# Python Join List Pairs

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Given a list of strings. Join the first with the second string, the second with the third, and so on. The one-liner `[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 `i=0, 2, 4, 5, ...` to concatenate the `i`-th and the `i+1`-th elements in a list comprehension expression with `lst[i] + lst[i+1]`.

You may already know the normal join function in Python:

## Intro: Python Join

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

Example: You want to convert list `['learn ', 'python ', 'fast']` to the string `'learn python fast'`.

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

• Call the `''.join(list)` method on the empty string `''` that glues together all strings in the `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: Let’s have a look at the code.

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

The output is:

`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:

`['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?

`['xy', 'vw']`

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

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

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

Exercise: What’s the most Pythonic method?

## Method 1: Zip() + List Comprehension

You can use the following smart one-liner solution

```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 `lst[::2]` and `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 `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 `zip()` skills, check out my blog tutorial here.
• Use list comprehension to iterate over each pair of values `x,y` and concatenate them using list concatenation `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:

```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 `iter()`.
• Use list comprehension to go over each element in the iterator.
• Concatenate each element with the return value of calling the `next()` function on the iterator. This ensures that the iterator moves one position further iterating over the list. So, the next element `x` won’t be a duplicate.

## Method 3: Use List Comprehension with Indexing

This method is the most straightforward one for Python beginners:

```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 range function to iterate over every other index `i=0, 2, 4, 5, ...` to access the `i`-th and the `i+1`-th elements at the same time in the expression statement of list comprehension (to concatenate those with `lst[i] + lst[i+1]`).

## Where to Go From Here?

Enough theory. Let’s get some practice!

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