Difference between 2D vector declarations

Question

It might sound quite simple question but I am a bit confused.What I want to know is difference between these two vector declarations in c++.

vector<  vector<int> >a(some_size)

vector<int>b[some_size]

I want to know situations in which we can use one of these but not other one. Please provide situations other than those dependent on fixed size of array and variable size of vector.Please provide sample code to prove your point.

Thanks in advance.

_'My point is that both of these can be understood as vector of vectors.'_ Not really, no. — πάντα ῥεῖ, Jul 11 '14 at 21:12
De nada! That's usual here. Keep watching your questions, you'll have responds within seconds. Coming back after a couple of minutes, may surprise you with a highly downvoted/closed ansewr :P ... — πάντα ῥεῖ, Jul 11 '14 at 21:17
No,I am not saying it to you!Now ,I think this questions quality is not good enough,so it would be better if it is closed,so I downvoted it. — charany1, Jul 11 '14 at 21:37
You could simply ask for deleting your question. Flag it ... — πάντα ῥεῖ, Jul 11 '14 at 21:41
`I want to know situations in which we can use one of these but not other one:` I find it hard to understand what's your point, use is specified by the context. Obviously if you want to have a "static" data structure that its size is going to be fixed you can use both, if your want the versatility of inserting and deleting elements you can only use a vector of vectors. — 101010, Jul 11 '14 at 21:43

score 5 · Answer 1 · edited Jul 11 '14 at 21:12

5

The second is an array of vectors. The size of the array has to be known at compile time. The first is a vector of vectors. The size can change at runtime.

If you try to look for commonality you could say that both are ranges with the same value_type and category (RandomAccessIterator).

edited Jul 11 '14 at 21:12

Serge Ballesta

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answered Jul 11 '14 at 21:05

pmr

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101010 · Accepted Answer · 2014-07-11T21:15:18.757

1

My point is that both of these can be understood as vector of vectors.

Absolutely not: vector<vector<int>>a(some_size) is a vector of vectors. vector<int>b[some_size] is an array of vectors.

Main difference is vector<int>b[some_size] is fixed sized, vector<vector<int>>a(some_size) is dynamic, meaning you canpush_backothervector` and generally manipulate its elements (e.g., erase them, insert other elements etc.).

edited Jul 11 '14 at 21:15

answered Jul 11 '14 at 21:06

101010

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ok this is right,can you please provide me with some practical use where one is preferred over other! – charany1 Jul 11 '14 at 21:08
@charany1 depends on what you want. If you want fixed sized use the second or even better use a `std::array, some_size>`. If you want insertion and deletion on demand use the first. If there a performance gain in using the one over the other is debatable. – 101010 Jul 11 '14 at 21:12

score 0 · Answer 3 · answered Jul 11 '14 at 21:08

The main difference between a vector and a C-style array is that the vector hasn´t a fixed size (ie. can be resized). Something similar can be achieved ith C pointers too, but more complicated, and you need to keep the size epartely etc.etc.

Your first code line creates "some_size" vectors of ints.
Each of this int-vectors can have an own size (different from others)
and you can add/remove whole vectors too.

The second line won´t let you add/remove vectors.

score 0 · Answer 4 · edited May 23 '17 at 11:50

@pmr, @40two, and @deviantfan are right. The first is a vector, with vectors as elements. The second is a c-style array of vectors.

I think the best way to think of this is just to look at the differences between arrays and vectors, which is nicely addressed in this question: Arrays vs Vectors: Introductory Similarities and Differences

Your decision to use one over the other will be whether you need the functionality of a full vector object, or if you can safely perform all the same processes using a simple c-style array.

Difference between 2D vector declarations

4 Answers4