Iterate through a pointer set that contains string vectors in C++

Question

This is my declaration of the set:

set< vector<string> >* tuples = new set< vector<string> >();

And this is how I am trying to iterate through it:

for(set< vector<string> >::iterator it = tuples->begin(); it != tuples->end(); it++){
    if(it[column] == value){
        rowResults->insert(*it);
    }
}

but I get an error

no match for ‘operator[]’ (operand types are ‘std::set<std::vector<std::__cxx11::basic_string<char> > >::iterator {aka std::_Rb_tree_const_iterator<std::vector<std::__cxx11::basic_string<char> > >}’ and ‘int’)
    if(it[column] == value){
         ^

a dynamic array of sets of vectors of strings named tuples? Why ? — Stack Danny, Oct 06 '16 at 17:16

Jean-François Fabre · Answer 1 · 2016-10-06T17:21:24.133

it is an iterator, not the vector object itself. To access the vector object just use *it

Even better: get rid of the confusing iterator type by defining a reference (here constant ref since we don't seem to need a non-const) to the element itself.

for(set< vector<string> >::iterator it = tuples->begin(); it != tuples->end(); it++){
    const vector<string> &v = *it;  // more readable
    if(v[column] == value){
        rowResults->insert(v);
    }
}

as no decent C++ answer cannot not mention the "new" C++11, note that if you use -std=c++11 option, the syntax is much better to iterate on a list

for(auto v : *tuples)
{
    if(v[column] == value){
        rowResults->insert(v);
    }
}

Thanks! This helped a lot! :) – Benjamin Rodriguez Oct 06 '16 at 17:31 — Benjamin Rodriguez, Oct 06 '16 at 17:31

score 0 · Accepted Answer · answered Oct 06 '16 at 17:17

0

You're applying [] to the iterator instead of to the object to which it points. You need to dereference the iterator (and mind operator precedence!):

for(set< vector<string> >::iterator it = tuples->begin(); it != tuples->end(); ++it){
    if((*it)[column] == value){
        rowResults->insert(*it);
    }
}

Note that with iterators, it's better to use ++it instead of it++ in loops, since the latter can be less efficient under insufficient optimisation.

answered Oct 06 '16 at 17:17

Angew is no longer proud of SO

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Thank you Angew! It makes a lot of sense haha. Thank you for the ++it tip :). Do you know where I can read more about the difference between ++it and it++? – Benjamin Rodriguez Oct 06 '16 at 17:29
Probably any [good C++ book](http://stackoverflow.com/q/388242/1782465), or just infer it from some [reference documentation](http://en.cppreference.com/). – Angew is no longer proud of SO Oct 06 '16 at 17:31

score 0 · Answer 3 · answered Oct 06 '16 at 17:59

You may avoid iterator with something like:

std::set<std::vector<std::string>>
computeRowResult(const std::set<std::vector<std::string>>& input,
                 int column,
                 const std::string& value)
{
    std::set<std::vector<std::string>> rowResults;

    for (const auto& v : input) {
        if (v[column] == value) {
            rowResults.insert(v);
        }
    }
    return rowResults;
}

or avoiding the manual loop with

std::set<std::vector<std::string>>
computeRowResult(const std::set<std::vector<std::string>>& input,
                 int column,
                 const std::string& value)
{
    std::set<std::vector<std::string>> rowResults;

    std::copy_if(input.begin(), input.end(),
                 std::inserter(rowResults, rowResults.end()),
                 [&](const auto& v) { return v[column] == value; });

    return rowResults;
}

Demo

Iterate through a pointer set that contains string vectors in C++

3 Answers3