Computer Science 200b Exam 2 April 14, 2016

Transcription

1 YOUR NAME PLEASE: NETID: Computer Science 200b Exam 2 April 14, 2016 Enter your netid at the bottom of each page. Closed book and closed notes. No electronic devices. Show ALL work you want graded on the test itself. You may not hand in a Blue Book. For problems that do not ask you to justify the answer, an answer alone is sufficient. However, if the answer is wrong and no derivation or supporting reasoning is given, there will be no partial credit. GOOD LUCK! Problem Points Actual Total 60 Page 1. NETID:

2 1. (10 points) Short answer. What is referential integrity? # If a given field has a finite number of possible entries, you # can specify that list of legal values and have the db detect # if a bad value has been entered. What is idempotence? Give an example from SQL or HTTP. Idempotence is the property that an operation can be applied to the same object multiple times without changing the initial result. For example, absolute value, abs(x), is idempotent, while increment, x += 1, is not. The SELECT operation is idempotent. Actually, it is nullipotent. UPDATE and DELETE are idempotent (but not nullipotent) assuming that the WHERE clause confirms the initial state of the database. The HyperText Transfer Protocol encourages idempotence for the verbs GET, PUT, and DELETE, but not POST. Which of the following real world actions are idempotent? An elevator call button YES Pushing "Place order" moves order 4567 from 'cart' to 'finalized' status. YES Make me a pizza. NO Check a password and increment the bad login attempts if it's wrong NO Page 2. NETID:

3 2. (10 points) Below we define several functions, which may or may not raise an exception when executed. For each function, indicate if an exception is raised always, sometimes, or never. For always and sometimes, indicate what exception is raised and for sometimes specify the conditions under which the exception occurs. def f1(): Import mymodule Sometimes: ImportError: No module named 'mymodule' def f2(): a = {} return a[ key ] Always: KeyError: 'key' def f3(): raise Exception("oops!") Always: Error: oops A = 9 def f4(): return a Always: (assuming a is not globally defined) NameError: name 'a' is not defined def f5(): for x in open("foo"): print (x) Sometimes: when file foo does not exist or does not have read permissions FileNotFoundError: [Errno 2] No such file or directory: 'foo' Page 3. NETID:

4 3. (10 points) Define a python procedure x() that generates the following bytecode >>> dis.dis(x) 20 0 LOAD_CONST 1 (7) 3 STORE_FAST 0 (a) 21 6 LOAD_FAST 0 (a) 9 LOAD_CONST 2 (2) 12 BINARY_MODULO 13 POP_JUMP_IF_FALSE LOAD_CONST 3 (1) 19 LOAD_CONST 4 (3) 22 LOAD_FAST 0 (a) 25 BINARY_MULTIPLY 26 BINARY_ADD 27 RETURN_VALUE 24 >> 28 LOAD_FAST 0 (a) 31 LOAD_CONST 2 (2) 34 BINARY_TRUE_DIVIDE 35 RETURN_VALUE 36 LOAD_CONST 0 (None) 39 RETURN_VALUE def y(): a = 7 if a % 2: return 1 + 3*a else: return a / 2 Page 4. NETID:

5 4. (10 points) Provide the bytecode generated for the following Python function. Use dis.dis() format, but without the source code line numbers from the first column. def y(): a = 2 b = 3 c = a + b d = c * c return d >>> dis.dis(x) 6 0 LOAD_CONST 1 (2) 3 STORE_FAST 0 (a) 7 6 LOAD_CONST 2 (3) 9 STORE_FAST 1 (b) 8 12 LOAD_FAST 0 (a) 15 LOAD_FAST 1 (b) 18 BINARY_ADD 19 STORE_FAST 2 (c) 9 22 LOAD_FAST 2 (c) 25 LOAD_FAST 2 (c) 28 BINARY_MULTIPLY 29 STORE_FAST 3 (d) LOAD_FAST 3 (d) 35 RETURN_VALUE Page 5. NETID:

6 5. (10 points) Write the UNIX command(s) corresponding to XXXX in the transcript below. You may not use echo. bash 4.2$ date > now bash 4.2$ cat now Wed Apr 6 14:56:53 EDT 2016 bash 4.2$ ls l total 4 rw rw r 1 sbs5 cs201ta 29 Apr 6 14:56 now bash 4.2$ touch now bash 4.2$ ls l total 4 rw rw r 1 sbs5 cs201ta 29 Apr 6 16:22 now bash 4.2$ cat now Wed Apr 6 14:56:53 EDT 2016 bash 4.2$ TODAY=Wednesday bash 4.2$ echo $TODAY Wednesday bash 4.2$ alias alias la='ls al' alias ll='ls l' alias p='python3.4' alias rm='rm i' bash 4.2$ sleep 100 & [1] bash 4.2$ jobs [1]+ Running sleep 100 & bash 4.2$ kill %1 bash 4.2$ jobs [1]+ Terminated sleep 100 bash 4.2$ ls now bash 4.2$ cp now now1 bash 4.2$ ls l total 8 rw rw r 1 sbs5 cs201ta 29 Apr 6 16:22 now rw rw r 1 sbs5 cs201ta 29 Apr 6 16:26 now1 bash 4.2$ diff now* bash 4.2$ Page 6. NETID:

7 6. (10 points) Define the decorator xxx() that has the following def fact(n): if n <= 1: return n else: return n * fact(n 1) >>> fact(3) fact 3 fact 2 fact 1 return 1 return 2 return 6 6 def xxx(f): f.indent = 0 def g(x): print(' ' * f.indent + ' ', f. name, x) f.indent += 1 value = f(x) print(' ' * f.indent + ' ', 'return', repr(value)) f.indent = 1 return value return g Page 7. NETID: