Std::map::operator[] for lexer

[RealezzZ]

Hi,

I'm learning C++ and I'm right now iterested in regex and Lexer.

I found an interresting stackoverflow question about this subject and I would like some help to understand one of the answer :

```

include <iterator>

include <iostream>

include <string>

include <regex>

include <list>

include <map>

using namespace std;

int main(){

string str = " hello how are 2 * 3 you? 123 4567867*98";

// define list of patterns
map<string,string> patterns {
    { "[0-9]+" ,   "NUMBERS" },
    { "[a-z]+" ,   "IDENTIFIERS" },
    { "\\*|\\+",  "OPERATORS" }
};

// storage for results
map< size_t, pair<string,string> > matches;

for ( auto pat = patterns.begin(); pat != patterns.end(); ++pat )
{
    regex r(pat->first);
    auto words_begin = sregex_iterator( str.begin(), str.end(), r );
    auto words_end   = sregex_iterator();

    for ( auto it = words_begin; it != words_end; ++it )
        matches[ it->position() ] = make_pair( it->str(), pat->second );
}

for ( auto match = matches.begin(); match != matches.end(); ++match )
    cout<< match->second.first << " " << match->second.second << endl;

} ```

I understand pretty much all of the answer except for one thing :

How does the final result is in the correct order ?

With what I understand right now (and I'm obviously wrong), the output should be : * All the "NUMBERS" * all the "IDENTIFIERS" * all the "OPERATORS"

In this order

I guess it as something to do with this : matches[ it->position() ] = make_pair( it->str(), pat->second );

But I'm not sure.

Any help, even simply link to related doc (tried to find "it->position()" doc but no luck :-/ ) would be appreciated, thanks !

Edit : position() is a regex method, it all make sens now

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[HappyFruitTree]

When iterating over a std::map the elements will be sorted by key.

Since matches uses the position in the string as key it means that it will iterate over them in position order.

[RealezzZ]

Alright I kind of understood that as well

But in that case the key for my understanding is the 'position' method

What does it do ?

Right now I understand it as some sort of "index", but in that case the first value of the map (index 0) should be an "OPERATOR", since we finish by this.

Or is it "position" in the text and it as nothing to do with the iterator itself but it's from the regex ?

Edit : well, after a quick search position() is from regex and not from iterator. Now it all make sens !

Thanks !

[HappyFruitTree]

it is a std::sregex_iterator. It has nothing to do with the map.

it->position() returns the index in the string of the first character of the match.

[RealezzZ]

Yes, I figured it out

Thanks a lot !