CS 2005, B Term 1999
Techniques of Programming
Lab 4 (Dec. 1)


In this lab session you will implement the middleindex function required in HW4, and you will make progress toward implementing the recursive buildtree function for HW4.

  1. Sign in with the TA. You should both print your name and sign the sheet.
  2. Listen to the TA's mini-lecture.
  3. Log onto your CCC Unix account and change the directory to /cs/cs2005/samples/lab4/. That directory contains copies of the files required for HW4, in particular a copy of the file named filetotree.cxx.
  4. Do the problems listed below. Feel free to ask the TA questions about the problems. Actively working on the lab assignment during the lab session is required. Don't worry if you can't finish the full assignment before the lab session is over. You can finish afterwards.


  1. Copy the files in /cs/cs2005/samples/lab4/ to one of your own personal directories. Be careful to choose a directory other than the one you're using for HW4, as otherwise you may overwrite your HW4 files.

  2. Examine the incomplete implementation file filetotree.cxx. Most of the functions needed for HW4 have already been implemented there, except for middleindex and buildtree. Make sure you understand the overall outline of filetotree.cxx. See HW4 for additional comments.

  3. Fill in the implementation of the middleindex function in filetotree.cxx. This can be done in a single line of C++ code. You may use the fact that the midpoint between numbers x and y is (x+y)/2. If x and y are integers, the result of this division will be rounded down to an integer. In order to achieve the desired rounding up effect, you may find it helpful to use the modular remainder operator a % b which gives the integer remainder in the integer division a/b; for example, 5%2 equals 1. Notice that rounding up is required precisely when the sum of the lowindex and highindex arguments to middleindex is odd, and this condition may be expressed in terms of the modular remainder %2, so you just need to add the appropriate modular remainder expression to the midpoint expression given above.

  4. Fill in the outline for buildtree provided in filetotree.cxx to obtain a complete implementation. You will need to use the binary tree toolkit that is available in the files bintree2.h and bintree2.template. The buildtree function must recursively construct a balanced binary search tree from the items stored in its data[] argument. The basic binary tree toolkit function needed for this is create_node. This function accepts three arguments: the entry to be stored in the new node, and pointers to the trees that will be the new node's left subtree and right subtree respectively. Study the code for create_node in the binary tree toolkit files for further details. For each case in the buildtree outline, assign appropriate values to the pointers left_ptr and right_ptr. You may find it easiest to fill in the last case first. Keep in mind that the final instruction in buildtree should return a pointer to the root of the newly constructed tree, which should have data[midindex] as the root entry, and left_ptr and right_ptr as pointers to the appropriate subtrees.

  5. Type make at the Unix prompt in the directory to which you copied the files provided in /cs/cs2005/samples/lab4/. This compiles and links the appropriate files, and produces an executable file named filetotree. Run the executable file filetotree and use it to debug your implementation. Modify your implementation as needed. Note that the integers needed to fill the balanced binary search tree constructed by filetotree must be placed in a file, with blanks between consecutive integers; there should be no spaces after the last integer.