问题
Suppose we have the following situation:
- A record struct is declared as follows
struct Person {
unsigned int id;
std::string name;
uint8_t age;
// ...
};
- Records are stored in a file using the following format:
ID Forename Lastname Age
------------------------------
1267867 John Smith 32
67545 Jane Doe 36
8677453 Gwyneth Miller 56
75543 J. Ross Unusual 23
...
The file should be read in to collect an arbitrary number of the Person records mentioned above:
std::istream& ifs = std::ifstream(\"SampleInput.txt\");
std::vector<Person> persons;
Person actRecord;
while(ifs >> actRecord.id >> actRecord.name >> actRecord.age) {
persons.push_back(actRecord);
}
if(!ifs) {
std::err << \"Input format error!\" << std::endl;
}
Question: (that\'s a frequently asked question, in the one or the other form)
What can I do to read in the separate values storing their values into the one actRecord variables\' fields?
The above code sample ends up with run time errors:
Runtime error time: 0 memory: 3476 signal:-1
stderr: Input format error!
回答1:
One viable solution is to reorder input fields (if this is possible)
ID Age Forename Lastname
1267867 32 John Smith
67545 36 Jane Doe
8677453 56 Gwyneth Miller
75543 23 J. Ross Unusual
...
and read in the records as follows
#include <iostream>
#include <vector>
struct Person {
unsigned int id;
std::string name;
uint8_t age;
// ...
};
int main() {
std::istream& ifs = std::cin; // Open file alternatively
std::vector<Person> persons;
Person actRecord;
unsigned int age;
while(ifs >> actRecord.id >> age &&
std::getline(ifs, actRecord.name)) {
actRecord.age = uint8_t(age);
persons.push_back(actRecord);
}
return 0;
}
回答2:
You have whitespace between firstname and lastname. Change your class to have firstname and lastname as separate strings and it should work. The other thing you can do is to read in two separate variables such as name1 and name2 and assign it as
actRecord.name = name1 + " " + name2;
回答3:
Here's an implementation of a manipulator I came up with that counts the delimiter through each extracted character. Using the number of delimiters you specify, it will extract words from the input stream. Here's a working demo.
template<class charT>
struct word_inserter_impl {
word_inserter_impl(std::size_t words, std::basic_string<charT>& str, charT delim)
: str_(str)
, delim_(delim)
, words_(words)
{ }
friend std::basic_istream<charT>&
operator>>(std::basic_istream<charT>& is, const word_inserter_impl<charT>& wi) {
typename std::basic_istream<charT>::sentry ok(is);
if (ok) {
std::istreambuf_iterator<charT> it(is), end;
std::back_insert_iterator<std::string> dest(wi.str_);
while (it != end && wi.words_) {
if (*it == wi.delim_ && --wi.words_ == 0) {
break;
}
dest++ = *it++;
}
}
return is;
}
private:
std::basic_string<charT>& str_;
charT delim_;
mutable std::size_t words_;
};
template<class charT=char>
word_inserter_impl<charT> word_inserter(std::size_t words, std::basic_string<charT>& str, charT delim = charT(' ')) {
return word_inserter_impl<charT>(words, str, delim);
}
Now you can just do:
while (ifs >> actRecord.id >> word_inserter(2, actRecord.name) >> actRecord.age) {
std::cout << actRecord.id << " " << actRecord.name << " " << actRecord.age << '\n';
}
Live Demo
回答4:
A solution would be to read in the first entry into an ID variable.
Then read in all the other words from the line (just push them in a temporary vector) and construct the name of the individual with all the elements, except the last entry which is the Age.
This would allow you to still have the Age on the last position but be able to deal with name like "J. Ross Unusual".
Update to add some code which illustrates the theory above:
#include <memory>
#include <string>
#include <vector>
#include <iterator>
#include <fstream>
#include <sstream>
#include <iostream>
struct Person {
unsigned int id;
std::string name;
int age;
};
int main()
{
std::fstream ifs("in.txt");
std::vector<Person> persons;
std::string line;
while (std::getline(ifs, line))
{
std::istringstream iss(line);
// first: ID simply read it
Person actRecord;
iss >> actRecord.id;
// next iteration: read in everything
std::string temp;
std::vector<std::string> tempvect;
while(iss >> temp) {
tempvect.push_back(temp);
}
// then: the name, let's join the vector in a way to not to get a trailing space
// also taking care of people who do not have two names ...
int LAST = 2;
if(tempvect.size() < 2) // only the name and age are in there
{
LAST = 1;
}
std::ostringstream oss;
std::copy(tempvect.begin(), tempvect.end() - LAST,
std::ostream_iterator<std::string>(oss, " "));
// the last element
oss << *(tempvect.end() - LAST);
actRecord.name = oss.str();
// and the age
actRecord.age = std::stoi( *(tempvect.end() - 1) );
persons.push_back(actRecord);
}
for(std::vector<Person>::const_iterator it = persons.begin(); it != persons.end(); it++)
{
std::cout << it->id << ":" << it->name << ":" << it->age << std::endl;
}
}
回答5:
Since we can easily split a line on whitespace and we know that the only value that can be separated is the name, a possible solution is to use a deque for each line containing the whitespace separated elements of the line. The id and the age can easily be retrieved from the deque and the remaining elements can be concatenated to retrieve the name:
#include <iostream>
#include <fstream>
#include <deque>
#include <vector>
#include <sstream>
#include <iterator>
#include <string>
#include <algorithm>
#include <utility>
struct Person {
unsigned int id;
std::string name;
uint8_t age;
};
int main(int argc, char* argv[]) {
std::ifstream ifs("SampleInput.txt");
std::vector<Person> records;
std::string line;
while (std::getline(ifs,line)) {
std::istringstream ss(line);
std::deque<std::string> info(std::istream_iterator<std::string>(ss), {});
Person record;
record.id = std::stoi(info.front()); info.pop_front();
record.age = std::stoi(info.back()); info.pop_back();
std::ostringstream name;
std::copy
( info.begin()
, info.end()
, std::ostream_iterator<std::string>(name," "));
record.name = name.str(); record.name.pop_back();
records.push_back(std::move(record));
}
for (auto& record : records) {
std::cout << record.id << " " << record.name << " "
<< static_cast<unsigned int>(record.age) << std::endl;
}
return 0;
}
回答6:
Another solution is to require certain delimiter characters for a particular field, and provide a special extraction manipulator for this purpose.
Let's suppose we define the delimiter character ", and the input should look like this:
1267867 "John Smith" 32
67545 "Jane Doe" 36
8677453 "Gwyneth Miller" 56
75543 "J. Ross Unusual" 23
Generally needed includes:
#include <iostream>
#include <vector>
#include <iomanip>
The record declaration:
struct Person {
unsigned int id;
std::string name;
uint8_t age;
// ...
};
Declaration/definition of a proxy class (struct) that supports being used with the std::istream& operator>>(std::istream&, const delim_field_extractor_proxy&) global operator overload:
struct delim_field_extractor_proxy {
delim_field_extractor_proxy
( std::string& field_ref
, char delim = '"'
)
: field_ref_(field_ref), delim_(delim) {}
friend
std::istream& operator>>
( std::istream& is
, const delim_field_extractor_proxy& extractor_proxy);
void extract_value(std::istream& is) const {
field_ref_.clear();
char input;
bool addChars = false;
while(is) {
is.get(input);
if(is.eof()) {
break;
}
if(input == delim_) {
addChars = !addChars;
if(!addChars) {
break;
}
else {
continue;
}
}
if(addChars) {
field_ref_ += input;
}
}
// consume whitespaces
while(std::isspace(is.peek())) {
is.get();
}
}
std::string& field_ref_;
char delim_;
};
std::istream& operator>>
( std::istream& is
, const delim_field_extractor_proxy& extractor_proxy) {
extractor_proxy.extract_value(is);
return is;
}
Plumbing everything connected together and instantiating the delim_field_extractor_proxy:
int main() {
std::istream& ifs = std::cin; // Open file alternatively
std::vector<Person> persons;
Person actRecord;
int act_age;
while(ifs >> actRecord.id
>> delim_field_extractor_proxy(actRecord.name,'"')
>> act_age) {
actRecord.age = uint8_t(act_age);
persons.push_back(actRecord);
}
for(auto it = persons.begin();
it != persons.end();
++it) {
std::cout << it->id << ", "
<< it->name << ", "
<< int(it->age) << std::endl;
}
return 0;
}
See the working example here.
NOTE:
This solution also works well specifying a TAB character (\t) as delimiter, which is useful parsing standard .csv formats.
回答7:
What can I do to read in the separate words forming the name into the one
actRecord.namevariable?
The general answer is: No, you can't do this without additional delimiter specifications and exceptional parsing for the parts forming the intended actRecord.name contents.
This is because a std::string field will be parsed just up to the next occurence of a whitespace character.
It's noteworthy that some standard formats (like e.g. .csv) may require to support distinguishing blanks (' ') from tab ('\t') or other characters, to delimit certain record fields (which may not be visible at a first glance).
Also note:
To read an uint8_t value as numeric input, you'll have to deviate using a temporary unsigned intvalue. Reading just a unsigned char (aka uint8_t) will screw up the stream parsing state.
回答8:
Another attempt at solving the parsing problem.
int main()
{
std::ifstream ifs("test-115.in");
std::vector<Person> persons;
while (true)
{
Person actRecord;
// Read the ID and the first part of the name.
if ( !(ifs >> actRecord.id >> actRecord.name ) )
{
break;
}
// Read the rest of the line.
std::string line;
std::getline(ifs,line);
// Pickup the rest of the name from the rest of the line.
// The last token in the rest of the line is the age.
// All other tokens are part of the name.
// The tokens can be separated by ' ' or '\t'.
size_t pos = 0;
size_t iter1 = 0;
size_t iter2 = 0;
while ( (iter1 = line.find(' ', pos)) != std::string::npos ||
(iter2 = line.find('\t', pos)) != std::string::npos )
{
size_t iter = (iter1 != std::string::npos) ? iter1 : iter2;
actRecord.name += line.substr(pos, (iter - pos + 1));
pos = iter + 1;
// Skip multiple whitespace characters.
while ( isspace(line[pos]) )
{
++pos;
}
}
// Trim the last whitespace from the name.
actRecord.name.erase(actRecord.name.size()-1);
// Extract the age.
// std::stoi returns an integer. We are assuming that
// it will be small enough to fit into an uint8_t.
actRecord.age = std::stoi(line.substr(pos).c_str());
// Debugging aid.. Make sure we have extracted the data correctly.
std::cout << "ID: " << actRecord.id
<< ", name: " << actRecord.name
<< ", age: " << (int)actRecord.age << std::endl;
persons.push_back(actRecord);
}
// If came here before the EOF was reached, there was an
// error in the input file.
if ( !(ifs.eof()) ) {
std::cerr << "Input format error!" << std::endl;
}
}
回答9:
When seeing such an input file, I think it is not a (new way) delimited file, but a good old fixed size fields one, like Fortran and Cobol programmers used to deal with. So I would parse it like that (note I separated forename and lastname) :
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <vector>
struct Person {
unsigned int id;
std::string forename;
std::string lastname;
uint8_t age;
// ...
};
int main() {
std::istream& ifs = std::ifstream("file.txt");
std::vector<Person> persons;
std::string line;
int fieldsize[] = {8, 9, 9, 4};
while(std::getline(ifs, line)) {
Person person;
int field = 0, start=0, last;
std::stringstream fieldtxt;
fieldtxt.str(line.substr(start, fieldsize[0]));
fieldtxt >> person.id;
start += fieldsize[0];
person.forename=line.substr(start, fieldsize[1]);
last = person.forename.find_last_not_of(' ') + 1;
person.forename.erase(last);
start += fieldsize[1];
person.lastname=line.substr(start, fieldsize[2]);
last = person.lastname.find_last_not_of(' ') + 1;
person.lastname.erase(last);
start += fieldsize[2];
std::string a = line.substr(start, fieldsize[3]);
fieldtxt.str(line.substr(start, fieldsize[3]));
fieldtxt >> age;
person.age = person.age;
persons.push_back(person);
}
return 0;
}
来源：https://stackoverflow.com/questions/23047052/why-does-reading-a-record-struct-fields-from-stdistream-fail-and-how-can-i-fi