PCEP Control Flow, Loops, Lists and Logic Practice Test 4 — 15 Questions

Question 1

A quality engineer stores measurement readings in the list `readings`. A sentinel value of `-1` marks the end of valid data, but later entries may still exist in the list. The engineer needs to total only the values before the first `-1`. Which code fragment produces the correct total in `total`?

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

total = 0
for r in readings:
    if r == -1:
        break
    total += r. The task requires summing elements up to but not including the first sentinel. A loop that adds each reading and executes `break` upon seeing `-1` terminates at exactly the right point, leaving post-sentinel values untouched. Skipping the sentinel with `continue`, filtering without stopping, or relying on `index()` all either include extra values or fail when the sentinel is absent.

Answer

total = 0
for r in readings:
    if r == -1:
        continue
    total += r

Answer

total = 0
for r in readings:
    if r != -1:
        total += r
    else:
        pass

Answer

total = sum(readings[:readings.index(-1)])

Question 2

A developer processes batches of readings stored as a list of lists named batches. She wants to print the value 7 only if the number 7 appears somewhere in the second batch, batches[1], and print nothing otherwise. Which code correctly tests that condition?

Accepted Answer

if 7 in batches[1]:
    print(7). Membership testing with in only compares the target value against the elements of the container it is applied to. Applying it to the indexed inner list batches[1] compares 7 against each reading in that batch, which is the intended check, whereas applying it to the outer list or a slice compares 7 against list objects and is always false.

Answer

if batches[1][7]:
    print(7)

Answer

if 7 in batches[1:]:
    print(7)

Answer

if 7 in batches:
    print(7)

Question 3

A data analyst has a list of daily temperatures in Celsius: `temps = [15, 22, 8, 30]`. She needs to create a new list containing each temperature converted to Fahrenheit using the formula `F = C * 9/5 + 32`. Which list comprehension achieves this?

Accepted Answer

[c * 9/5 + 32 for c in temps]. A list comprehension that converts all Celsius temperatures to Fahrenheit should simply apply the formula to each element without any filtering. The correct option does this. Adding an `if` clause, whether `if c > 0` or `if c`, incorrectly removes some temperatures from the result, which is not requested.

Answer

[c * 9/5 + 32 in temps]

Answer

[c * 9/5 + 32 for c in temps if c > 0]

Answer

[c * 9/5 + 32 for c in temps if c]

Question 4

A Python programmer is analyzing a list of test scores and wants to determine how many scores are above the average. The programmer writes a loop that first calculates the average, then iterates again to count scores greater than that average. Which code snippet correctly counts the number of scores above the average?

Accepted Answer

count = 0; avg = sum(scores) / len(scores); for s in scores: if s > avg: count += 1. To count scores above the average, the code must compute the average, initialize a counter, iterate over all scores, increment the counter when a score exceeds the average, and then display the final count. The correct snippet does exactly that, using valid Python syntax and a strict greater-than comparison.

Answer

count = 0; avg = sum(scores) / len(scores); for s in scores: if s > avg: count = count + 1; else: break

Answer

count = 0; avg = sum(scores) / len(scores); for s in scores: if s >= avg: count++

Answer

count = 0; avg = sum(scores) / len(scores); for s in scores: if s > avg: count += 1; else: continue; print(count)

Question 5

A programmer is analyzing a list of integers `nums = [4, 7, 2, 9, 5]`. She wants to determine which of the following code snippets will correctly compute and print the sum of all even numbers in the list. (Choose two.)

Accepted Answer

print(sum(n for n in nums if n % 2 == 0)). The sum of even numbers can be computed either by an explicit loop that adds even numbers to a total, or by using a generator expression with `sum()`. Both approaches correctly filter and sum the even numbers. The other options either sum odd numbers, use incorrect assignment, or employ an indirect method that, while mathematically valid, is not a direct computation as required by the question's phrasing.

Answer

total = 0
for n in nums:
    if n % 2 != 0:
        total += n
print(total)

Answer

print(sum(nums) - sum(n for n in nums if n % 2 != 0))

Answer

total = 0
for n in nums:
    if n % 2 == 0:
        total = n
print(total)

Question 6

A developer is writing a Python script to analyze a list of daily temperatures. The script must print the first temperature that is below freezing (less than 0 degrees Celsius). If no such temperature exists, it should print 'No freezing temperatures'. Which code correctly implements this logic?

Accepted Answer

for temp in temperatures:
    if temp < 0:
        print(temp)
        break
else:
    print('No freezing temperatures'). The for-else construct is designed for search loops: the else block runs only if the loop completes without a break. When a freezing temperature is found, printing it and breaking prevents the else from executing. If the loop finishes without finding one, the else prints the fallback message. The other options either always print the fallback or never stop after the first match.

Answer

for temp in temperatures:
    if temp < 0:
        print(temp)
else:
    print('No freezing temperatures')

Answer

for temp in temperatures:
    if temp < 0:
        print(temp)
        continue
else:
    print('No freezing temperatures')

Answer

for temp in temperatures:
    if temp < 0:
        print(temp)
        break
print('No freezing temperatures')

Question 7

A developer is writing a Python script to simulate a simple game where a player can move on a grid. The script uses a while loop to repeatedly ask for a direction until the player enters 'quit'. The loop must also validate that the input is one of 'north', 'south', 'east', 'west', or 'quit'. If the input is invalid, it should print an error and ask again. Which code correctly implements this loop?

Accepted Answer

command = ''
while command != 'quit':
    command = input('Direction: ')
    if command in ('north', 'south', 'east', 'west'):
        print('Moving', command)
    elif command != 'quit':
        print('Invalid direction'). The correct implementation uses a while loop with a condition that checks for 'quit', reads input inside the loop, and validates the command. It prints movement for valid directions and an error for invalid ones, while exiting on 'quit'. The other options either loop infinitely, fail to handle 'quit' correctly, or misorder conditions.

Answer

command = input('Direction: ')
while command != 'quit':
    if command in ('north', 'south', 'east', 'west'):
        print('Moving', command)
    else:
        print('Invalid direction')
    command = input('Direction: ')

Answer

while True:
    command = input('Direction: ')
    if command in ('north', 'south', 'east', 'west'):
        print('Moving', command)
    elif command == 'quit':
        break
    else:
        print('Invalid direction')

Answer

while True:
    command = input('Direction: ')
    if command == 'quit':
        break
    elif command not in ('north', 'south', 'east', 'west'):
        print('Invalid direction')
    else:
        print('Moving', command)

Question 8

A programmer writes code that uses a while loop to process user input until the user types 'exit'. The code currently prints 'Done' after the loop, but it never exits. What is the most likely cause?

Accepted Answer

The variable controlling the loop is not updated inside the loop. If the variable controlling the loop (e.g., the user's input) is never updated inside the while loop, the loop condition will never become false, causing an infinite loop. In this scenario, the programmer likely reads input once before the loop but does not call input() again inside the loop to update the variable, so the loop never sees the 'exit' value.

Answer

The loop condition is 'while True'

Answer

The print statement is indented incorrectly

Answer

The input function is called outside the loop

Question 9

A list of numbers is defined as nums = [1, 2, 3, 4, 5]. Which expression returns the last element?

Accepted Answer

nums[-1]. Python uses zero-based indexing, so the first element is at index 0 and the last element is at index -1. Negative indices count from the end of the list, so nums[-1] directly accesses the last element (5) without needing to know the list length.

Answer

nums[5]

Answer

nums[0]

Answer

nums[-2]

Question 10

A developer writes the following code snippet:
for i in range(3):
    for j in range(2):
        if i == j:
            break
    else:
        print(i, 'outer')
What is the output?

Accepted Answer

2 outer. The code uses nested loops with a `for-else` construct. The `else` block executes only if the inner loop completes without a `break`. When `i == j`, the `break` exits the inner loop, skipping the `else`. For `i=0`, `j=0` triggers `break`; for `i=1`, `j=1` triggers `break`; for `i=2`, the inner loop runs `j=0,1` without any `i==j` (since 2 != 0 and 2 != 1), so the `else` executes, printing `2 outer`. Thus, only option B is correct.

Answer

0 outer
2 outer

Answer

0 outer
1 outer
2 outer

Answer

No output

Question 11

Given the code:
x = [1, 2, 3]
y = x
y.append(4)
print(x)
What is the output?

Accepted Answer

[1, 2, 3, 4]. In Python, variables hold references to objects, not the objects themselves. When `y = x` is executed, both `x` and `y` point to the same list object in memory. The `y.append(4)` method modifies that shared list in-place, so the change is reflected when `x` is printed, outputting `[1, 2, 3, 4]`.

Answer

Error

Answer

[1, 2, 3]

Answer

[1, 2, 3, 4, 5]

Question 12

A programmer wants to create a list of even numbers from 0 to 10 inclusive. Which list comprehension is correct?

Accepted Answer

[x for x in range(0,11) if x%2==0]. It uses `range(0,11)` to generate numbers from 0 to 10 inclusive, and the condition `if x%2==0` selects only even numbers (where the remainder when divided by 2 is 0). This matches the requirement exactly.

Answer

[x for x in range(0,10) if x%2==0]

Answer

[x for x in range(0,11) if x%2]

Answer

[x for x in range(0,11) if x%2==1]

Question 13

Which THREE of the following are valid ways to create a list with elements 10, 20, 30?

Accepted Answer

my_list = [x for x in (10,20,30)]. Option A is correct because a list comprehension iterating over the tuple (10,20,30) produces a new list [10, 20, 30]. Option B is correct because list((10, 20, 30)) converts the tuple into a list with those three elements. Option E is correct because a list literal [10, 20, 30] directly creates the desired list. Option C is invalid because list() accepts at most one iterable argument, so list(10, 20, 30) raises a TypeError. Option D is invalid because semicolons are not valid separators inside a Python list literal; elements must be comma-separated.

Answer

my_list = list(10, 20, 30)

Answer

my_list = [10; 20; 30]

Question 14

Which TWO of the following code snippets will print the numbers 0, 1, 2, 3, 4?

Accepted Answer

for i in range(0,5): print(i). Option A is correct because range(0,5) generates the sequence 0,1,2,3,4 — starting at 0 and stopping before the exclusive end value 5 — so print(i) outputs exactly those numbers. Option C is correct because range(5) is equivalent to range(0,5), producing the same 0 through 4 sequence. Option B is wrong because range(5+1) equals range(6), which yields 0,1,2,3,4,5 and thus also prints 5. Option D is wrong because range(0,5,2) uses a step of 2, producing only 0,2,4. Option E is wrong because range(1,6) starts at 1 and ends before 6, printing 1,2,3,4,5 instead of starting at 0.

Answer

for i in range(5+1): print(i)

Answer

for i in range(0,5,2): print(i)

Answer

for i in range(1,6): print(i)

Question 15

Which THREE of the following are valid list operations?

Accepted Answer

my_list.extend([5,6]). Option B, my_list.extend([5,6]), is valid because Python lists provide the extend() method, which appends each element of the given iterable to the end of the list, so [5,6] adds two separate items. Option D, my_list.insert(0,5), is valid because insert(index, value) is a standard list method that places the value 5 at index 0, shifting existing elements to the right. Option E, my_list.append(5), is valid because append() is the canonical list method for adding a single element to the end of the list. Option A, my_list.add(5), is not a list operation; add() belongs to set objects, not lists. Option C, my_list.push(5), is not a Python list method; push() is associated with stack-like structures in other languages (e.g., JavaScript arrays), whereas Python lists use append().

Answer

my_list.add(5)

Answer

my_list.push(5)