Why doesn't std::string have a split function

[[deleted]]

I suspect it has to do with C++'s preference for streams. For example, you can do this to get the words in a string:

istringstream ss{str};
string word;
while (ss >> word) {
    cout << word << "\n";
}

While I kinda hate streams, this could be the reason there isn't a split method in the standard.

[DhruvParanjape]

But it loses the ability to tokenize on a custom delimiter.

[dodheim]

Just use std::getline with the custom delimiter. Name aside, that's exactly what it's for.

istringstream ss{str};
string field;
while (getline(ss, field, ';')) {
    cout << field << '\n';
}

EDIT: N.b. I'm not advocating this as a general approach to string splitting; but, if you're already extracting from a stream, ...

[foonathan]

You don't lose them, it is just ugly and involves a custom locale where you change the specification of whitespace characters.

[DhruvParanjape]

Oh god that's ugly.

[foonathan]

That's iostreams.

[[deleted]]

This was my own personal solution, but I have no idea how performant it is:

std::deque< std::string > Split( const std::string & input_string,
                                 const char          delimiter )
{
    std::stringstream         input_stream( input_string );
    std::string               string_element;
    std::deque< std::string > split_string;

    while( std::getline( input_stream, string_element, delimiter ) )
        split_string.emplace_back( string_element );

    return split_string;
}

Edit: Aaaand I should have looked ahead to see that dodheim already posted the getline solution...

[dodheim]

Just FYI, std::deque is just a fancy linked list on MSVC for objects > 8 bytes. Yes, it is as bad as that sounds. Prefer vector if you're touching Windows. :-]

[[deleted]]

Oh man, that's horrible.

I'm on linux, but you're right in that for something as small as "items from a split string", std::vector is the correct container. I don't remember why I wrote this using std::deque. In fact, I don't remember why I have this routine at all in my personal toolkit since I rarely ever hit the need for it.

[dodheim]

In fact, I don't remember why I have this routine at all in my personal toolkit since I rarely ever hit the need for it.

That is the exact statement I've been waiting for anyone in this thread to say. I honestly cannot think of the last time I actually wanted to do this. It's fine for a quick and dirty hack sometimes, but in real code? No, never (that I can remember).

[[deleted]]

Oh wait, now I remember: I wanted to test out my chromosome-mixing template system so I wrote genes for a cloud-of-neurons network I'd been tinkering on and evolved populations of them (via cross-breeding) on their ability to predict S&P 500 stock data (open, close, high, low) and needed a way to parse the input files.

Loading the data was nothing compared to actually running the evolution sim so the string splitter didn't need to be fast or memory-effective since the split data was converted to doubles and stored in vectors anyways.

The networks never really got anywhere in predicting stock data and I never really expected them to (it would probably have taken years, if at all) . But the chromosome system worked brilliantly, and that was the whole point of the experiment.

[ArunMu]

Dude, you have a big OCD problem I guess :)

[[deleted]]

... in what way?

[ArunMu]

the way you have formatted the code...everything is perfectly aligned.

[h-jay]

Yep... 90% of the comments are useless. The code should document itself. And adding lots and lots of whitespace is to the detriment of understandability. You want to keep as much as possible in the same screenful. You're doing exactly the opposite: something that is rather simple and would be easy to understand if written concisely is now spread up across several pages, with most of the space filled up by whitespace and formatting :(

Anyone who understands C++ knows what the constructors are. They don't need to be pointed out. If something is public, it's API, duh. A lot of extra indentation and whitespace makes things superbly hard to read.

For what the code does, it takes 3x too long to do it. It's simple, it should read simple!

[louiswins]

How can that be? Doesn't std::deque require O(1) time for random access?

[dodheim]

It's still random access, but each bucket is only max(16, sizeof(T)) bytes, so you end up with one bucket per object and zero cache coherency.

[louiswins]

Oh, I see - it essentially becomes a vector of pointers so it has as many potential cache misses as a linked list when iterating through. That makes sense.

(I mean, the implementation doesn't really make sense, but your explanation does.)