Sequence 序列

a sequence means a data container that has content is continuous and ordering, having index.

Slicing 切片

Means that, it get a Subsequence 子序列 from a sequence.

Ex: a list, tuple, string, etc that support slicing operation

Syntax

sequence[start_items : end_items : step]

Info

• Step = 1 (default value), means that operation is 1 by 1
• Step = 2, means that operation will always skip 1
• Step = n, means that operation will always skip n-1
• if step is negatives value, Its start from right to left (Reversed).

Also, with slicing operation will get an new sequence instead of modify it

Usage

sequence-slicing.py

# List
list_list = [0, 1, 2, 3, 4, 5, 6]
# index:     0  1  2  3  4  5  6

# Slicing:
# 1. From 1 to 4, step = 1 (can be ignored)
print(list_list[1:4:1]) # Expected: [1, 2, 3]
# Without step
print(list_list[1:4]) # Expected: [1, 2, 3]

# Tuple
tuple_list = (0, 1, 2, 3, 4, 5, 6)
# 2. from 0 to end, 7th item , step = 1 (can be ignored)

# 7 - 0 = 7 items.
print(tuple_list[0:7:1]) # (0, 1, 2, 3, 4, 5, 6)
# For starting to end can use:
print(tuple_list[:]) # (0, 1, 2, 3, 4, 5, 6)

# String
str_dc = "0123456789"
# Slicing From start to end step = 2
print(str_dc[::2]) # 02468

# Using negative step, -1 (Reversed)
print(str_dc[::-1]) # 9876543210
# Negative step, -2
print(str_dc[::-2]) # 97531

Output:

[1, 2, 3]
[1, 2, 3]
(0, 1, 2, 3, 4, 5, 6)
(0, 1, 2, 3, 4, 5, 6)
02468
9876543210
97531

Comprehensive Example

Given:

text = str("Hi, I'm John and he is Will")

• Define an original text
• Make text in reversed style.
• Get John in a original style and print it on console.

sequence-slicing-ex.py

# Define an original text
text = str("Hi, I'm John and he is Will")

# Make text in reversed style.
reversed_text = text[::-1]

print(reversed_text) # lliW si eh dna nhoJ m'I ,iH

# Get John in a original style
split_reversed_text = reversed_text.split()
splited_john = split_reversed_text[-3]

print(splited_john[::-1]) # John

Output:

lliW si eh dna nhoJ m'I ,iH
John