2D矩阵声明差异

Question

I noticed something strange about my code, and the amount of time it was taking to declare and initialize a 2D matrix of mine.

First way :

vector< vector<double> > gblStiff(dOF, vector<double>(dOF, 0));

dOF was some determined value earlier in the code. This first way took approximately 3 seconds when dOF = 30000!!

Second way :

double** gblStiff = new double*[dOF];
for (i=0; i < dOF; i++) 
    gblStiff[i] = new double[dOF];

This second way takes 0.063 seconds for the same dOF!!

Anyone able to shed light onto why this is happening? I am extremely curious.

Answer 1

This is really a C++ question. First, you probably didn't compile with optimisation. The vector template really needs a little compiler help to be usable. Second, vector 's constructor initialises all of the elements; in your case, it initialises dOF*dOF double s to zero. new double[dOF] does no such thing; that memory can contain anything at all.