Loops — Python 练习题

Grades 9–12 中等难度 10 道题

Loops repeat code while a condition is true (while) or iterate over a sequence (for). They eliminate repetitive code and are essential for processing collections and running simulations.

Python for loop: for i in range(5) runs i = 0,1,2,3,4. range(start, stop) excludes stop. while loops need a condition that eventually becomes false to avoid infinite loops.

Common patterns: accumulator (total += x), counter (count += 1), and finding max/min. Always check loop bounds and off-by-one errors.

练习的技能

  • Writing for and while loops
  • Using range() correctly
  • Tracing loop output
  • Avoiding infinite loops
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练习题：Loops

说明： 仔细解答每一道题。清晰地展示所有解题过程。在指定位置或按指示在另一张纸上写出最终答案。

  1. 1.Write a for loop that prints the numbers from 1 to 10, each on a new line.

Loops — 练习题（续）

  1. 2.What is the output of the following code? 
    total = 0
for i in range(5):
    total += i
print(total)

Loops — 练习题（续）

  1. 3.Write a while loop that repeatedly asks the user to enter a number. The loop should stop when the user enters the number 0. For each number entered (except 0), print its square.

Loops — 练习题（续）

  1. 4.How many times will the following loop execute? 
    count = 0
while count < 10:
    print("Hello")
    count += 2

Loops — 练习题（续）

  1. 5.Use a for loop to calculate and print the sum of all even numbers between 1 and 20 (inclusive).

Loops — 练习题（续）

  1. 6.What is the output of this code? 
    for i in range(3, 0, -1):
    print(i)

Loops — 练习题（续）

  1. 7.Write a loop that prints the multiplication table for the number 7 (from 1 to 10). Each line should show the multiplication and result, e.g., “7 x 1 = 7”.

Loops — 练习题（续）

  1. 8.Which of the following loops will run infinitely? (Circle all that apply)
    1. A.
    2. B.
    3. C.
    4. D.

Loops — 练习题（续）

  1. 9.Write a for loop that iterates over a list names = ["Alice", "Bob", "Charlie"] and prints each name in uppercase.

Loops — 练习题（续）

  1. 10.The following code is supposed to print the numbers 1 through 5, but it has a bug. Identify the bug and write the corrected code. 
    num = 1
while num <= 5:
    print(num)

参考答案

  1. 1.
    最终答案： 
    for i in range(1, 11):
    print(i)
  2. 2.
    The loop sums 0 + 1 + 2 + 3 + 4 = 10.
    最终答案： 10
  3. 3.
    最终答案： 
    num = int(input("Enter a number: "))
while num != 0:
    print(num ** 2)
    num = int(input("Enter a number: "))
  4. 4.
    The loop executes 5 times (count = 0, 2, 4, 6, 8).
    最终答案： 5

参考答案（续）

  1. 5.
    最终答案： 
    total = 0
for num in range(2, 21, 2):
    total += num
print(total)
    The sum is 110.
  2. 6.
    最终答案： 
    3
2
1
  3. 7.
    最终答案： 
    for i in range(1, 11):
    print(f"7 x {i} = {7 * i}")

参考答案（续）

  1. 8.
    最终答案： Options (a) and (d) will run infinitely.
  2. 9.
    最终答案： 
    names = ["Alice", "Bob", "Charlie"]
for name in names:
    print(name.upper())
  3. 10.
    最终答案： The bug is that num is never incremented, so the loop runs infinitely. Corrected code: 
    num = 1
while num <= 5:
    print(num)
    num += 1
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需要避免的常见错误

  • Off-by-one errors with range boundaries
  • Infinite while loops when condition never changes
  • Modifying loop variable incorrectly
  • Using for when while is clearer for unknown iterations

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