如何从C ++中的字符串中提取子字符串？

Question

I've been looking thousand of questions and answers about what I'm going to ask, but I still didn't find the way to do what I'm gonna to explain.

I have a text file from which I have to extract information about several things, all of them with the following format:

"string1":"string2"

And after that, there is more information, I mean:

The text file is something like this:

LINE 1 XXXXXXXXXXXXXXXXXXXXXXXXXXXX"string1":"string2"XXXXXXXXXXXXXXXXXXXXXXXXXX"string3":"string4"XXXXXXXXXXXXXXXXXXXXXXXXXXXX...('\\n')

LINE 2 XXXXXXXXXXXXXXXXXXXXXXXXXXXX"string5":"string6"XXXXXXXXXXXXXXXXXXXXXXXXXX"string7":"string8"XXXXXXXXXXXXXXXXXXXXXXXXXXXX...

XXX represents irrelevant information I do not need, and theEntireString (string used in the code example) stores all the information of a single line, not all the information of the text file.

I have to find first the content of string1 and store the content of string2 into another string without the quotes. The problem is that I have to stop when I reache the last quote and I don't know how exactly do this. I suppose I have to use the functions find() and substr(), but despite having tried it repeatedly, I did not succeed.

What I have done is something like this:

string extractInformation(string theEntireString)
{
  string s = "\"string1\":\"";    
  string result = theEntireString.find(s);
  return result;
}

But this way I suppose I store into the string the last quote and the rest of the string.

Answer 1

"find" function just give you the position of matched string to get the resulting string you need to use the "subst" function. Try This

string start,end;
start = theEntireString.substr(1,theEntireString.find(":")-2);
end = theEntireString.substr(theEntireString.find(":")+2,theEntireString.size()-1);

That will solve you problem

Best of Luck...

Answer 2

Two steps:

First we have to find the position of the : and splice the string into two parts:

string first = theEntireString.substr(0, theEntireString.find(":"));
string second = theEntireString.substr(theEntireString.find(":") + 1);

Now, we have to remove the "" :

string final_first(first.begin() + 1, first.end() - 1);
string final_second(second.begin() + 1, second.end() - 1);

Answer 3

Assuming either the key or value contains a quotation mark. The following will output the value after the ":". You can also use it in a loop to repeatedly extract the value field if you have multiple key-value pairs in the input string, provided that you keep a record of the position of last found instance.

#include <iostream>
using namespace std;

string extractInformation(size_t p, string key, const string& theEntireString)
{
  string s = "\"" + key +"\":\"";
  auto p1 = theEntireString.find(s);
  if (string::npos != p1)
    p1 += s.size();
  auto p2 = theEntireString.find_first_of('\"',p1);
  if (string::npos != p2)
    return theEntireString.substr(p1,p2-p1);
  return "";
}

int main() {
  string data = "\"key\":\"val\" \"key1\":\"val1\"";
  string res = extractInformation(0,"key",data);
  string res1 = extractInformation(0,"key1",data);
  cout << res << "," << res1 << endl;
}

Outputs:

val,val1

Answer 4

#include <regex>
#include <iostream>

using namespace std;

const string text = R"(
XXXXXXXXXXXXXXXXXXXXXXXXXXXX"string1":"string2"XXXXXXXXXXXXXXXXXXXXXXXXXX"string3"  :"string4" XXXXXXXXXXXXXXXXXXXXXXXXXXXX...
XXXXXXXXXXXXXXXXXXXXXXXXXXXX"string5":  "string6"XXXXXXXXXXXXXXXXXXXXXXXXXX"string7"  :  "string8" XXXXXXXXXXXXXXXXXXXXXXXXXXXX...
)";

int main() {
    const regex pattern{R"~("([^"]*)"\s*:\s*"([^"]*)")~"};
    for (auto it = sregex_iterator(begin(text), end(text), pattern); it != sregex_iterator(); ++it) {
        cout << it->format("First: $1, Second: $2") << endl;
    }
}

Output:

First: string1, Second: string2
First: string3, Second: string4
First: string5, Second: string6
First: string7, Second: string8

• Running (with clang and libc++): http://coliru.stacked-crooked.com/a/f0b5fd383bc227fc
• This is how raw string literals look in an editor that understand them: http://bl.ocks.org/anonymous/raw/9442865/

Answer 5

You don't need any string operation. I hope the XXXXX doesn't contain any '"', so You can read the both strings directly from the file:

ifstream file("input.txt");
for( string s1,s2; getline( getline( file.ignore( numeric_limits< streamsize >::max(), '"' ), s1, '"' ) >> Char<':'> >> Char<'"'>, s2, '"' ); )
    cout << "S1=" << s1 << " S2=" << s2 << endl;

the little help-function Char is:

template< char C >
std::istream& Char( std::istream& in )
{
    char c;
    if( in >> c && c != C )
        in.setstate( std::ios_base::failbit );
    return in;
}