c++ - Displaying vectors on Screen -

i have been doing reading how create set number of arrays , populate arrays using loops in c++. everywhere says need use vectors. have created arrays , populated them using vectors (i think). when compile , run displaying arrays, ok. when try display contents of arrays, program compiles ok crashes when run it.

i trying create 100 arrays size increasing 500 elements each time. (this assignment need test efficacy of algorithm.)

unsigned int numarrays = 100; unsigned int arraysize = 0; //create holder variable: unsigned int randomint = 0; //create arrays: std:vector<std::vector<int> > arrays(numarrays);  for(std::size_t i=0; i< numarrays; i++){      arraysize = * 500;     vector<int> temp;     temp.reserve(arraysize);      for(int j=0; j< arraysize; j++){          //generate random number:         randomint = rand();         //add number array:         temp.push_back(randomint);      }     arrays.push_back(temp); }   for(int i=0; i<arrays.size(); i++){      cout << "{";     for(int j=0; j<arrays.at(i).size() - 1; j++){          cout << arrays.at(i).at(j) << ",";      }     cout << arrays.at(i).back() << "}" << endl; } 

i assume error somewhere in final nested loops not sure. in advance.

std:vector<std::vector<int> > arrays(numarrays); 

here initialize vector numarrays empty vector<int>s.
append new vectors. meaning have @ end 2*numarrays vectors.

the crash comes first half of vectors:

• the minus 1 in j<arrays.at(i).size() - 1: size() returns unsigned 0, minus 1 maximum value.
• arrays.at(i).back() trying non existing last element.

to fix problem, replace line

std::vector<std::vector<int> > arrays(numarrays); 

by

std::vector<std::vector<int> > arrays; arrays.reserve(numarrays); 

and add test non-empty()ness before computing size() - 1.