How to get c++ vector bigger?

Question

I'm using the following:

vector < vector < unsigned int > > paths;

But it seems I only can add 647 278 rows there. I have 10 numbers on each row.

I call this on each iteration:

paths.resize(paths.size() + 1, vector < unsigned int >(10));

Is there a better way to do it than call it every iteration? And do I always have to give the column count since it doesn't change?

Why does it seem that you can only add that many rows? What happens? — sth, Nov 26 '12 at 22:28
After 647 278 rows it stops increasing. This number is from paths.size() — sleepless_in_seattle, Nov 26 '12 at 22:30
http://stackoverflow.com/questions/131803/unsigned-int-vs-size-t related — pyCthon, Nov 26 '12 at 22:31
Please give more info, I'm having hard time understanding the topic. — sleepless_in_seattle, Nov 26 '12 at 22:35
What kind of error are you getting? Are you checking for errors? — David Hammen, Nov 26 '12 at 22:36
Please reduce your program to the smallest complete program that demonstrates the error, and copy-paste that entire program into your question. See http://sscce.org/. — Robᵩ, Nov 26 '12 at 22:45

score 1 · Accepted Answer · edited May 23 '17 at 11:51

As pyCthon points out, this other question explains that size_t is the right type to use for the size here, as it is guaranteed to be big enough to allow up to the maximum for your architecture.

Secondly, the .resize() method doesn't really need calling each time. Instead construct the new vector and .push_back(newvec) to add it to the vector. The internal allocator will allocate space as it sees fit, and is generally the best option there - it will generally require O(log n) reallocations, which are important here; if the vector has to reallocate because it needs more space, you could end up continuously re-copying the entire array to new blocks of memory.

Even better, if you could work out the total size of the arrays at the start, do so. That way there will only be 1 allocation at the start if you call .reserve(size), and then use .push_back() for each element, as it will allocate the whole block at the beginning.

If you want to know the maximum number of elements a vector can take on your architecture, call vector::max_size(). Example from cplusplus.com:

// comparing size, capacity and max_size
#include <iostream>
#include <vector>
using namespace std;

int main ()
{
  vector<int> myvector;
  cout << "max_size: " << myvector.max_size() << "\n";
  return 0;
}

Running this on ideone.com quickly gets me a max size of 1,073,741,823, and if the vector is a vector< vector< unsigned int > > instead, I get 357,913,941.

I changed the int to size_t but I'm stuck with the same problem. Am I wrong or doesn't the int on my example handle the size of the second vector inside the first one? How do I set the size for the outer (first) vector? — sleepless_in_seattle, Nov 26 '12 at 22:38
The problem was on my end. Figured it out with the max_size, thanks. — sleepless_in_seattle, Nov 27 '12 at 20:31

score 1 · Answer 2 · edited Nov 26 '12 at 22:43

1

not sure but in this std::vector description it points out that the vector storage is in one continues location in memory. That means that the vector is stored in one big block in your memory.

Did you tried to split this into different vectors (the big array)?

edited Nov 26 '12 at 22:43

Robᵩ

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answered Nov 26 '12 at 22:36

Lukas

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Since this is vectors within a vector it will be one large block and lots of little ones. – Mark Ransom Nov 26 '12 at 22:40
i checked it, if he is sure to have everytime 10 elements in the inner vector its better to use an array within the outer vector. The outer vector will store a lot of unneeded information cause of using vectors. – Lukas Nov 26 '12 at 23:22

How to get c++ vector bigger?

2 Answers2