how to add element into the nested vector - c++

Question

Hello sorry for the stupid question but I am very beginner in c++. I cannot describe the problem well because of my bad English. I'll add my code here that i have tried so far.
vector< vector<string> >allData;
int main(){
vector<string>test;
for(int i = 0; i<allData.size(); i++){
test = allData[i];
}
int id;
cout<<"enter Id"<<endl;
cin>>id;
if (id == test[2]){
string desc;
cout<<"enter ur description"<<endl;
cin>>description;
allData.push_back(description);
} else {
cout<<"there is no data with the id u have entered"<<endl;
}
}
Above code is just an example code. Lets say that there are 2 vectors inside the vector named allData, so when i enter the ID of a first vector i can add description into the vector which i have chosen by choosing it by its Id. Somehow i cannot do that thing like choosing the first or second vector by inputing their ids then add more datas into chosen 1. So please someone tell me what should i do. I know that above code is incomplete and awful but as i said before i am very beginner in c++

Answer 1

You can add values into nested vector just like a normal vector. So you could do allData[i].push_back(description) or even allData[i][j] = description to overwrite an existing value. However you are trying to push a string into allData which does not contain strings but rather Vectors of strings.
Additionally your first for loop runs through allData and saves each to test, overwriting test each time. When the loop is done test will simply contain the last element of allData. Thus your for loop is the equivalent of doing test = allData.back()

Answer 2

Well, if you need to access your child vectors directly, you need to declare your vector saying how many child vectors it has (without this, you have segmentation fault when trying to access it ie. allData[0] will be unitialised).
Let's say you have 2, so they will be allData[0] and allData[1], just like a normal array.
std::vector<std::vector<std::string>> allData (2);
allData[0].push_back("string0a");
allData[0].push_back("string0b");
allData[1].push_back("string1");
for(auto & a : allData[0])
std::cout << a << "\n";
Of course, you can add vectors dinamically too.
std::vector<std::vector<std::string>> allData;
std::vector<std::string> child0;
child0.push_back("string0");
allData.push_back(child0);
And to check if the id the user input is valid, you can do
if(id >= 0 && id < allData.size()) {
....

Answer 3

Your variable all_data is a vector of vectors representing rows and columns. You can create a scratch vector for a row, push_back() data on that scratch vector, then push_back() that scratch vector onto all_data like so:
#include <vector>
int main() {
std::vector<std::vector<int>> all_data;
for (int ii = 0; ii < 10; ii++) {
std::vector<int> row_data;
for (int jj = 0; jj < 10; jj++) {
row_data.push_back(ii * jj);
}
all_data.push_back(row_data);
}
return (0);
}