{"id":25066,"date":"2020-02-25T12:15:10","date_gmt":"2020-02-25T10:15:10","guid":{"rendered":"https:\/\/www.webcodegeeks.com\/?p=25066"},"modified":"2020-06-23T14:55:45","modified_gmt":"2020-06-23T11:55:45","slug":"how-to-compare-strings-in-python-equality-and-identity","status":"publish","type":"post","link":"https:\/\/www.webcodegeeks.com\/python\/how-to-compare-strings-in-python-equality-and-identity\/","title":{"rendered":"How to Compare Strings in Python: Equality and Identity"},"content":{"rendered":"\nOnce again, we\u2019re back with another Python topic. Today, we\u2019ll talk about how to compare strings in Python. Typically, I try to stay away from strings because they have a lot of complexity (e.g. different languages, implementations, etc.). That said, I decided to take a risk with this one. Hope you like it!<\/p>\n\n\n\n
As a bit of a teaser, here\u2019s what you can expect in this article. We\u2019ll be looking at a few different comparison operators in Python including ==<\/code>, <<\/code>, <=<\/code>, >=<\/code>, and ><\/code> as well as is<\/code>. In addition, we\u2019ll talk about how these operators can be used to compare strings and when to use them. If you want to know more, you\u2019ll have to keep reading.<\/p>\n\n\n\nProblem Description<\/h2>\n\n\n\n
Let\u2019s imagine we\u2019re building up a simple search engine. For example, we have a bunch of files with text in them, and we want to be able search through those documents for certain keywords. How would we do that?<\/p>\n\n\n\n
At the core of this search engine, we\u2019ll have to compare strings. For instance, if we search our system for something about the Pittsburgh Penguins (say, Sidney Crosby), we\u2019ll have to look for documents that contain our keyword. Of course, how do we know whether or not we have a match?<\/p>\n\n\n\n
Specifically, we want to know how we can compare two strings for equality. For example, is \u201cSidney Crosby\u201d the same as \u201cSidney Crosby\u201d? How about \u201csidney crosby\u201d? Or even \u201cSiDnEy CrOsBy\u201d? In other words, what constitutes equality in Python?<\/p>\n\n\n\n
Of course, equality isn\u2019t the only way to compare strings. For example, how can we compare strings alphabetically\/lexicographically? Does \u201cMalkin\u201d come before or after \u201cLetang\u201d in a list?<\/p>\n\n\n\n
If any of these topics sound interesting, you\u2019re in luck. We\u2019ll cover all them and more in this article.<\/p>\n\n\n\n
Solutions<\/h2>\n\n\n\n
In this section, we\u2019ll take a look at a few different ways to compare strings. First, we\u2019ll look at a brute force solution which involves looping over each character to check for matches. Then, we\u2019ll introduce the comparison operators which abstract away the brute force solution. Finally, we\u2019ll talk about identity.<\/p>\n\n\n\n
Compare Strings by Brute Force<\/h3>\n\n\n\n
Since strings are iterables, there\u2019s nothing really stopping us from writing a loop to compare each character:<\/p>\n\n\n\n
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\n
\n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n 4<\/div>\n 5<\/div>\n 6<\/div>\n 7<\/div>\n<\/td>\n \n\n penguins_87 = <\/code>\"Crosby\"<\/code><\/div>\npenguins_71 = <\/code>\"Malkin\"<\/code><\/div>\nis_same_player = True<\/code><\/div>\nfor<\/code> a, b in zip(penguins_87, penguins_71):<\/code><\/div>\n <\/code>if<\/code> a != b:<\/code><\/div>\n <\/code>is_same_player = False<\/code><\/div>\n <\/code>break<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nIn this example, we zip both strings and loop over each pair of characters until we don\u2019t find a match. If we break before we\u2019re finished, we know we don\u2019t have a match. Otherwise, our strings are \u201cidentical.\u201d<\/p>\n\n\n\n While this gets the job done for some strings, it might fail in certain scenarios. For example, what happens if one of the strings is longer than the other?<\/p>\n\n\n\n \n \n \n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n<\/td>\n \n\n penguins_87 = <\/code>\"Crosby\"<\/code><\/div>\npenguins_71 = <\/code>\"Malkin\"<\/code><\/div>\npenguins_59 = <\/code>\"Guentzel\"<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nAs it turns out, zip()<\/code> will actually truncate the longer string. To deal with that, we might consider doing a length check first:<\/p>\n\n\n\n\n \n \n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n 4<\/div>\n 5<\/div>\n 6<\/div>\n 7<\/div>\n 8<\/div>\n 9<\/div>\n<\/td>\n \n\n penguins_87 = <\/code>\"Crosby\"<\/code><\/div>\npenguins_71 = <\/code>\"Malkin\"<\/code><\/div>\npenguins_59 = <\/code>\"Guentzel\"<\/code><\/div>\nis_same_player = len(penguins_87) == len(penguins_59)<\/code><\/div>\nif<\/code> is_same_player:<\/code><\/div>\n <\/code>for<\/code> a, b in zip(penguins_87, penguins_59):<\/code><\/div>\n <\/code>if<\/code> a != b:<\/code><\/div>\n <\/code>is_same_player = False<\/code><\/div>\n <\/code>break<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nOf course, even with the extra check, this solution is a bit overkill and likely error prone. In addition, this solution only works for equality. How do we check if a string is \u201cless\u201d than another lexicographically? Luckily, there are other solutions below.<\/p>\n\n\n\n Compare Strings by Comparison Operators<\/h3>\n\n\n\nFun fact: we don\u2019t have to write our own string equality code to compare strings. As it turns out, there are several core operators that work with strings right out of the box: ==<\/code>, <<\/code>, <=<\/code>, >=<\/code>, ><\/code>.<\/p>\n\n\n\nUsing our Penguins players from above, we can try comparing them directly:<\/p>\n\n\n\n \n \n \n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n 4<\/div>\n 5<\/div>\n 6<\/div>\n 7<\/div>\n 8<\/div>\n<\/td>\n \n\n penguins_87 = <\/code>\"Crosby\"<\/code><\/div>\npenguins_71 = <\/code>\"Malkin\"<\/code><\/div>\npenguins_59 = <\/code>\"Guentzel\"<\/code><\/div>\n <\/div>\n penguins_87 == penguins_87 # True<\/code><\/div>\npenguins_87 == penguins_71 # False<\/code><\/div>\npenguins_87 >= penguins_71 # False<\/code><\/div>\npenguins_59 <= penguins_71 # True<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nNow, it\u2019s important to note that these comparison operators work with the underlying ASCII representation of each character. As a result, seemingly equivalent strings might not appear to be the same:<\/p>\n\n\n\n \n \n \n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n 4<\/div>\n<\/td>\n \n\n penguins_87 = <\/code>\"Crosby\"<\/code><\/div>\npenguins_87_small = <\/code>\"crosby\"<\/code><\/div>\n <\/div>\n penguins_87 == penguins_87_small # False<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nWhen we compare \u201cCrosby\u201d and \u201ccrosby\u201d, we get False<\/code> because \u201cc\u201d and \u201cC\u201d aren\u2019t equivalent:<\/p>\n\n\n\n\n \n \n\n\n\n1<\/div>\n 2<\/div>\n<\/td>\n \n\n ord(<\/code>'c'<\/code>) # <\/code>99<\/code><\/div>\nord(<\/code>'C'<\/code>) # <\/code>67<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nNaturally, this can lead to some strange behavior. For example, we might say \u201ccrosby\u201d is less than \u201cMalkin\u201d because \u201ccrosby\u201d comes before \u201cMalkin\u201d alphabetically. Unfortunately, that\u2019s not how Python interprets that expression:<\/p>\n\n\n\n \n \n \n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n 4<\/div>\n<\/td>\n \n\n penguins_87_small = <\/code>\"crosby\"<\/code><\/div>\npenguins_71 = <\/code>\"Malkin\"<\/code><\/div>\n <\/div>\n penguins_87_small < penguins_71 # False<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nIn other words, while these comparison operators are convenient, they don\u2019t actually perform a case-insensitive comparison. Luckily, there are all sorts of tricks we can employ like converting both strings to uppercase or lowercase:<\/p>\n\n\n\n \n \n \n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n 4<\/div>\n 5<\/div>\n<\/td>\n \n\n penguins_87_small = <\/code>\"crosby\"<\/code><\/div>\npenguins_71 = <\/code>\"Malkin\"<\/code><\/div>\n <\/div>\n penguins_87_small.lower() < penguins_71.lower()<\/code><\/div>\npenguins_87_small.upper() < penguins_71.upper()<\/code><\/div>\n<\/div>\n<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<\/div>\n\n\n\nSince strings in Python are immutable like most languages, these methods don\u2019t actually manipulate the underlying strings. Instead, the return new ones.<\/p>\n\n\n\n All that said, strings are inherently complex. I say that has a bit of a warning because there are bound to be edge cases where the solutions in this article don\u2019t work as expected. After all, we\u2019ve only scratched the surface with ASCII characters. Try playing around with some strings that don\u2019t include English characters (e.g. \ud83e\udd10, \u6c49, etc.). You may be surprised by the results.<\/p>\n\n\n\n Compare Strings by Identity<\/h3>\n\n\n\nBefore we move on, I felt like it was important to mention another way of comparing strings: identity. In Python, ==<\/code> isn\u2019t the only way to compare things; we can also use is<\/code>. Take a look:<\/p>\n\n\n\n\n \n \n\n\n\n1<\/div>\n 2<\/div>\n 3<\/div>\n 4<\/div>\n | | | | | | | | | | | | | | | | |