Put values from Vector to txt file in C++

i have a problem with putting values stored in Vector to txt file. This is my declared vector:

 vector<float> spectrum;
 z = sqrt( (xreal[k] * xreal[k]) + (ximagine[k] * ximagine[k]) );
 spectrum.push_back(z);    #here i put values inside vector
 -> Here i want to use stored values in vector spectrum to put them to txt file, starting from 1st.