Accessing data inside std::vector container

Question

I have three std::vector.

typedef std::pair< double,double > A;
typedef std::vector< A > B;
typedef std::vector< B > C;

I know how to access the element inside B like

B b;
b.at(0).first;
b.at(0).second;
And
C c;

How can I access the element of b using the container variable c? Thanks

if A and B are variables as well, this isn't valid. Are you missing some `typedef`s in there? — WhozCraig, Feb 19 '14 at 02:29
I made a comment at the same time as WhozCraig, when you posted code that didn't make sense. I deleted my comment when it was no longer relevant, and might confuse you, which it looks like it did even 10 minutes after being deleted. — paddy, Feb 19 '14 at 02:41
So that makes sense to everyone wondering "huh?" : [prior post before edit](http://stackoverflow.com/revisions/21869771/2) — WhozCraig, Feb 19 '14 at 02:42

score 2 · Accepted Answer · answered Feb 19 '14 at 02:30

2

C.at(0).at(0) will access first element of B. Because C is B's container.

answered Feb 19 '14 at 02:30

Pranit Kothari

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4

I would recommend `C[0][0]` because `at()` performs additional range checks. It's C++ afterall, you have to have the freedom to shoot-off your leg :) – user3159253 Feb 19 '14 at 02:33
Agreed with @user3159253. – rockinfresh Feb 19 '14 at 02:36
@user3159253 Yes right, but OP has some preferred way, so. – Pranit Kothari Feb 19 '14 at 02:41

score 2 · Answer 2 · answered Feb 19 '14 at 03:02

(I know you (or someone) updated the question to fix the lack of typedefs - just leaving this as relevant to the original question in case it helps someone else).

You need to understand the difference between a type and an object/variable/instance of that type. For example, double is a type, and if you say:

double x;

Then you've defined a variable x of type double in which you can store one number.

In a sense, types are saying how to use some memory to record a value of that type. Type's aren't told where the memory they're to manage is - that's only done when you define one or more variables of that type.

The templates std::pair<> and std::vector<> expect their parameters to be types: for example std::vector<int> is an instantiation of the std::vector template for the parameter int, and is itself a new type which you can use to create a variable:

std::vector<int> vi;

So, to your code:

std::pair< double,double > A;
std::vector< A > B;
std::vector< B > C;

The first line creates a variable called A of type std::pair<double,double>. The second line tries to instantiate std::vector<> with the variable from the last line, which won't work - the std::vector<> template must be instantiated with a type. To make it work, we need:

typedef std::pair< double,double > A;
std::vector< A > B;

The typedef keyword indicates that the first line is creating an alternative name for std::pair<double, double>, so the above definition for the variable B is entirely equivalent to:

std::vector< std::pair<double, double> > B;

Then your next line repeats the just-fixed mistake the second line made, of instantiating a template with a variable. To fix the code to create a two-dimensional vector:

typedef std::pair< double,double > A;
typedef std::vector< A > B;
std::vector< B > C;

Which creates a variable C of type std::vector<std::vector<std::pair<double,double>>>.

If you're certain the vectors inside C have sufficient size() for i and j to be valid indices, then you can access the individual doubles using:

C[i][j].first;
C[i][j].second;

Otherwise, you can attempt to access indices i, j and have the compiler throw an exception if either the outer or inner vector is too small:

try
{
    std::cout << C.at(i).at(j).first << ' ' << C.at(i).at(j).second << '\n';
}
catch (const std::exception& e)
{
    std::cerr << "caught an exception " << e.what() << '\n';
}

score 1 · Answer 3 · answered Feb 19 '14 at 02:34

1

C.at(i) is the *i*th B item's reference in C.

C.at(i).at(j) is the *j*th A item's reference in *i*th B item's reference in C.

You should refer to std::vector's use specification.

answered Feb 19 '14 at 02:34

Charlie

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Accessing data inside std::vector container

3 Answers3