Understanding Python if...else Statements with Practical Examples

Python's if...else statement is a fundamental control flow construct that executes different code blocks based on boolean conditions. The basic syntax is:

if condition:
    # code when condition is True
    pass
else:
    # code when condition is False
    pass

Below are several illustrative examples.

Example 1: Simple if‑else to check sign of a number

number = int(input("请输入一个整数："))
if number > 0:
    print("这是一个正数。")
elif number < 0:
    print("这是一个负数。")
else:
    print("这是零。")

Example 2: Grade classification

score = 75
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "D"
print(f"成绩等级为: {grade}")

Example 3: Leap year determination

year = 2024
if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
    print(f"{year} 是闰年")
else:
    print(f"{year} 不是闰年")

Example 4: Weather‑based output

weather = "sunny"
if weather == "rainy":
    print("带伞出门")
elif weather == "cloudy":
    print("可能需要外套")
else:
    print("享受好天气")

Example 5: Simple login verification

username = input("请输入用户名：")
password = input("请输入密码：")
if username == "admin" and password == "123456":
    print("登录成功")
else:
    print("登录失败")

More advanced conditional examples include:

Recursive Fibonacci

def fibonacci(n):
    if n <= 0:
        print("输入应为正整数")
    elif n == 1:
        return 0
    elif n == 2:
        return 1
    else:
        return fibonacci(n-1) + fibonacci(n-2)
print(fibonacci(10))

Bubble sort implementation

def bubble_sort(arr):
    n = len(arr)
    for i in range(n):
        for j in range(0, n-i-1):
            if arr[j] > arr[j+1]:
                arr[j], arr[j+1] = arr[j+1], arr[j]
    return arr
numbers = [64, 34, 25, 12, 22, 11, 90]
sorted_numbers = bubble_sort(numbers)
print("排序后的数组:", sorted_numbers)

Number guessing game with limited attempts

import random
secret_number = random.randint(1, 20)
guesses_taken = 0
print('我想了一个1到20之间的数字')
while guesses_taken < 6:
    guess = int(input('请猜这个数字: '))
    guesses_taken += 1
    if guess < secret_number:
        print('太小了')
    elif guess > secret_number:
        print('太大了')
    else:
        break
if guess == secret_number:
    print(f'干得漂亮，你在第{guesses_taken}次尝试中猜到了数字!')
else:
    print(f'很遗憾，你没有在6次机会内猜到。我想的数字是{secret_number}')

Age‑based population categorization

ages = [24, 55, 18, 30, 68, 15, 75]
categories = []
for age in ages:
    if age < 18:
        categories.append("未成年人")
    elif 18 <= age < 60:
        categories.append("成年人")
    else:
        categories.append("老年人")
print(categories)

Bank account balance check with multiple conditions

class BankAccount:
    def __init__(self, balance):
        self.balance = balance
    def withdraw(self, amount):
        if amount > self.balance:
            print("余额不足")
        elif amount < 0:
            print("取款金额不能为负")
        else:
            self.balance -= amount
            print(f"成功取出{amount}元，剩余{self.balance}元")
account = BankAccount(1000)
account.withdraw(500)   # 成功
account.withdraw(-100) # 错误：取款金额不能为负
account.withdraw(1500) # 错误：余额不足

These examples demonstrate how conditional statements can be combined with other Python constructs to solve a wide range of practical programming problems.