Python if else 條件判斷

條件判斷讓程式根據不同的條件執行不同的程式碼。

if 語法

if 條件:
    # 條件為 True 時執行

age = 20

if age >= 18:
    print("You are an adult")

if...else

if 條件:
    # 條件為 True 時執行
else:
    # 條件為 False 時執行

age = 15

if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

輸出：

You are a minor

if...elif...else

當有多個條件要判斷時，使用 elif（else if 的縮寫）：

score = 85

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
else:
    print("F")

輸出：

B

程式會從上到下依序檢查條件，一旦某個條件為 True，就執行該區塊的程式碼，然後跳過剩餘的 elif 和 else。

巢狀 if

if 語句可以巢狀使用：

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("You need a license to drive")
else:
    print("You are too young to drive")

條件運算

比較運算子

邏輯運算子

age = 25
income = 50000

if age >= 18 and income >= 30000:
    print("Eligible for credit card")

if age < 18 or income < 20000:
    print("Not eligible")

if not (age < 18):
    print("You are an adult")

成員運算子

fruits = ["apple", "banana", "orange"]

if "apple" in fruits:
    print("Apple is in the list")

if "grape" not in fruits:
    print("Grape is not in the list")

身份運算子

x = None

if x is None:
    print("x is None")

if x is not None:
    print("x has a value")

三元運算子 (Conditional Expression)

一行寫完簡單的 if-else：

# 語法: value_if_true if condition else value_if_false

age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

# 等同於
if age >= 18:
    status = "adult"
else:
    status = "minor"

# 實際應用
score = 75
result = "Pass" if score >= 60 else "Fail"
print(result)  # Pass

x = 10
abs_x = x if x >= 0 else -x
print(abs_x)  # 10

Truthy 和 Falsy

條件不一定要是布林值，Python 會自動將值轉換為布林值：

# 這些值是 Falsy（會被當作 False）
# False, None, 0, 0.0, "", [], {}, set()

name = ""
if name:
    print(f"Hello, {name}")
else:
    print("Name is empty")  # 會執行這行

my_list = [1, 2, 3]
if my_list:
    print("List is not empty")  # 會執行這行

實際範例

判斷奇偶數

num = 7

if num % 2 == 0:
    print(f"{num} is even")
else:
    print(f"{num} is odd")

判斷閏年

year = 2024

if (year % 4 == 0 and year % 100 != 0) or (year % 400 == 0):
    print(f"{year} is a leap year")
else:
    print(f"{year} is not a leap year")

找出最大值

a, b, c = 10, 25, 15

if a >= b and a >= c:
    max_value = a
elif b >= a and b >= c:
    max_value = b
else:
    max_value = c

print(f"Maximum: {max_value}")  # Maximum: 25

# 當然，Python 有內建函數
print(max(a, b, c))  # 25