Peer Instruction: Strings Multiple Choice Questions

11-9-1: What is the value of s after the following code runs?

s = "abc"
s = "d" * 3 + s
s = s + "" * 3
s = s + "q"

• "abcddd q"
• Incorrect! The value of s changes in the following order from the first line- s="abc", s="dddabc", s="dddabc" and s="dddabcq".
• "abcddd" "" "" "q"
• Incorrect! The value of s changes in the following order from the first line- s="abc", s="dddabc", s="dddabc" and s="dddabcq".
• "qdddabc"
• Incorrect! The value of s changes in the following order from the first line- s="abc", s="dddabc", s="dddabc" and s="dddabcq".
• "dddabcq"
• Correct! The value of s changes in the following order from the first line- s="abc", s="dddabc", s="dddabc" and s="dddabcq".
• I don't know
• Incorrect! The value of s changes in the following order from the first line- s="abc", s="dddabc", s="dddabc" and s="dddabcq".

11-9-2: What is the value of y after the following code runs?

s = "Cats"
y = s[len(s)-1]

• C
• Incorrect! Here, len(s) = 4 and s[len(s)-1] = s[3]. So, the value of y will be the fourth character in the string i.e. "s".
• t
• Incorrect! Here, len(s) = 4 and s[len(s)-1] = s[3]. So, the value of y will be the fourth character in the string i.e. "s".
• s
• Correct! Here, len(s) = 4 and s[len(s)-1] = s[3]. So, the value of y will be the fourth character in the string i.e. "s".
• Nothing, this will cause an error
• Incorrect! Here, len(s) = 4 and s[len(s)-1] = s[3]. So, the value of y will be the fourth character in the string i.e. "s".
• I don't know
• Incorrect! Here, len(s) = 4 and s[len(s)-1] = s[3]. So, the value of y will be the fourth character in the string i.e. "s".

11-9-3: What will the following code print?

s = "bats"
print(s[0:len(s)])

• ats
• Incorrect! Here len(s) = 4. So, s[0:4] = "bats" i.e. the first four characters.
• bat
• Incorrect! Here len(s) = 4. So, s[0:4] = "bats" i.e. the first four characters.
• bats
• Correct! Here len(s) = 4. So, s[0:4] = "bats" i.e. the first four characters.
• This will throw an exception
• Incorrect! Here len(s) = 4. So, s[0:4] = "bats" i.e. the first four characters.
• I don't know
• Incorrect! Here len(s) = 4. So, s[0:4] = "bats" i.e. the first four characters.

11-9-4: What will the following code print?

s = "Vampires"
print(s[3:-1]

• mpire
• Incorrect! Negative index- "-1" has been used in the slice. So, the slice will end at the second last character.
• pires
• Incorrect! Negative index- "-1" has been used in the slice. So, the slice will end at the second last character.
• pire
• Correct! Negative index- "-1" has been used in the slice. So, the slice will end at the second last character.
• pir
• Incorrect! Negative index- "-1" has been used in the slice. So, the slice will end at the second last character.
• I don't know
• Incorrect! Negative index- "-1" has been used in the slice. So, the slice will end at the second last character.

11-9-5: What does the following code return?

def mystery(s):
   new_s = ""
   for c in s:
      new_s = c + new_s
   return new_s

• Returns a copy of s
• Incorrect! For example, take s = "xyz". With each step in the loop, the value of new_s will change in the order- new_s = x, new_s = yx and new_s = zyx.
• Returns the reverse of s
• Correct! For example, take s = "xyz". With each step in the loop, the value of new_s will change in the order- new_s = x, new_s = yx and new_s = zyx.
• Returns a string with only the last character of s
• Incorrect! For example, take s = "xyz". With each step in the loop, the value of new_s will change in the order- new_s = x, new_s = yx and new_s = zyx.
• Returns a string with only the first character of s
• Incorrect! For example, take s = "xyz". With each step in the loop, the value of new_s will change in the order- new_s = x, new_s = yx and new_s = zyx.
• I don't know
• Incorrect! For example, take s = "xyz". With each step in the loop, the value of new_s will change in the order- new_s = x, new_s = yx and new_s = zyx.

11-9-6: What is the value of s after the following code runs?

s = 'abc'
s = 'd' * 3 + s
s = s + ' ' * 3
s = s + 'q'

• "abcddd q"
• Incorrect! Here, 'd' * 3 + s = 'ddd' + 'abc' = 'dddabc'. Then ' ' adds three spaces at the end of the string followed by a 'q' at the end.
• "abcddd q"
• Incorrect! Here, 'd' * 3 + s = 'ddd' + 'abc' = 'dddabc'. Then ' ' adds three spaces at the end of the string followed by a 'q' at the end.
• "abcdddq"
• Incorrect! Here, 'd' * 3 + s = 'ddd' + 'abc' = 'dddabc'. Then ' ' adds three spaces at the end of the string followed by a 'q' at the end.
• "qdddabc"
• Incorrect! Here, 'd' * 3 + s = 'ddd' + 'abc' = 'dddabc'. Then ' ' adds three spaces at the end of the string followed by a 'q' at the end.
• "dddabc q"
• Correct! Here, 'd' * 3 + s = 'ddd' + 'abc' = 'dddabc'. Then ' ' adds three spaces at the end of the string followed by a 'q' at the end.

11-9-7: What is a good description of what the following function does?

def mystery(s):
   new_s = ''
   for c in s:
      new_s = c + new_s
   return new_s

• Returns a copy of s
• Incorrect! Consider s = 'abc'. In the first iteration, new_s = c + new_s = 'a' + '' = 'a'. In the second iteration, new_s = 'b' + 'a' = 'ba' and so on. So, it will return the reverse of s.
• Returns the reverse of s
• Correct! Consider s = 'abc'. In the first iteration, new_s = c + new_s = 'a' + '' = 'a'. In the second iteration, new_s = 'b' + 'a' = 'ba' and so on. So, it will return the reverse of s.
• Returns a string consisting of only the final character of s
• Incorrect! Consider s = 'abc'. In the first iteration, new_s = c + new_s = 'a' + '' = 'a'. In the second iteration, new_s = 'b' + 'a' = 'ba' and so on. So, it will return the reverse of s.
• Returns a string consisting of only the first character of s
• Incrrect! Consider s = 'abc'. In the first iteration, new_s = c + new_s = 'a' + '' = 'a'. In the second iteration, new_s = 'b' + 'a' = 'ba' and so on. So, it will return the reverse of s.

11-9-8: What is the value of val after this code executes?

val = 0
for i in 'ab':
   for j in 'cd':
      val += 1

• 1
• Incorrect! For each 'a' and 'b', the nested for loop will run twice. Thus, val gets incremented by 1 four times resulting in val = 4.
• 2
• Incorrect! For each 'a' and 'b', the nested for loop will run twice. Thus, val gets incremented by 1 four times resulting in val = 4.
• 4
• Correct! For each 'a' and 'b', the nested for loop will run twice. Thus, val gets incremented by 1 four times resulting in val = 4.
• 8
• Incorrect! For each 'a' and 'b', the nested for loop will run twice. Thus, val gets incremented by 1 four times resulting in val = 4.
• 16
• Incorrect! For each 'a' and 'b', the nested for loop will run twice. Thus, val gets incremented by 1 four times resulting in val = 4.

11-9-9: What is the value of val after this code executes?

val = 0
for i in 'abc':
   for j in 'def':
   val += 1

• 1
• Incorrect! For each 'a', 'b' and 'c', the nested for loop will run thrice. Thus, val gets incremented by 1 nine times resulting in val = 9.
• 3
• Incorrect! For each 'a', 'b' and 'c', the nested for loop will run thrice. Thus, val gets incremented by 1 nine times resulting in val = 9.
• 6
• Incorrect! For each 'a', 'b' and 'c', the nested for loop will run thrice. Thus, val gets incremented by 1 nine times resulting in val = 9.
• 9
• Correct! For each 'a', 'b' and 'c', the nested for loop will run thrice. Thus, val gets incremented by 1 nine times resulting in val = 9.
• 27
• Incorrect! For each 'a', 'b' and 'c', the nested for loop will run thrice. Thus, val gets incremented by 1 nine times resulting in val = 9.

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