How can I get the current position without another variable? - c++

Question

Many times when creating a grammatical list (with comas), I use similar code to the following:
std::stringstream list;
int i = 0;
for (auto itemListIt = itemList.begin(); itemListIt != itemList.end(); itemListIt++)
{
list << *itemListIt;
if (i < itemList.size() - 1) list << ", ";
i++;
}
Is there some more concise way do this, perhaps without the extra variable - 'i'?

Answer 1

Why not test what you're really interested in; "Is there another element after this one?".
std::stringstream list;
for (auto it = roomList.begin(); it != itemList.end(); it++)
{
list << *it;
if ( it+1 != itemList.end() ) list << ", ";
}

Answer 2

There are two simple solutions for this. The first is to use a while
loop:
auto itemListIt = itemList.begin();
while ( itemListIt != itemList.end() ) {
list << *itemListIt;
++ itemListIt;
if ( itemListIt != itemList.end() ) {
list << ", ";
}
}
The second solution is to change the logic slightly: instead of
appending a ", " if there is more to follow, prefix one if you're not
the first element:
for ( auto itemListIt = itemList.begin(); itemListIt != itemList.end(); ++ itemListIt ) {
if ( itemListIt != itemList.begin() ) {
list << ", ";
}
list << *itemListIt;
}

Answer 3

You can loop over everything up to the next to last using --items.end().
Then output the last one using items.back().
#include <algorithm>
#include <iterator>
#include <sstream>
#include <vector>
#include <iostream>
int main()
{
std::ostringstream oss;
std::vector<int> items;
items.push_back(1);
items.push_back(1);
items.push_back(2);
items.push_back(3);
items.push_back(5);
items.push_back(8);
if(items.size() > 1)
{
std::copy(items.begin(), --items.end(),
std::ostream_iterator<int>(oss, ", "));
oss << "and ";
}
// else do nothing
oss << items.back();
std::cout << oss.str();
}
Output:
1, 1, 2, 3, 5, and 8

Answer 4

The following will work with any InputIterator input:
std::stringstream list;
auto it(std::begin(input)); //Or however you get the input
auto end(std::end(input));
bool first(true);
for (; it != end; ++it)
{
if (!first) list << ", ";
else first = false;
list << *it;
}
Or without an extra variable:
std::stringstream list;
auto it(std::begin(input)); //Or however you get the input
auto end(std::end(input));
if (it != end)
{
list << *it;
++it;
}
for (; it != end; ++it)
{
list << ", " << *it;
}

Answer 5

And if you want to do that with a map or other iterators that cannot do random access, as others have suggested, check for the first element:
std::stringstream query;
query << "select id from dueShipments where package in (";
for (auto it = packageList.begin(); it != packageList.end(); it++)
{
if (it != packageList.begin()) query << ", ";
query << it->second;
}
query << ")";