Midterm Exam

1
Midterm Exam

• COS 217

2
Midterm Exam

• Statistics
• Average score 72
• Median score 76
• Range of scores 29-101
• More detailed breakdown
• 6 people scored 90-101
• 15 people scored 80-89
• 15 people scored 70-79
• 9 people scored 60-69
• 14 people scored below 60
• So, this test was hard, and a little long
• Quite a few students didnt finish the last
  question
• Questions 4 and 5 were pretty challenging

3
Common Mistakes

• Question 1
• Treating as string concatenation, rather than
  numerical addition
• E.g., putchar(0 (837 10)) does not print
  07
• For example, 0 7 is 7, not 07
• Question 2
• Misunderstanding the idiom while (s) s
• The boolean s is true until you reach the \0
  character
• The s keeps incrementing, one character at a
  time, till \0
• Meaning of const char
• Common mistake to assume that the pointer cannot
  change
• When actually it is the memory contents that
  cannot change
• Question 3c
• Not understanding that heap and stack could grow
  in same direction
• Heap could easily start at one location, and
  stack at another
• But, this would reduce flexibility in sharing the
  memory space

4
Common Mistakes (Continued)

• Question 4
• Certain valid strings were not handled,
  including
• . Numbers ending with a decimal
  point
• . Sign followed by a dot
• E Sign in the exponent
• Some invalid strings were accepted, including
• .. Multiple decimal points (or
  Es, or signs)
• . Just a decimal point, or
  sign with decimal point
• E. Decimal in the exponent
• Finally, some did not clearly indicate their DFA
• A few wrote non-deterministic finite automata
• Many forgot to label transitions, or mark final
  states

5
Common Mistakes (Continued)

• Question 5c Queue_empty
• Misinterpreting the functionality of Queue_empty
• Several students emptied the Queue of its
  elements
• Rather than checking whether the Queue is empty
• Checking for queue with one entry, rather than
  empty
• Several students checked for queue-gthead
  queue-gttail
• Rather than checking that the head and/or tail
  are NULL
• Missed opportunity for concise code (no points
  removed)
• Return value determined via if/else block
• Rather than doing return (queue-gthead NULL)

6
Common Mistakes (Continued)

• Question 5d Queue_add
• Implementing a stack instead of a queue
• Adding element to the head, rather than the tail
• Missing corner case
• When the queue is initially empty
• Missing assert after call to malloc()
• Forgetting the assert() when calling malloc for
  new entry
• Question 5e Queue_remove
• Implementing a stack instead of a queue
• Removing element from the tail, rather than the
  head
• Missing corner cases
• Removing an element from an empty queue
• Removing the last element in the queue
• Not copying the head entry to enable return of
  the item
• Not making a separate copy of the head element,
  and item
• Not doing a free() to return the space used by
  head element