Back to Basics: More Strings in Python

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(Image: HowtoGeek.com)

Last week we implemented a blockchain, which was quite intensive, so this week I opted to give you a break with a simple string problem. It's good practice, right? Let's take a look.

# Write a method to replace all spaces in a string with '%20'. 
# Ignore any additional white space at the beginning or end of the string.

If you've been on the internet, which I imagine you have, you may have noticed that URLs never have spaces in them. If you try to type a URL in your browser and include a space, you might notice it converts it to an escape character: '%20'. Try typing "http://google.com/ /" in Chrome, and you'll see it converts it to "http://google.com/%20/". Not that that page goes anywhere, but you get the point.

For this challenge, we're going to write a method to do what your browser just did, i.e. take a url as a string and return the encoded form with spaces replaced by '%20'. Let's start by defining our method which takes in a string s.

def urlify(s):
    pass

Next, let's take a look at the fine print: ignore any additional white space at the beginning or end of the string. Let's take care of this right away by using the Python strip() method to remove any leading or trailing whitespace on our string.

def urlify(s):
    s = s.strip()

Let's think about addressing these spaces. We could loop through the string and look at each character, checking if it's a space, and if so, replace it. However, there's an easier way to do this. Python has some built in methods to split a string based on a given character. Then, we can join them back together with "%20" replacing the spaces.

Start by calling the split() method. We can call this after our .strip(). The method takes one argument, the divider character, which in our case is " ", a single space.

def urlify(s):
  s = s.strip().split(" ")

This should give us a list of strings. For example, if the string was "Hello my name is Erik", now s is a list containing ["Hello", "my", "name", "is", "Erik"].

Now we just have to join the list items together. The .join() method is a little strange; you call it on the string you want to put in between the words, and pass in the list as an argument. So, if our replacement string is "%20", we call .join() after it (I'm just wrapping it with parentheses), and pass it s, which is now a list. We could redefine this variable to s, but I'm just going to return it.

def urlify(s):
  s = s.strip().split(" ")
  return ("%20").join(s)

And that's it! To recap:

1. Strip whitespace.
2. Split on the space character.
3. Join with "%20" in between each item.

Test it out

Pass our urlify method a string containing some spaces. It should return a string with those spaces replaced by the URL escape character.

print(urlify("My name is Erik "))
# -> My%20name%20is%20Erik

That's it for this week; hopefully we gave your brain a rest after all those linked lists. Don't forget about them for too long, as next week we'll be tackling binary trees merge sort. See you then!

<< Week 9: Blockchain | View Solution on GitHub | Week 11: Merge Sort >>

Sheamus Heikkila is formerly a Teaching Assistant at General Assembly Seattle. This blog is not associated with GA.