IndexOf seems like a fairly straight forward method, it looks for the index of a character (or set of characters - aka. a string) and tells you the...
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I'd assume that the contract of
s.indexOf(b)is to return the smallest integerrsuch thats.substring(r, r + b.length) == b.When
bis"", that result is0.In my opinion, most string functions like this shouldn't allow
""as an argument, because they are at least somewhat ambiguous. However, the unformity of allowing any string as the needle to search for is also nice, and in this particular case returning0is not so strange. I have more distaste for.replace("", "x")and.split("")which have truly ambiguous meanings.You're right,
.replace("", "x")and.split("")definitely have ambiguous meanings.One thought I just had now though is thinking what if
replacewas usingindexOfinternally. Without a special case for an empty string, it could easily get stuck replacing the character at index0becauseindexOfsaid it found it there.You can give position as
indexOfsecond parameter. This gives a plausible little case where you can figure out the length of a string without usinglength:Most likely useful use case I can think of would be when doing code golfing.
Other than that, I think the current functionality is perfectly fine. It is often a silly case, but I don't see a reason to block it. It does give a valid match to positions between and around characters.
Interesting idea using it to find out the length though that trick only works for JavaScript. In C#, you get an
ArgumentOutOfRangeExceptionunless you specify the start position to be within the length of the string.I'm not sure how useful it would be for finding the position between characters as it can only tell you what the start position was.
I suspect it originated as an implementation detail or performance optimization, and is staying around mostly for backwards compatibility.
I can't comment on C# (I've never used C# before), but in JavaScript,
String.prototype.indexOf()performs a rather fast substring search for a fixed string (usually faster than the same search made with a regex in fact).Given that behavior, it follows logically that an empty string should return index 0, because that's the first place in the string being searched that you can find an empty string (you can technically find one at every index between 0 and the length of the string). If we assume that the original implementors just thought you would never call it with an empty string (which actually makes some sense, the only case I can think of for this happening is if you're passing user input directly to
indexOf()), then it would make sense that they wouldn't implement any checks for that case (and that would explain why it isn't well documented, it was thought to either be a use case that would never come up, or was thought to be self-evident).At this point, it doesn't matter though, because it's been a language 'feature' for long enough that it can't be chanted without significant risk of breaking something.
If you were writing an IndexOf function, say it was a for-loop, you'd be comparing characters between the two strings. Comparing an empty string to any other value would be false. It would likely mean gettting to the end, not finding anything and then returning.
That said, I could see it returning
0if the implement substring inside ofIndexOfas as it might substring the length of the input and then compare two empty strings...Still, it just feels wrong but I agree, even if people thought it was a problem, I doubt it will be fixed because of backwards compatibility.
I think in order to have an opinion, I'd have to understand why I'd be passing an empty string into IndexOf. The only reason I can think you might encounter that is if you are working with dynamic input either from a user or from an external source, but I can't think of a reason you'd be using IndexOf for that scenario. Can you give a real world example in which you would ever do this?
I'm thinking things like a basic text search function where you might want to not only say that it is found but where in the document. Returning
0gives the false impression it is found in the string.The only time I feel like it could return
0is if the string it was looking the index up in itself was empty. Even then, it is still searching for nothing and finding an index.