CSCI 333 Homework 4 Solution

Problem 1

Show the comparisions and exchanges made by the bad sorts, selection and insertion, on the following input set of six values:

1. 3
2. 7
3. 15
4. 13
5. 2
6. 5

Problem 2

Show how quicksort will partition the following array of nine values into two parts. Show the exchange and comparisions made in the partitioning. You may choose the pivot.

1. 3
2. 19
3. 7
4. 13
5. 15
6. 2
7. 17
8. 5
9. 23

We're going to choose the middle element, 15, as the pivot.

Problem 3

Show how the good sorts, heapsort, and mergesort will sort the following list of eight values.

1. 3
2. 7
3. 15
4. 20
5. 13
6. 10
7. 2
8. 5

Since insertion sort beats the O(n log n) sorts for small data sets, in our examples we'll assumer we're using insertion sort where there are only four unsorted elements remain.

Problem 4

A file contains a sequence of integers, possibly separated by commas. Write a C++ subroutine which is passed an istream that has been opened to this file and returns the sum of all the integers in the file. Return -1 is the file is not in the correct format.

Here is an example of the file input:

   35  67, 89
13 5

13, 67

int SumEm(istream NumS) {
  int retSum = 0 ;
  int nxtNum ;
  while (!NumS.eof()) {
    NumS >> nxtNum ;
    if (!NumS.eof()) {
      if (NumS.fail())
        return -1 ;
      else {
        retSum += nxtNum ;
	// keep reading integers until you get a comma or EOF
	while (NumS >> nxtNum && !NumS.fail())
	  retSum += nxtNum ;
	if (!NumS.eof()) {
	  NumS.clear() ;
	  // This better be a comma
	  char c = 'X' ;
	  NumS >> c ;
	  if (c != ',')
	    return -1 ;
        }
      }
    }
  }
  return retSum ;
}

Problem 5

Write some C++ statements to replace the fifth character of a file with the character 'X'.

// Assume the file has been opened to ostream F
char OutBuff[1] = { 'X' } ;
F.seekp(5, ios::beg) ;
F.write(OutBuff, 1) ;