In my code there is a loop that adds sth like that "number," to stringstream. When it ends, I need to extract ',' add '}' and add '{' if the loop is to repeated.
I thought i can use ignore() to remove ',' but it didn't work. Do you know how I can do what I describe?
example:
douCoh << '{';
for(unsigned int i=0;i<dataSize;i++)
if(v[i].test) douCoh << i+1 << ',';
douCoh.get(); douCoh << '}';
You can seek the stringstream and go back 1 character, using stringstream::seekp. Note that it does not remove the last character, but only moves the write head. This is sufficient in this case, as we overwrite the last character with an }.
douCoh << '{';
for(unsigned int i=0;i<dataSize;i++)
if(v[i].test) douCoh << i+1 << ',';
douCoh.seekp(-1,douCoh.cur); douCoh << '}';
You can extract the string (with the str() member), remove the last char with std::string::erase and then reset the new string as buffer to the std::ostringstream.
However, a better solution would be to not insert the superfluous ',' in the first place, by doing something like that :
std::ostringstream douCoh;
const char* separator = "";
douCoh << '{';
for (size_t i = 0; i < dataSize; ++ i)
{
if (v[i].test)
{
douCoh << separator << i + 1;
separator = ",";
}
}
douCoh << '}';
U+3001 or even \r\n. I'd argue any microoptimisations are almost always unnecessary, fragile and confusing, if checking a boolean is slowing things down I'd be far more concerned of how long the string being shown to the user is!I have had this very problem and I found out that you can simply do:
douCoh.seekp(-1, std::ios_base::end);
And the keep inserting data. As others stated, avoiding inserting the bad data in the first place is probably the ideal solution, but in my case was the result of a 3rd party library function, and also I wanted to avoid copying the data to strings.
stringstream douCoh;
for(unsigned int i=0;i<dataSize;i++)
if(v[i].test)
douCoh << ( douCoh.tellp()==0 ? '{' : ',' ) << i+1;
douCoh << '}';
I've found this way using pop_back() string's method since c++11. Probably not so good as smarter ones above, but useful in much more complicated cases and/or for lazy people.
douCoh << '{';
for(unsigned int i=0;i<dataSize;i++)
if(v[i].test) douCoh << i+1 << ',';
string foo(douCoh.str());
foo.pop_back();
douCoh.str(foo);
douCoh.seekp (0, douCoh.end);
douCoh << '}';
Have fun with std::copy, iterators and traits. You either have to assume that your data is reverse iterable (end - 1) or that your output can be rewinded. I choose it was easier to rewind.
#include <ostream>
#include <algorithm>
#include <iterator>
namespace My
{
template<typename Iterator>
void print(std::ostream &out, Iterator begin, Iterator end)
{
out << '{';
if (begin != end) {
Iterator last = end - 1;
if (begin != last) {
std::copy(begin, last, std::ostream_iterator< typename std::iterator_traits<Iterator>::value_type >(out, ", "));
}
out << *last;
}
out << '}';
}
}
#include <iostream>
int main(int argc, char** argv)
{
My::print(std::cout, &argv[0], &argv[argc]);
std::cout << '\n';
}
You could use std::string::erase to remove the last character directly from the underlying string.
std::ostringstream::str function that allow to change the buffer. However, this solution cost two copy of the buffer content, which can be coslty.#include <sstream>
#include <vector>
#include <iterator>
#include <algorithm>
template<typename T>
std::string implode(std::vector<T> vec, std::string&& delim)
{
std::stringstream ss;
std::copy(vec.begin(), vec.end(), std::ostream_iterator<std::string>(ss, delim.c_str()));
if (!vec.empty()) {
ss.seekp(-1*delim.size(), std::ios_base::end);
ss<<'\0';
}
return ss.str();
}
int main()
{
std::cout<<implode(std::vector<std::string>{"1", "2", "3"}, ", ");
return 0;
}
Why not just check the counter? And not insert the ','
douCoh << '{';
for(unsigned int i=0;i<dataSize;i++){
if(v[i].test){
douCoh << i+1;
if(i != dataSize - 1) douCoh << ',';
}
}
/*douCoh.get();*/ douCoh << '}';
