Re: help with vector<vector<double>>

T. Crane wrote:
:: On Aug 10, 2:47 pm, "Bo Persson" <b...@gmb.dk> wrote:
::: BobR wrote:
:::
:::::
::::: So, if you did something like:
:::::
::::: std::vector<int> VecInt;
::::: VecInt.reserve( 100 ); // sets 'capacity', not 'size'
:::::
::::: ... and then somewhere you did:
:::::
::::: VecInt.push_back( 1 ); // this is the 101st push_back
:::::
::::: ... you kind-of shot yourself in the foot (it needs to realloc
::::: and copy). No big thing for small vectors, but costs time on
::::: very big vectors. (....and you could run out of memory during
::::: the realloc/copy).
:::
::: If you know that you actually need 101 push_backs, you could do a
::: reserve(101) :-)
:::
::: If you don't know, and the numbers are reasonably random, you
::: will get a good performance *on average*. Note that after
::: "shooting yourself in the foot" at 101, you get the next 99
::: push_backs for free. If you reallocate at a-million-and-one, you
::: get the next million or so push_backs for free. That's cheap!
:::
::: Bo Persson
::
:: So I was curious to see which is faster -- pushing back onto a
:: vector or using direct index access of the elements. I declare a
:: std::vector<int> v, setting the number of elements to vSize. I set
:: the values using a for-loop. Then, I declare two
:: std::vector<vector<int> > objects, m1 & m2. The first of these I
:: initialize with mSize elements, and the second I use the
:: std::vector::reserve method to claim mSize space. The using two
:: for- loops I populate m1 & m2 using direct index accessing of the
:: elements to set them equal to v, and then I use push_back to fill
:: m2. I time these two for-loops as well as the whole function.
:: What I find (perhaps not suprisingly) is that I fill m1 much, much
:: faster, i.e. push_back() with reserve() are SLOW. Anyway, here's
:: the code. Nothing to special.
::
:: #include <vector>
:: #include <iostream>
:: #include <iomanip>
:: #include <time.h>
::
:: using namespace std;
::
:: int main(){
:: time_t t0_0,t1_0,t2_0;
:: time_t t0_f,t1_f,t2_f;
::
:: t0_0 = time(NULL);
:: int vSize = 10000000;
:: int mSize = 10;
::
:: vector<int> v(vSize);
::
:: for (int i=0;i<vSize;i++){v.at(i) = i;}
::
:: vector<vector<int> > m1(mSize);
:: vector<vector<int> > m2;
:: m2.reserve(mSize);
:: t1_0 = time (NULL);
:: for (int j=0;j<mSize;j++){m1.at(j) = v;}
::
:: t1_f = time(NULL);
:: t2_0 = time(NULL);
::
:: for (int i=0; i<mSize;i++){ m2.push_back(v);}
::
:: t2_f = time(NULL);
:: t0_f = time(NULL);
:: cout << "testTime0 = " << t0_f-t0_0 << endl;
:: cout << "testTime1 = " << t1_f-t1_0 << endl;
:: cout << "testTime2 = " << t2_f-t2_0 << endl;
::
:: return 0;
:: }

Strange!

I get about equal time for both versions. In release mode it in fact
runs so fast that I get 0-1 second for all results. Changing time() to
clock(), I get something like

testTime0 = 890
testTime1 = 422
testTime2 = 406

You don't run with iterator debugging enabled, or anything?

Bo Persson