CS 303E Fall 2011 Exam 2 Solutions and Criteria November 2, Good Luck!

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1 CS 303E Fall 2011 Exam 2 Solutions and Criteria November 2, 2011 Name: EID: Section Number: Friday discussion time (circle one): Friday discussion TA(circle one): Wei Ashley Answer all questions. Please give clear answers. If you give more than one answer, we will grade only the first answer, so please clearly mark out any answer you do not want us to grade. Use extra paper to determine your answers and copy them neatly on these sheets. Scratch paper is included at the end of the exam and more is available on request. You are not allowed to use concepts or programming constructs or functions that have not been discussed in class on this exam. Make sure you clearly write your name, EID, lecture and discussion times, and TA on this page. Good Luck! Question Points Lost Points Earned Maximum 1 22 (2 pts ea) 2 18 (3 pts ea) Total 100 1

2 1. What is the value of each Python expression? Write the value in the proper form to indicate its type (int, float, str, etc.) you can use single or double quotes around a string, a decimal point for a float, square brackets around a list, etc. If the expression is illegal, just write illegal. For all questions referencing mystring use this definition: mystring = Hello, World! These questions are two points each. (a) car + go cargo (b) 42 + str(6) 426 (c) 12 + car illegal (d) **2/4 14 (e) % 4 7 (f) mystring[4] (g) mystring[-3] (h) mystring[3:7] (i) mystring[6:] o l lo, World! What is the result when the following code is executed? (j) mystring[3] = x an error (k) if( orl in mystring): yes! print yes! else: print no 2

3 2. Please answer each with the value of variable x after the code is executed or the expected output, as appropriate. Write the value of x in the proper form to indicate its type. If the execution of the code produces an error, just write error. The code for each problem is following its blank. Assume all functions are defined correctly. These questions are 3 points each (a) x = 3 x = addone(3) def addone(num): return num num = num + 1 (b) s = 0 [Code below (c)] (c) output: 3 x = 3 y = 1 s = sum(x,y) print x def sum(x,y): x=-1 return x + y (d) x = 12 a = b = 4 x,y = a,b (e) x = 50 x = 0 for i in range(10): for j in range(5): x = x + 1 (f) output: 1234 for i in range(1,5): sys.stdout.write(i) 3

4 3. Short Answer. (a) (3 pts) What is a variable s scope? What is the scope of a function parameter? A variable s scope is the part of the program where it may be accessed. A function parameter s scope is its function. -2 variable scope wrong -1 function parameter scope wrong (b) (3 pts) What does it mean for an operator to be overloaded? Give an example. An operator is overloaded if it has different meanings in different contexts. + and * both overloaded and are likely to be the most common examples. -2 overload definition wrong -1 example wrong (c) (2 pts) How is the empty string represented? "" All or nothing (d) (2 pts) If a function does not have a return statement, what value is returned? None All or nothing 4

5 4. Write a segment of code that accepts a string from the user and then sums each character s location in the alphabet. It should then print the sum. For example: The string abc would result in a sum of 3 since a is at location 0 in the alphabet, b is at location 1, and c is at location 2, and = 3. You may use find() from the string library. Its documentation is below. string.find(s, sub) Return the lowest index in s where the substring sub is found such that sub is wholly contained in s. Return -1 on failure. import string alphabet="abcdefghijklmnopqrstuvwxyz" mystring = raw_input("enter a string: ") mysum = 0 for ch in mystring: mysum = mysum + string.find(alphabet, ch) print mysum Lowercase only is fine. This question is worth 15 points. -5 incorrect determination of location in alphabet -2 incorrect parameters to string.find -2 no alphabet -1 doesn t get return value -2 sum does not accumulate -5 missing loop -2 incorrect indices/range -3 loops over the wrong string -1 print missing or in wrong location -2 raw input missing -1 wrong input 5

6 5. In the space provided below, write: (a) A function isodd() that accepts a number parameter, calculates whether that number is odd, and returns one of the Boolean values indicating the result. (b) A function oddcalcs() that accepts two numbers and calculates their sum. It should then use isodd() to determine whether the sum is odd and whether each of the parameters is odd. It should return a tuple with three Boolean values: the first Boolean value should indicate whether the first parameter is odd, the second Boolean value should indicate whether the second parameter is off, and the last should indicate whether the sum is odd. (c) A segment of code that calls the oddcalcs() function with the arguments x and y and stores the result appropriately. def isodd(num): return num%2==1 def oddcalcs(x,y): sum = x + y return isodd(x),isodd(y),isodd(sum) a,b,c=oddcalcs(x,y) This question is worth 15 points. isodd: -1 wrong parameter -2 incorrect odd calculation -2 wrong or missing return oddcalcs: -1 wrong parameters -3 wrong return -1 each missing value -1 incorrect use of isodd() call: -3 missing call -2 incorrect assignment 6

7 6. A science lab has a yeast colony whose population doubles every two hours. Unfortunately, a random percentage of the population dies at the end of every day. Write a Python program that prompts the user for the original yeast population and then prints the new population every two hours for a week. For example, if the original population is 2, then the population after two hours will be 4, after four hours will be 8, after six hours will be 16, after eight hours will be 32,... Use random.random() to determine the percentage that dies at the end of each day. Its documentation is below. random.random() Return a random floating point number in the range [0.0, 1.0). import random def main(): main() pop = input("please enter current yeast population: ") for day in range(7): for hour in range(2,25,2): pop = pop*2 print "for hour %d: population is %d" % (hour, pop) percent_dead = random.random() pop = pop * (1-percent_dead) This question is worth 20 points. -5 missing days in week loop or failure to check for end of week or misplaced loop/checking for end -4 bad loop -2 wrong range -5 missing hours in day loop or failure to check for each two hours or misplaced loop/hour check -4 bad loop -2 wrong range -2 failure to update value -3 missing or wrong population doubling calculation -2 misplaced population doubling 7

8 -1 incorrect math -3 missing killing off calculation -2 misplaced killing off -1 for killing off wrong percentage -2 missing input -1 wrong input -2 missing or misplaced print statement 8

9 Scratch Paper 9

10 Scratch Paper 10