CODEWITHSUNDEEP
c++vectorperformance
jmasterx
jmasterx

Reputation: 54193

std::vector is very slow?

Here is what I'm doing.

I have a class which I instance and it has a std::vector.

when I first instance the class this std::vector is empty.

The way I use it is I exponentially add to it and clear. Ex:

Add a number, clear the vector:

Add 2 numbers, clear the vector:

Add 3 numbers, clear the vector,

Add 4 numbers, clear the vector. ......

Is a std::vector the bst way to do what I'm doing? I tried to do reserve(100,000) in the constructor but this did not help.

Is there maybe a better container for my usage?

Thanks

Upvotes: 1

Views: 1548

Answers (2)

anon
anon

Reputation:

Something like:

struct A {
   std::vector <int> v;
   A() : v(100000) {}
};

is probably what you want. When you say:

A a;

This creates an instance the struct A (works for classes too) containing a vector of the required size.

Upvotes: 2

Thomas
Thomas

Reputation: 182083

Your algorithm appears to be quadratic. If you really need 100,000 elements, you're adding an element 1 + 2 + 3 + ... + 100,000 times. That's about 5,000,000,000 operations. That many operations, no matter how trivial they are, will take a while on a standard pc, regardless of whether you're using std::vector or hand-crafted assembly language.

Upvotes: 11

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