std::vector to string with custom delimiter

Question

I would like to copy the contents of a vector to one long string with a custom delimiter. So far, I\'ve tried:

// .h
string getLabe         


        

Answer 1

This is an extension to the two answers already provided above as run-time performance seemed to be a theme in the comments. I would have added it as comments, but I do not have that privilege yet.

I tested 2 implementations for run-time performance using Visual Studio 2015:

Using stringstream:

std::stringstream result;
auto it = vec.begin();
result << (unsigned short)*it++;
for (; it != vec.end(); it++) {
    result << delimiter;
    result << (unsigned short)*it;
}
return result.str();

Using accumulate:

std::string result = std::accumulate(std::next(vec.begin()), vec.end(),
    std::to_string(vec[0]),
    [&delimiter](std::string& a, uint8_t b) {
    return a + delimiter+ std::to_string(b);
});
return result;

Release build run-time performance was close with a couple subtleties.

The accumulate implementation was slightly faster (20-50ms, ~10-30% of the overall run-time (~180ms) on 1000 iterations over a 256 element vector). However, the accumulate implementation was only faster when the a parameter to the lambda function was passed by reference. Passing the a parameter by value resulted in a similar run-time difference favoring the stringstream implementation. The accumulate implementation also improved some when the result string was returned directly rather than assigned to a local variable that was immediately returned. YMMV with other C++ compilers.

The Debug build was 5-10 times slower using accumulate so I think the extra string creation noted in several comments above is resolved by the optimizer.

I was looking at a specific implementation using a vector of uint8_t values. The full test code follows:

#include 
#include 
#include 
#include 
#include 

using namespace std;
typedef vector uint8_vec_t;

string concat_stream(const uint8_vec_t& vec, string& delim = string(" "));
string concat_accumulate(const uint8_vec_t& vec, string& delim = string(" "));

string concat_stream(const uint8_vec_t& vec, string& delimiter)
{
    stringstream result;

    auto it = vec.begin();
    result << (unsigned short)*it++;
    for (; it != vec.end(); it++) {
        result << delimiter;
        result << (unsigned short)*it;
    }
    return result.str();
}

string concat_accumulate(const uint8_vec_t& vec, string& delimiter)
{
    return accumulate(next(vec.begin()), vec.end(),
        to_string(vec[0]),
        [&delimiter](string& a, uint8_t b) {
        return a + delimiter + to_string(b);
    });
}

int main()
{
    const int elements(256);
    const int iterations(1000);

    uint8_vec_t test(elements);
    iota(test.begin(), test.end(), 0);

    int i;
    auto stream_start = chrono::steady_clock::now();
    string join_with_stream;
    for (i = 0; i < iterations; ++i) {
        join_with_stream = concat_stream(test);
    }
    auto stream_end = chrono::steady_clock::now();

    auto acc_start = chrono::steady_clock::now();
    string join_with_acc;
    for (i = 0; i < iterations; ++i) {
        join_with_acc = concat_accumulate(test);
    }
    auto acc_end = chrono::steady_clock::now();

    cout << "Stream Results:" << endl;
    cout << "    elements: " << elements << endl;
    cout << "    iterations: " << iterations << endl;
    cout << "    runtime: " << chrono::duration(stream_end - stream_start).count() << " ms" << endl;
    cout << "    result: " << join_with_stream << endl;

    cout << "Accumulate Results:" << endl;
    cout << "    elements: " << elements << endl;
    cout << "    iterations: " << iterations << endl;
    cout << "    runtime: " << chrono::duration(acc_end - acc_start).count() << " ms" << endl;
    cout << "    result:" << join_with_acc << endl;

    return 0;
}