Python [:-1], [::-1], [n::-1]用法

List & String index rules

List和 String的 index rules都相同

• 從左到右, index從 0開始遞增
• 從右到左, index從 -1開始遞減

[:-1], [::-1], [n::-1]用法

str[start:end] or list[start:end]

從 start到 end - 1

# String
str = "damon"
print(str[0:3])  # dam  -> from 0 to 2
print(str[2:4])  # mo   -> from 2 to 3
print(str[:-1])  # damo -> from 0 to -2
# List
list = ['d', 'a', 'm', 'o', 'n']
print(list[0:4])  # ['d', 'a', 'm', 'o'] -> from 0 to 3
print(list[2:5])  # ['m', 'o', 'n']      -> from 2 to 4
print(list[:-2])  # ['d', 'a', 'm']      -> from 0 to -3

Default value

• start default value = 0, 即 str[:3]相當於 str[0:3]
• enddefault value = len(str), 即 str[2:]相當於 str[2:len(str)]
• 當 start, end都為 default value, 即 str[:]相當於 str[0:len(str)], 表示複製一份 str

# String
str = "damon"
print(str[:3])  # dam   -> from 0 to 2
print(str[2:])  # mon   -> from 2 to 4
print(str[:])   # damon -> from 0 to 4
# List
list = ['d', 'a', 'm', 'o', 'n']
print(list[:4])  # ['d', 'a', 'm', 'o']      -> from 0 to 3
print(list[2:])  # ['m', 'o', 'n']           -> from 2 to 4
print(list[:])   # ['d', 'a', 'm', 'o', 'n'] -> from 0 to 4

str[start:end:step] or list[start:end:step]

Step正負的意義

• step > 0, 從左到右
• step < 0, 從右到左

Step value

• 向左或向右 n步取一個值

Default value

• start和 end 的 default value同 str[start:end], step default value = 1, 即 str[0:3:1]相當於 str[0:3]
• 當 step < 0時, start使用 default value, 預設為 -1, end使用 default value, 預設為 -len(str) - 1, 即 str[::-1]相當於 str[-1:-len(str) - 1:-1], 表示從最後一個 element到第一個 element複製一遍 (reverse)

# String
str = "damon"
print(str[::])     # damon -> from 0 to 4, step 1
print(str[::2])    # dmn   -> from 0 to 4, step 2
print(str[::-1])   # nomad -> from -1 to -5, step -1
print(str[::-2])   # nmd   -> from -1 to -5, step -2
print(str[2::-1])  # mad   -> from 2(-3) to -5, step -1
# List
list = ['d', 'a', 'm', 'o', 'n']
print(list[2::2])   # ['m', 'n']      -> from 2 to 4, step 2
print(list[::-2])   # ['n', 'm', 'd'] -> from -1 to -5, step -2
print(list[::-3])   # ['n', 'a']      -> from -1 to -5, step -3
print(list[2::-1])  # ['m', 'a', 'd'] -> from 2(-3) to -5, step -1