[Tut] Python Join List Pairs

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Python Join List Pairs

Given a list of strings. Join the first with the second string, the second with the third, and so on. The one-liner

Code:

[lst[i] + lst[i+1] for i in range(0, len(lst), 2)]

solves the problem by using the range function to iterate over every other index

Code:

i=0, 2, 4, 5, ...

to concatenate the

Code:

i

-th and the

Code:

i+1

-th elements in a list comprehension expression with

Code:

lst[i] + lst[i+1]

.

You may already know the normal join function in Python:

Intro: Python Join

Problem: Given a listof elements. How to join the elements by concatenating all elements in the list?

Example: You want to convert list

Code:

['learn ', 'python ', 'fast']

to the string

Code:

'learn python fast'

.

Quick Solution: to convert a list of strings to a string, do the following.

• Call the
  Code:

  ''.join(list)
  method on the empty string
  Code:

  ''
  that glues together all strings in the
  Code:

  list
  and returns a new string.
  
  
• The string on which you call the join method is used as a delimiter between the list elements.
  
  
• If you don’t need a delimiter, just use the empty string
  Code:

  ''
  .
  

Code: Let’s have a look at the code.

Code:

lst = ['learn ', 'python ', 'fast'] print(''.join(lst))

The output is:

Code:

learn python fast

However, what if you want to do something different. Rather than joining all strings in the list to a single string, you want to join the strings in the list in pairs.

Problem: Python Join List Pairs

Problem: Given a list of strings. Join the first with the second string, the second with the third, and so on.

Example: Let’s consider the following minimal example:

Code:

['x', 'y', 'v', 'w']

Is there any simple way to pair the first with the second and the third with the fourth string to obtain the following output?

Code:

['xy', 'vw']

Note that the length of the strings in the list is variable so the following would be a perfectly acceptable input:

Code:

['aaaa', 'b', 'cc', 'dddd', 'eee', 'fff']

You can play with all three methods before diving into each of them:

https://trinket.io/embed/python/1a68a6e75f

Exercise: What’s the most Pythonic method?

Method 1: Zip() + List Comprehension

You can use the following smart one-liner solution

Code:

lst = ['aaaa', 'b', 'cc', 'dddd', 'eee', 'fff'] out = [x + y for x,y in zip(lst[::2], lst[1::2])] print(out) # ['aaaab', 'ccdddd', 'eeefff']

The one-liner uses the following strategy:

• Obtain two slices
  Code:

  lst[::2]
  and
  Code:

  lst[1::2]
  of the original list over every other element starting from the first and the second elements, respectively. If you need to refresh your slicing skills, check out my detailed blog article.
  
  
• Zip the two slices to a sequence of tuples using the
  Code:

  zip(...)
  function. This aligns the first with the second elements from the original list, the third with the forth, and so on. To refresh your
  Code:

  zip()
  skills, check out my blog tutorial here.
  
  
• Use list comprehension to iterate over each pair of values
  Code:

  x,y
  and concatenate them using list concatenation
  Code:

  x+y
  . For a refresher on list comprehension, check out this free tutorial—and for a refresher on list concatenation, check out this one.
  

Method 2: Iterator + List Comprehension

You can also use an iterator to accomplish this:

Code:

lst = ['aaaa', 'b', 'cc', 'dddd', 'eee', 'fff'] it = iter(lst) out = [x + next(it, '') for x in it] print(out) # ['aaaab', 'ccdddd', 'eeefff']

Here’s the idea:

• Create an iterator object it using the built-in function
  Code:

  iter()
  .
  
  
• Use list comprehension to go over each element in the iterator.
  
  
• Concatenate each element with the return value of calling the
  Code:

  next()
  function on the iterator. This ensures that the iterator moves one position further iterating over the list. So, the next element
  Code:

  x
  won’t be a duplicate.
  

Method 3: Use List Comprehension with Indexing

This method is the most straightforward one for Python beginners:

Code:

lst = ['aaaa', 'b', 'cc', 'dddd', 'eee', 'fff'] out = [lst[i] + lst[i+1] for i in range(0, len(lst), 2)] print(out) # ['aaaab', 'ccdddd', 'eeefff']

The idea is simply to use the rangefunction to iterate over every other index

Code:

i=0, 2, 4, 5, ...

to access the

Code:

i

-th and the

Code:

i+1

-th elements at the same time in the expression statement of list comprehension (to concatenatethose with

Code:

lst[i] + lst[i+1]

).

Where to Go From Here?

Enough theory, let’s get some practice!

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