Multidimensional Dynamic Memory Vector c++

Question

I do this program in c++. I want to build a table with random rows and 3 columns. When I compile there is no mistake but I execute the program do nothing and write nothing. Can you help me to find my error?

Thanks

int main(){
srand(time(0));
int project = (rand() % 5) + 1 ;   
std::cout << "The Number is " << project << "." << std::endl;

vector<vector<int> > P(project, vector<int>(3));

for(int i = 0; i <= P.size(); ++i){
  for(int j = 0;j <= P[i].size();++j){
        P[i][j] = (rand() % 50)+10;
        P[i][j] = (rand() % 2)+1;
        P[i][j] = (rand() % 50)+1;
  }
 }

 for(int a = 0;a <= P.size();++a){           
  for(int j = 0;j <= P[a].size();++j){
      std::cout << "Project "<< a <<" :"<< P[a][j] ;
        std::cout << P[a][j] <<" , ";
        std::cout << P[a][j] <<" ."<< std::endl;  
 }
 }

I get another problem it writes the same value for all the columns. So I need to change something in my loop right?

What happens when you step through your program in the debugger? — johnsyweb, Mar 19 '13 at 04:05

Shafik Yaghmour · Accepted Answer · 2013-03-19T03:19:46.633

1

There are a couple of issues, you should be using size_t for the loops instead of int and you were going over by one in the loops. For example it should be i < P.size() instead of i <= P.size():

for(size_t i = 0; i < P.size(); ++i)
{
  for(size_t j = 0;j < P[i].size();++j)
  {
    P[i][j] = (rand() % 50)+10;
    P[i][j] = (rand() % 2)+1;
    P[i][j] = (rand() % 50)+1;
  }
 }

Also, did you mean to write to the same entry, three times, what was the purpose of that? Also, you output the same entry three times as well. I modified to only output once:

for(size_t a = 0;a < P.size();++a)
{           
 for(size_t j = 0;j < P[a].size();++j)
 {
   std::cout << "Project "<< a <<" :"<< P[a][j] << std::endl;  
 }
}

Also if you are have using namespace std in your code I would advise against it. Just use std::vector explicitly.

edited Mar 19 '13 at 03:19

answered Mar 19 '13 at 02:50

Shafik Yaghmour

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this is not very important in most cases but maybe you would like to also note that one should use MAX_RANGE to ensure that your random numbers are uniformly distributed, now you have slightly skewed distribution – 4pie0 Mar 19 '13 at 02:57
thanks now something appears. But i have the problem that it write all the time 3 times the same thing like 000111222... – usertfwr Mar 19 '13 at 02:58
size_t is just a typedef int(unsigned) – TravellingGeek Mar 19 '13 at 03:00
@user2152947 I amended my answer to address that issue, I was a little confused about what you intended. – Shafik Yaghmour Mar 19 '13 at 03:20
@GeraldSv see previous thread http://stackoverflow.com/a/1114031/1708801 – Shafik Yaghmour Mar 19 '13 at 03:37
I can't find how to write different things in each columns. is it possible? – usertfwr Mar 19 '13 at 03:49

Multidimensional Dynamic Memory Vector c++

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