C++ - Parsing a .txt-file with mulltiple delimiters, display string - c++

Question

everyone!
I'm new to C++, alas I make silly mistakes.
This is a snippet of a .txt-file's content:
<tag attr1="value1" attr2="value2" ... >
What I'm trying to accomplish is parsing through the .txt-file, generating the following output:
Tag: tag
name: attr1
value: value1
name: attr2
value: value2
What I've done so far didn't work (my problem is the delimiters):
#include<iostream>
#include <sstream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
struct tagline{
string tag;
string attributeN;
string attributeV;
};
int main(){
vector<tagline> information;
string line;
tagline t;
ifstream readFile("file.txt");
while(getline(readFile,line)){
stringstream in(line);
getline(in,t.tag);
getline(in,t.attributeN,'=');
getline(in,t.attributeV,'"');
information.push_back(t);
}
vector<tagline>::iterator it = information.begin();
for(; it != information.end(); it++){
cout << "Tag: " << (*it).tag << " \n"
<< "name: " << (*it).attributeN << " \n"
<< "value: " << (*it).attributeV << " \n";
}
return 0;
}
All I get is a plain display of the snippet as it's formatted in the .txt-file:
<tag attr1="value1" attr2="value2" ... >
I would be happy if someone could help. Thank you!

Answer 1

This would be better handled using an HTML/XML parser (depending on what your file actually contains).
That being said, you are not parsing the lines correctly.
Your first call to getline(in,t.tag); is not specifying a delimiter, so it reads the entire line, not just the first word. You would have to use getline(in, t.tag, ' '); instead.
Also, your tags can have multiple attributes, but you are only reading and storing the first attribute, ignoring the rest. You need a loop to read all of them, and a std::vector to store them all into.
Try something more like this instead:
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
struct tagattribute {
string name;
string value;
};
struct tagline {
string tag;
vector<tagattribute> attributes;
};
int main() {
vector<tagline> information;
string line;
ifstream readFile("file.txt");
while (getline(readFile, line)) {
istringstream in(line);
tagline t;
tagattribute attr;
in >> ws;
char ch = in.get();
if (ch != '<')
continue;
if (!(in >> t.tag))
continue;
do
{
in >> ws;
ch = in.peek();
if (ch == '>')
break;
if (getline(in, attr.name, '=') &&
in.ignore() &&
getline(in, attr.value, '"'))
{
t.attributes.push_back(attr);
}
else
break;
}
while (true);
information.push_back(t);
}
vector<tagline>::iterator it = information.begin();
for(; it != information.end(); ++it) {
cout << "Tag: " << it->tag << "\n";
vector<tagattribute>::iterator it2 = it->attributes.begin();
for(; it2 != it->attributes.end(); ++it2) {
cout << "name: " << it2->name << "\n"
<< "value: " << it2->value << "\n";
}
cout << "\n";
}
return 0;
}
Live demo
Alternatively, consider writing some custom operator>> to help with the parsing, eg:
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
struct tagattribute {
string name;
string value;
};
istream& operator>>(istream &in, tagattribute &attr)
{
getline(in, attr.name, '=');
in.ignore();
getline(in, attr.value, '"');
return in;
}
struct tagline {
string tag;
vector<tagattribute> attributes;
};
istream& operator>>(istream &in, tagline &t)
{
tagattribute attr;
in >> ws;
char ch = in.get();
if (ch != '<')
{
in.setstate(ios_base::failbit);
return in;
}
if (!(in >> t.tag))
return in;
do
{
in >> ws;
ch = in.peek();
if (ch == '>')
{
in.ignore();
break;
}
if (!(in >> attr))
break;
t.attributes.push_back(attr);
}
while (true);
return in;
}
int main() {
vector<tagline> information;
string line;
ifstream readFile("file.txt");
while (getline(readFile, line)) {
istringstream in(line);
tagline t;
if (in >> t)
information.push_back(t);
}
vector<tagline>::iterator it = information.begin();
for(; it != information.end(); ++it) {
cout << "Tag: " << it->tag << "\n";
vector<tagattribute>::iterator it2 = it->attributes.begin();
for(; it2 != it->attributes.end(); ++it2) {
cout << "name: " << it2->name << "\n"
<< "value: " << it2->value << "\n";
}
cout << "\n";
}
return 0;
}
Live demo

Answer 2

Well, I would try to do something like this using this wonderful answer:
struct xml_skipper : std::ctype<char> {
xml_skipper() : ctype(make_table()) { }
private:
static mask* make_table() {
const mask* classic = classic_table();
static std::vector<mask> v(classic, classic + table_size);
v[','] |= space;
v['"'] |= space;
v['='] |= space;
v['<'] |= space;
v['>'] |= space;
return &v[0];
}
};
Then, what you can do is just keep reading:
ifstream readFile("file.txt");
while(getline(readFile,line)){
istringstream in(line);
in.imbue(std::locale(in.getloc(), new xml_skipper));
in >> t.tag >> t.attributeN >> t.attributeV;
information.push_back(t);
}
//...
Do note that this will break if values or attribute names have whitespaces.
If you want something more serious, you will need to write lexer, syntax tree builder and semantics tree builder.
Full code
#include<iostream>
#include <sstream>
#include <string>
#include <vector>
#include <fstream>
#include <sstream>
using namespace std;
struct tagline{
string tag;
string attributeN;
string attributeV;
};
struct xml_skipper : std::ctype<char> {
xml_skipper() : ctype(make_table()) { }
private:
static mask* make_table() {
const mask* classic = classic_table();
static std::vector<mask> v(classic, classic + table_size);
v[','] |= space;
v['"'] |= space;
v['='] |= space;
v['<'] |= space;
v['>'] |= space;
return &v[0];
}
};
int main(){
vector<tagline> information;
string line;
tagline t;
std::istringstream readFile{"<tag attr1=\"value1\" attr2=\"value2\" ... >"};
while(getline(readFile,line)){
istringstream in(line);
in.imbue(std::locale(in.getloc(), new xml_skipper));
in >> t.tag >> t.attributeN >> t.attributeV;
information.push_back(t);
}
vector<tagline>::iterator it = information.begin();
for(; it != information.end(); it++){
cout << "Tag: " << (*it).tag << " \n"
<< "name: " << (*it).attributeN << " \n"
<< "value: " << (*it).attributeV << " \n";
}
}
Live on Wandbox.

Answer 3

If your input may vary within the boundaries of the xml specification, an XML-parser might be a better approach than parsing the string "manually".
Just to show how this could look like, see the following code. It is based on tinyxml2, which just requires to include a single .cpp / .h-file in your project. You could - of course - also use any other xml libraries; this is just for demonstration purpose:
#include <iostream>
#include "tinyxml2.h"
using namespace tinyxml2;
int main()
{
const char* test = "<tag attr1='value1' attr2 = \"value2\"/>";
XMLDocument doc;
doc.Parse(test);
XMLElement *root = doc.RootElement();
if (root) {
cout << "Tag: " << root->Name() << endl;
const XMLAttribute *attrib = root->FirstAttribute();
while (attrib) {
cout << "name: " << attrib->Name() << endl;
cout << "value : " << attrib->Value() << endl;
attrib = attrib->Next();
}
}
}