Com Sci 105
Fundamentals of Computer Programming (Scheme)
Autumn 1998

College Instruction
Department of Computer Science
The University of Chicago

Midterm Exam Report, Section 2, Mike O'Donnell

The scores

I scored the exam out of 35. The actual scores are 35 34 32 31 29x2 26 25 24x3 23x3 22x3 21 19x2 18x3 17x2 16x3 15x4 14x4 13x3 12x3 11x3 10 9x3 8x2. The mean is 18 and the median is 16. I will discuss the scores with the other two instructors on Friday 13 November or Monday 16 November, and then I'll say something more definite about the impact. Section 3 had similar scores. Don't panic yet.

Comments on scoring

To my surprise, you had a harder time with the higher-order/lambda questions than with recursion. I will look for a good time to review that material.

• Question 1 is mostly straightforward, 1 point per item. You can run the expressions for yourself to figure out what's right. o and (o) are not at all the same thing, so I didn't give credit for that sort of approximation. A few people made the same mistake repeatedly, and I accumulated a partial credit of about 1 point for 4 approximate answers. square is not defined by Scheme or by Simply Scheme, so ``error'' is a correct answer for the 3rd item. I also gave credit if you assumed that square was defined, and calculated the list (2 4).



• Question 2 you can work on with the Scheme interpreter. There are several correct answers. My favorite has a cond with 4 branches, including the else branch. The key to simplicity is to check for divisibility in the order 400, 100, 4. There are shorter solutions, and they all got full credit, but the 4-branch solution is the clearest, and the easiest to adjust if you use leap-year? in a larger program.



• Questions 3 and 4 required a sensible use of accumulate and every, respectively. I only gave partial credit for substantial progress toward the solutions with accumulate and every. A lot of people gave the simple solution:

  
  (define (name-game wd)
    (se (wd 'b (bf wd)) (wd 'f (bf wd)) (wd 'm (bf wd))))
  

  for Question 4. I gave no credit for this. The point of the problem is an exercise in the use of every. You should recognize that the collection of things to which you do the same thing is (b f m), and look for a solution of the form

  
  (define (name-game wd)
    (every
      (lambda (let) body)
      '(b f m))
  

  Aside from the requirement of the exam question, this is the right way to solve the problem. You would not be satisfied with the simple name-game function defined above, and you would want to add the refinements of the real song: omit the first consonant when it is already one of b, f, m (so mary produces (bary fary ary)), don't remove initial vowels, remove all initial consonants, instead of just one. You are much better off with the solution using every when you make such refinements.



• Question 6 needs a basis case where the item number is 1.