Understanding Loop Structures in Python: for, while, break, continue, and Nested Loops

When writing programs we often encounter situations that require repeating a command or a group of commands, such as continuously moving a robot toward a goal or printing "hello, world" every second for an hour; loops provide a structured way to handle these repetitions.

Python offers two primary loop constructs: the for‑in loop , suitable when the number of iterations is known, and the while loop , used when the iteration count is uncertain.

for‑in loop

If the number of iterations is predetermined, a for‑in loop is recommended, for example to compute the sum of numbers from 1 to 100:

<code>"""\n用for循环实现1~100求和\n"""
total = 0
for x in range(1, 101):
    total += x
print(total)
</code>

The range function can generate various sequences; examples include range(101) (0‑100), range(1, 101) (1‑100), range(1, 101, 2) (odd numbers), and range(100, 0, -2) (even numbers descending).

Using range(2, 101, 2) we can sum the even numbers between 1 and 100:

<code>"""\n用for循环实现1~100之间的偶数求和\n"""
total = 0
for x in range(2, 101, 2):
    total += x
print(total)
</code>

while loop

When the exact number of iterations is unknown, a while loop controls execution with a boolean expression; the loop continues while the expression evaluates to True and stops when it becomes False . An example is a "guess the number" game:

<code>"""\n猜数字游戏\n"""
import random
answer = random.randint(1, 100)
counter = 0
while True:
    counter += 1
    number = int(input('请输入: '))
    if number < answer:
        print('大一点')
    elif number > answer:
        print('小一点')
    else:
        print('恭喜你猜对了!')
        break
print(f'你总共猜了{counter}次')
</code>

Keywords break and continue allow early exit from a loop or skipping the remainder of the current iteration, respectively.

Nested loops

Loops can be nested inside one another; a classic example is printing the multiplication table (九九表):

<code>"""\n打印乘法口诀表\n"""
for i in range(1, 10):
    for j in range(1, i + 1):
        print(f'{i}*{j}={i * j}', end='\t')
    print()
</code>

In this code the outer loop creates nine rows, while the inner loop generates the columns for each row.

Practice exercises

1. Determine whether a given positive integer is a prime number.

<code>"""\n输入一个正整数判断它是不是素数\n"""
num = int(input('请输入一个正整数: '))
end = int(num ** 0.5)
is_prime = True
for x in range(2, end + 1):
    if num % x == 0:
        is_prime = False
        break
if is_prime and num != 1:
    print(f'{num}是素数')
else:
    print(f'{num}不是素数')
</code>

2. Compute the greatest common divisor (GCD) and least common multiple (LCM) of two positive integers.

<code>"""\n输入两个正整数计算它们的最大公约数和最小公倍数\n"""
x = int(input('x = '))
y = int(input('y = '))
if x > y:
    x, y = y, x
for factor in range(x, 0, -1):
    if x % factor == 0 and y % factor == 0:
        print(f'{x}和{y}的最大公约数是{factor}')
        print(f'{x}和{y}的最小公倍数是{x * y // factor}')
        break
</code>

Summary

By mastering Python's branching and looping structures—using for when the iteration count is known, while when it is not, and employing break and continue for flow control—you can solve a wide range of practical problems efficiently.