CODEWITHSUNDEEP
c++classvector
Doombot
Doombot

Reputation: 573

Vector of object from a custom class

In C++, I declare a custom class to store some values for an object. Then, I declare a vector of said object. Finally, I iterate through the vector to assign values to the fields.

#include <vector>

using namespace std;

class Custom
{ 
    public:
        int metric,nX,nY;
    private:

};

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

vector<Custom> MyWonderfulVector;

// Some code//

for(int i=0 ; i<10 ; i++){

MyWonderfulVector[i].metric = computation1();
MyWonderfulVector[i].nX= computation2();
MyWonderfulVector[i].nY= computation3();
}

return 0;

}

It throws a vector subscript out of range when it tries to evaluate MyWonderfulVector[i].metric = computation1();. metric is an int and computation1() too. at the first iteration, i=0 so it should be ok. Curiously, somewhere else in the code, I have a vector of another class (included in a library) and this syntax works for it, so I don't understand why it doesn't work here.

EDIT :

Ok with the comments I changed to following line: vector MyWonderfulVector(10);

So my problem is that I did not initialize the size of the vector (bad habit from Matlab ;) ) From what I understand, if I don't initialize the vector's to a fixed size, I must push_back the objects to "increase" the size of the vector. So, I should create a temporary Custom Object to assign the fields, then push_back this temp object into the vector. If one of the commenter wants to put this into an answer...

Upvotes: 10

Views: 31323

Answers (5)

R Sahu
R Sahu

Reputation: 206607

You declare a vector of Customs in the line

   vector<Custom> MyWonderfulVector;

but it is an empty vector. There are no items in it. When you try to access the elements of the vector in the for loop, you are accessing the vector using out of bounds indices.

I can think of the following options for fixing that problem.

  1. Create the vector with an initial size.

     vector<Custom> MyWonderfulVector(10);

  2. Add to the vector in the for loop.

    for(int i=0 ; i<10 ; i++){
  Custom c;
  c.metric = computation1();
  c.nX= computation2();
  c.nY= computation3();
  MyWonderfulVector.push_back(c);

    }

Upvotes: 9

Mihai Popescu
Mihai Popescu

Reputation: 25

Try using resize() before accesing an element. So your code will become:

for(int i=0 ; i<10 ; i++){

MyWonderfulVector.resize(i);

MyWonderfulVector[i].metric = computation1();
MyWonderfulVector[i].nX= computation2();
MyWonderfulVector[i].nY= computation3();
}

If you further want to add some other elements, you can store the size of the vector in size_t variable, and increment it every time you want to add other element.

size_t my_vector_size = MyWonderfulVector.size();
for(int i=0 ; i<10 ; i++)
{
    my_vector_size++;
    MyWonderfulVector.resize(my_vector_size);
}

This is a way to do it.

Upvotes: 0

Vlad from Moscow
Vlad from Moscow

Reputation: 310990

You defined a vector with no elements

vector<Custom> MyWonderfulVector;

If you call its member function empty like

std::cout << std::boolalpha << MyWonderfulVector.empty() << std::endl;

then you will get true

So you may not use the subscript operator applied to an ampty vector except with index 0 but in any case you may not assign a value.

You could either define the vector initially with some_variable elements like

vector<Custom> MyWonderfulVector( some_variable );

and then you could use your loop. Or you could reserve space for some_variable elements in the vector and in this case use member function push_back instead of the subscript operator. For example

vector<Custom> MyWonderfulVector;
MyWonderfulVector.reserve( some_variable );


for ( int i=0 ; i<some_variable ; i++ )
{
    Custom obj;
    obj.metric = computation1();
    obj.nX= computation2();
    obj.nY= computation3();

    MyWonderfulVector.push_back( obj );
}

Upvotes: 10

Tristan Djahel
Tristan Djahel

Reputation: 1212

I got the same problem before. I tried using the push_back function from vector class and it worked. Maybe it will solve your problem

Upvotes: 2

Wajeb
Wajeb

Reputation: 83

Vectors are not like arrays. You need to use push_back

Upvotes: 1

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