[Tut] Python Join List Slice

[xSicKxBot]

Python Join List Slice

To join and replace all strings in a list [b]

Code:

lst

between start index 1 and end index 5, use the one-liner

Code:

lst[1:5] = [''.join(lst[1:5])]

. This solution is based on slice assignment that selects the slice you want to replace on the left and the values to replace it on the right side of the assignment.[/b]

Problem: Given a list of strings. How to join all strings in a given list slice and replace the slice with the joined string?

Example:You start with the following list:

Code:

lst = ['i', 'l', 'o', 'v', 'e', 'u']

You want to join the slice

Code:

['l', 'o', 'v', 'e']

with start index 1 and end index 4 (included) and replace it in the list to obtain the result:

Code:

# Output: ['i', 'love', 'u']

You can get a quick overview of all three methods in the following interactive Python shell. If you’re short on time—method 1 using slice assignment is the most Pythonic solution!

https://trinket.io/embed/python/a7c4fa6ef4

Exercise: Modify the code so that only elements with start index 2 and end index 4 (included) are replaced by the joined string!

Method 1: Slice Assignment

Slice assignment is a little-used, beautiful Python feature to replace a slice with another sequence. Select the slice you want to replaceon the left and the values to replace it on the right side of the equation.

Here’s how you can join and replace all strings in a list slice (between indices 1 and 5) in a single line of Python code:

Code:

# Method 1: Slice Assignments lst = ['i', 'l', 'o', 'v', 'e', 'u'] lst[1:5] = [''.join(lst[1:5])] print(lst) # ['i', 'love', 'u']

The one-liner

Code:

lst[1:5] = [''.join(lst[1:5])]

performs the following steps:

• Select the slice to be replaced on the left-hand side of the equation with
  Code:

  lst[1:5]
  . Read more about slicing on my Finxter blog tutorial.
  
  
• Create a listthat replaces this slice on the right-hand side of the equation with
  Code:

  [...]
  .
  
  
• Select the slice of string elements to be joinedtogether (
  Code:

  'l', 'o', 'v', 'e'
  ) with
  Code:

  lst[1:5]
  .
  
  
• Pass this slice into the join function to create a new string with all four characters
  Code:

  'love'
  .
  
  
• This string replaces all four selected positions in the original list.
  

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Method 2: List Concatenation + Slicing + Join

A simpler but quite readable method is to use simple list concatenation. Read more on my Finxter blog tutorial to master all different ways to concatenate lists in Python.

Code:

# Method 2: List Concatenation + Join lst = ['i', 'l', 'o', 'v', 'e', 'u'] lst = lst[:1] + [''.join(lst[1:5])] + lst[5:] print(lst) # ['i', 'love', 'u']

This approach uses three slicing calls to select (or create) three lists. Then, it glues them together using the

Code:

+

operator. This approach has the slight disadvantage that a new list is created in memory (rather than working on the old list). Thus, it’s slightly less memory-friendly as the first method.

Method 3: Naive

This method is what a non-Python coder (maybe coming from Java or C++) would use. It’s NOT the recommended way though.

Code:

# Method 3: Naive lst = ['i', 'l', 'o', 'v', 'e', 'u'] new = '' for i in range(1,5): new += lst[i] lst = lst[:1] + [new] + lst[5:] print(lst) # ['i', 'love', 'u']

Instead of selecting the slice of strings to be joined using slicing, the coder creates a string variable

Code:

new

and adds one character at-a-time. This is very inefficient as many different strings are created—each time one adds one more character to the string.

Where to Go From Here?

Enough theory, let’s get some practice!

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