the code and its caller. Often we simply determine the valid input ranges and require the caller to ensure that the parameters are within these ranges. For more some code, sometimes we are more careful, and perform run-time checks to be sure that the inputs do not fall outside of the range of valid input values.

Proofs of Correctness

We will use this sorting procedure to examine proofs as well as for testing. More in upcoming lectures.
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