To join specific list elements (e.g., with indices 0, 2, and 4) and return the joined string that’s the concatenation of all those, use the expression ''.join([lst[i] for i in [0, 2, 4]]). The list comprehension statement creates a list consisting of elements lst[0], lst[2], and lst[4]. The ''.join() method concatenates those elements using the empty string as a delimiter.
Today, I stumbled upon the following problem—and I found it quite interesting and of high educational value.
Problem: Given a list of strings. How to join specific list elements and return the joined string that’s the concatenation of all those specific list elements?
Example: You’ve got the following list of strings.
lst = ['hello ', 'bye', 'world', '?', '!']
You want to join and concatenate the first, third, and fourth elements of the list.
'hello world!'
Related article: A variant of this problem is to join all element in a slice and replace the slice with the joined string. Read about this solution on my blog article.
Here’s a quick overview of the methods, I’m going to discuss in this tutorial:
Exercise: Concatenate elements with indices 1, 2, and 3 with each of the methods!
Method 1: Indexing + Concatenation
If you have just a few elements, it’s best to use simple indexing in combination with string concatenation +. You just won’t find a more readable solution:
lst = ['hello ', 'bye', 'world', '?', '!'] print(lst[0] + lst[2] + lst[4]) # hello world!
However, if there’s a regularity in your indices in that you want to concatenate every i-th element in your list, you can use slicing.
Method 2: Slicing + Join
You’ll use two concepts in this method:
- Slicing to select the string elements you want to concatenate.
- The
string.join(...)method to concatenate the elements in the slice.
Slicing is a concept to carve out a substring from a given string. Use slicing notation s[start:stop:step] to access every step-th element starting from index start (included) and ending in index stop (excluded). All three arguments are optional, so you can skip them to use the default values (start=0, stop=len(lst), step=1). For example, the expression s[2:4] from string 'hello' carves out the slice 'll' and the expression s[:3:2] carves out the slice 'hl'.
The string.join(iterable) method joins the string elements in the iterable to a new string by using the string on which it is called as a delimiter.
lst = ['hello ', 'bye', 'world', '?', '!']
print(''.join(lst[::2]))
# hello world!This method is more effective because it doesn’t involve concatenating multiple immutable data types (string). This avoids the creation of multiple strings in memory. Also, it’s more concise—even if you want to concatenate large sublists.
Method 3: Itemgetter + Join
If there’s no index pattern of elements you want to concatenate, you cannot use slicing. In this case, it’s best to access arbitrary elements from your list. You can use the itemgetter module:
from operator import itemgetter
lst = ['hello ', 'bye', 'world', '?', '!']
print(''.join(itemgetter(0, 2, 4)(lst)))
# hello world!The itemgetter returns a function that returns the elements with 0, 2, and 4 from the list (or, more general, iterable) you pass into the function.
Method 4: List Comprehension + Join
To create a manual list with only elements 0, 2, and 4, you can also use a short list comprehension call.
lst = ['hello ', 'bye', 'world', '?', '!']
print(''.join([lst[i] for i in [0, 2, 4]]))
# hello world!This is concise, efficient, and readable (for Python coders). You don’t even need to create a list but use a generator expression to pass into the join() method, which may be slightly more Pythonic:
lst = ['hello ', 'bye', 'world', '?', '!']
print(''.join(lst[i] for i in [0, 2, 4]))
# hello world!Where to Go From Here?
Enough theory. Let’s get some practice!
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