CS 361: About Test #1

Here is the breakdown of 48 of the 49 grades in Test #1 (scores are over 120), along with a rough interpretation as letter grades:

A+: over 115 (1 student)
A:  from 110 to 115 (2 students)
A-: from 101 to 109 (3 students)
B+: from  95 to 100 (3 students)
B:  from  90 to  94 (4 students)
B-: from  85 to  89 (4 students)
C+: from  80 to  84 (5 students)
C:  from  75 to  79 (7 students)
C-: from  70 to  74 (5 students)
D+: from  65 to  69 (3 students)
D:  from  60 to  64 (3 students)
D-: from  50 to  59 (6 students)
F:  below 50 (2 students)

A: over 100 (6 students)
B: from  85 to 100 (11 students)
C: from  70 to  84 (17 students)
D: from  50 to  69 (12 students)
F: below 50 (2 students)

Comments:
The single most common mistake was, surprisingly enough, in solving recurrences: many of you ignored the driving term when solving a recurrence that involved a change of variables (setting n=c^k), looking only at the homogeneous part as changed, then using the driving term directly as a particular solution. This, of course, fails when the driving term has a radix in common with the characteristic equation, which was the case in at least one of the recurrences.

Another common failing was not to solve recurrences that arose in one of Problems 3 or 4, even the very simple first recurrence of Problem 4 (describing the number of lines printed). Most of you lost points on the last two problems by oversimplifying the analysis in Problem 3 (a common error was to ignore the for loop entirely) or not realizing that logarithmns were the key to Problem 4 (although even there the problem is entirely solvable without any logs, just by writing a recurrence that expresses f(n) in terms of f(n/10), and getting the logs out of the solution to the recurrence).