Q31. Which of the following list comprehensions is equivalent to this loop?

result = []
for x in range(5):
result.append(x * 2)

Question

Q31. Which of the following list comprehensions is equivalent to this loop?

result = []
for x in range(5):
    result.append(x * 2)

BlackTom AI · Accepted Answer

List comprehensions evaluate an expression for each element in an iterable, so the loop that appends x * 2 for every x from range(5) becomes [x * 2 for x in range(5)].

1) Relevant knowledge points and concepts
- A list comprehension has the form: [expression for variable in iterable if condition].  
- The expression (what you append) comes before the for-clause.  
- range(5) produces the sequence 0, 1, 2, 3, 4.

2) Step-by-step reasoning
- Original loop:
  result = []
  for x in range(5):
      result.append(x * 2)
- For every x yielded by range(5), we compute x * 2 and append it. In a comprehension, we move that computation to the front: [x * 2 for x in range(5)].

3) Intermediate steps (iteration details)
- x = 0 -> x * 2 = 0 -> append 0  
- x = 1 -> x * 2 = 2 -> append 2  
- x = 2 -> x * 2 = 4 -> append 4  
- x = 3 -> x * 2 = 6 -> append 6  
- x = 4 -> x * 2 = 8 -> append 8  
Resulting list: [0, 2, 4, 6, 8]

4) Why the correct answer is correct
- The comprehension [x * 2 for x in range(5)] exactly mirrors the loop: it evaluates x * 2 for each x in range(5) and collects the results in a list. The order and computed values match the loop.

5) Why the other options are incorrect
- [x for x in range(5) * 2]: range(5) is not multiplied this way; even if list(range(5))*2 duplicates a list, the expression yields x not x*2—behavior differs.  
- [x * 2 in range(5)]: This is an expression using the membership operator 'in' (would produce booleans if x defined), not a comprehension with a for-clause; it does not produce the intended numeric values.  
- [x * 2 for range(5)]: Syntax error—for requires a target variable ("for x in ..."), not just the iterable.  
- [x for x * 2 in range(5)]: Invalid target in the for-clause ("x * 2 in ...") is not a valid assignment target; also the expression yields x, not x*2.

6) Conclusion
The equivalent list comprehension is [x * 2 for x in range(5)], which matches the provided correct answer. Confirmed: [x * 2 for x in range(5)].

Q31. Which of the following list comprehensions is equivalent to this loop? result = [] for x in range(5): result.append(x * 2)单项选择题

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