Using 2D vectors

[glasses]

I want to construct two 2D vectors to which I can add vectors to in the future. However, even the beginning attempt to push_back a 1D vector as the first row of a 2D vector doesn't give the results I expect. THe following shows my code which attempts to construct two 2D vectors (i.e., R_pl and T_pl) and add one vector to each in the 0th row position. I fully expect my code to give a result for R_pl which only has elements in R_pl[0] (or for T_pl, elements in T_pl[0]), but I do not find this. THe following shows the code I am using:

#include <iostream>
#include <vector>
using namespace std;

int main(int argc, char** argv){

     float R_pl_initial;
     int nr;
     nr=3000;
     R_pl_initial=30.0;
     double dr = (R_pl_initial)/(nr-1);

     float T_max=165.0;
     float T_min=1.0e-9;
     double LB=1.0e-9;
     double dT=(T_max-T_min)/(nr-1);
     vector <vector<double>> R_pl;
     vector <double> newrow(nr);   //vector I want to push back into R_pl and T_pl as an initialization vector. This newrow I want to be the first row as it were of the 2D matrix of rows. I set its length to nr which is 3000.

     cout<<"size"<< newrow.size()<<endl; //this is 3000 as expected
     R_pl.push_back(newrow);  //does this make R_pl have 3000 elements initialized in R_pl[0]?

     for (int i1=0; i1<newrow.size(); i1++){
     	if (i1==0){
     		R_pl[0].push_back(LB);
     		cout<<"rplsize"<<(R_pl[0]).size()<<endl;
     		}

     	else
     		R_pl[0].push_back(LB+(i1*dr));
     }
     vector <vector<double>> T_pl;
     T_pl.push_back(newrow);
     for (int i2=0; i2<newrow.size(); i2++){

     	if (i2==0){
     		T_pl[0].push_back(LB);
     		cout<<"tplsize"<<(T_pl[0]).size()<<endl;
     		}
     	else
     		T_pl[0].push_back(LB+(i2*dT));
     }

    printf("printing tpl %lf\n", T_pl[0][1000]);
    return 0}

I have two questions. First, why is it that running 'cout<<"rplsize"<<(R_pl[0]).size()<<endl;' in the code gives 3001 for the size instead of the expected 3000 from pushing in the newrow into R_pl[0]? Second, why is it that 'printf("printing tpl %lf\n", T_pl[0][1000]);' gives 0.0 for the temperature element, when it should be some value greater than zero but below 165?

[firedraco]

1) Please put [code][/code] tags around your code in the future so it is formatted/indented nicely.

2) Your size becomes 3001 because when you construct the vector with a size that means it has 3000 intialized elements in it like you said. Then when you push_back, this creates a new element and appends it to the end, so now you have 3001 elements.

3) Printing T_pl[0][1000] gives 0.0 because the 1000th element in the first row of T_pl is 0.0 since that is was was in newrow.

[glasses]

Doing the R_pl[0].push_back(LB) does not redefine elements of the pushed in newrow? How do I make this work?

[TarikNeaj]

Can you like not ignore @firedraco's first point? Use code tags, people are much less likely to help you if you dont use them, people like me, because I can't read the code.

http://www.cplusplus.com/articles/jEywvCM9/

[Chervil]

If you want to modify an existing element, use the array syntax [] or the member function at() - you must specify the subscript of the element you want to access.

See reference pages:
http://www.cplusplus.com/reference/vector/vector/push_back/
http://www.cplusplus.com/reference/vector/vector/operator%5B%5D/
http://www.cplusplus.com/reference/vector/vector/at/