C++ Unhandled exception for large vector/array - c++

I keep getting an unhandled exception in my code and it has me stumped.
I am sure it is in the way I have my variables declared.
Basically I am attempting to create 3 arrays, M rows, N columns of random variables.
If I set my N = 1,000 and M = 10,000, not a problem.
If I then change M = 100,000 I get an Unhandled exception memory allocation error.
Can someone please help me understand why this is happening.
Parts of the code was written on VS2010. I have now moved on to VS2013, so any additional advice on the usage of newer functions would also be appreciated.
cheers,
#include <cmath>
#include <iostream>
#include <random>
#include <vector>
#include <ctime>
#include <ratio>
#include <chrono>
int main()
{
using namespace std::chrono;
steady_clock::time_point Start_Time = steady_clock::now();
unsigned int N; // Number of time Steps in a simulation
unsigned long int M; // Number of simulations (paths)
N = 1000;
M = 10000;
// Random Number generation setup
double RANDOM;
srand((unsigned int)time(NULL)); // Generator loop reset
std::default_random_engine generator(rand()); // Seed with RAND()
std::normal_distribution<double> distribution(0.0, 1.0); // Mean = 0.0, Variance = 1.0 ie Normal
std::vector<std::vector<double>> RandomVar_A(M, std::vector<double>(N)); // dw
std::vector<std::vector<double>> RandomVar_B(M, std::vector<double>(N)); // uncorrelated dz
std::vector<std::vector<double>> RandomVar_C(M, std::vector<double>(N)); // dz
// Generate random variables for dw
for (unsigned long int i = 0; i < M; i++)
{
for (unsigned int j = 0; j < N; j++)
{
RANDOM = distribution(generator);
RandomVar_A[i][j] = RANDOM;
}
}
// Generate random variables for uncorrelated dz
for (unsigned long int i = 0; i < M; i++)
{
for (unsigned int j = 0; j < N; j++)
{
RANDOM = distribution(generator);
RandomVar_B[i][j] = RANDOM;
}
}
// Generate random variables for dz
for (unsigned long int i = 0; i < M; i++)
{
for (unsigned int j = 0; j < N; j++)
{
RANDOM = distribution(generator);
RandomVar_C[i][j] = RANDOM;
}
}
steady_clock::time_point End_Time = steady_clock::now();
duration<double> time_span = duration_cast<duration<double>>(End_Time - Start_Time);
//Clear Matricies
RandomVar_A.clear();
RandomVar_B.clear();
RandomVar_C.clear();
std::cout << std::endl;
std::cout << "its done";
std::cout << std::endl << std::endl;
std::cout << "Time taken : " << time_span.count() << " Seconds" << std::endl << std::endl;
std::cout << "End Of Program" << std::endl << std::endl;
system("pause");
return 0;
}
// *************** END OF PROGRAM ***************

Three 100,000 x 1,000 arrays of doubles represents 300 million doubles. Assuming 8 byte doubles, that's around 2.3 GB of memory. Most likely your process is by default limited to 2 GB on Windows (even if you have much more RAM installed on the machine). However, there are ways to allow your process to access a larger address space: Memory Limits for Windows.

I'm experienced something similar then my 32-bit application allocates more than 2Gb memory.
Your vectors require about 2.1Gb memory, so it might be same problem.
Try to change platform of your application to x64. This may solve problem.

Related

c++ random engines not really random

I'm playing a bit with c++ random engines, and something upsets me.
Having noticed that the values I had were roughly of the same order, I did the following test:
#include <random>
#include <functional>
#include <iostream>
int main()
{
auto res = std::random_device()();
std::ranlux24 generator(res);
std::uniform_int_distribution<uint32_t> distribution;
auto roll = std::bind(distribution, generator);
for(int j = 0; j < 30; ++j)
{
double ssum = 0;
for(int i = 0; i< 300; ++i)
{
ssum += std::log10(roll());
}
std::cout << ssum / 300. << std::endl;
}
return 0;
}
and the values I printed were all about 9.2 looking more like a normal distribution, whatever the engine I used.
Is there something I have not understood correctly?
Thanks,
Guillaume
Having noticed that the values I had were roughly of the same order
This is exactly what you'd expect with a uniform random number generator. There are 9 times as many integers in the range [10^(n-1),10^n) as there are in the range [0,10^(n-1)).

Using structs / genetic algorithm

As practice for myself I'm trying to create a genetic algorithm that will solve equations. So far my program can generate random "genes", fill up individuals with these "genes", and do some basic calculations with the genes (at the moment, simply summing the "genes").
However, I've realised now that I want to implement my fitness function that I would have been better off creating a struct for individual, since I need to keep the genes and the fitness outcome together to have the fittest genes reproduce again.
Anyway, here's my code:
// GA.cpp : Defines the entry point for the console application.
//
#include "stdafx.h"
#include <iostream>
#include <vector>
#include <random>
#include <string>
const int population_size = 10;
const int number_of_variables = 7;
struct one_individual
{
std::vector<std::vector<double>>individual;;
double evaluation = 0;
double fit = 0;
};
int main()
{
// Generate random number
std::random_device rd;
std::mt19937 rng(rd()); // random-number engine (Mersenne-Twister in this case)
std::uniform_real_distribution<double> dist(-10.0, 10.0);
// Create vector that holds vectors called individual and fill size it to the amount of individuals I want to have.
std::vector<std::vector<double>>individual;
for (int i = 0; i < population_size; i++)
{
std::vector<double>variables;
for (int j = 0; j < number_of_variables; j++)
{
variables.push_back(dist(rng));
}
individual.push_back(variables);
}
// Display entire population
for (auto &count : individual)
{
for (auto &count2 : count)
{
std::cout << count2 << " ";
}
std::cout << "\n";
}
// Do calculation with population. At the moment I just add up all the genes (sum) and display the sum for each individual.
for (int i = 0; i < population_size; i++)
{
int j = 0;
std::cout << "Organism "<< i;
double sum = individual[i].at(j) + individual[i].at(j + 1) + individual[i].at(j + 2) + individual[i].at(j + 3) + individual[i].at(j + 4) + individual[i].at(j + 5) + individual[i].at(j + 6);
std::cout << " is " << sum << "\n";
}
std::cout << "\n";
return 0;
}
What I think I should be doing is something like this:
for (int i = 0; i < population_size; i++)
{
one_individual individual;
std::vector<double>variables;
for (int j = 0; j < number_of_variables; j++)
{
variables.push_back(dist(rng));
}
one_individual.individual.push_back(variables);
}
The above code is not working. What happens when I try to compile is I get a list of errors, I just pasted it into pastebin since it's a pretty big list: www.pastebin.com/EVJaV0Ex. If I remove everything except the parts needed for the "creating individuals part" the errors that remain are: www.pastebin.com/djw6JmXZ. All errors are on line 41 which is the final line one_individual.individual.push_back(variables);
Edited for clarity, apologies that it was unclear.
Consider the instruction
one_individual.individual.push_back(variables);
where one_individual is a type (struct one_individual).
I suppose you should use the defined variable of type one_individual, so
individual.individual.push_back(variables);

Vector inside vector (creating chromosomes)

I'm attempting to build a genetic algorithm that can take a certain amount of variables (say 4), and use these in a way so that you could have 2a + 3b + c*c + d = 16. I realise there are more efficient ways to calculate this, but I want to try and build a genetic algorithm to expand later.
I'm starting by trying to create "organisms" that can compete later. What I've done is this:
#include "stdafx.h"
#include <iostream>
#include <vector>
#include <random>
// Set population size
const int population_size = 10;
const int number_of_variables = 4;
int main()
{
// Generate random number
std::random_device rd;
std::mt19937 rng(rd()); // random-number engine (Mersenne-Twister in this case)
std::uniform_int_distribution<int> uni(-10, 10);
// Set gene values.
std::vector<int>chromosome;
std::vector<int>variables;
for (int i = 0; i < number_of_variables; ++i)
{
double rand_num = uni(rng);
variables.push_back (rand_num);
std::cout << variables[i] << "\n";
}
return 0;
}
What happens is it will fill up the number_of_variables vector, and output these just because that makes it clear for me that it's actually doing what I intend for it to do. What I want it to do however is to fill up each "chromosome" with one variables vector, so that for example chromosome 0 would have the values {1, 5, -5, 9} etc.
The following code obviously isn't working, but this is what I'd like it to do:
for (int j = 0; j < population_size; ++j)
{
for (int i = 0; i < number_of_variables; ++i)
{
double rand_num = uni(rng);
variables.push_back(rand_num);
}
chromosome.push_back(variables[j]);
std::cout << chromosome[j] << "\n";
}
Meaning it'd fill up the variables randomly, then chromosome1 would take those 4 values that "variables" took, and repeat. What actually happens is that (I think) it only takes the first value from "variables" and copies that into "chromosome" rather than all 4.
If anyone could help it'd be very much appreciated, I realise this might be simply a rookie mistake that is laughably simply in the eyes of someone more experienced with vectors (which would probably be 99% of the people on this website, hah).
Anyway, thanks :)
#include <iostream>
#include <vector>
#include <random>
// Set population size
const int population_size = 10;
const int number_of_variables = 4;
int main()
{
// Generate random number
std::random_device rd;
std::mt19937 rng(rd()); // random-number engine (Mersenne-Twister in this case)
std::uniform_int_distribution<int> uni(-10, 10);
// Set gene values.
std::vector< std::vector<int>>chromosome;
for( int kp = 0; kp < population_size; kp++ )
{
std::vector<int>variables;
for (int i = 0; i < number_of_variables; ++i)
{
double rand_num = uni(rng);
variables.push_back (rand_num);
}
chromosome.push_back( variables );
}
// display entire population
for( auto c : chromosome )
{
for( auto v : c )
{
std::cout << v << " ";
}
std::cout << "\n";
}
// display 4th member of population
for( auto v : chromosone[ 3 ] )
{
std::cout << v << " ";
}
std::cout << "\n";
return 0;
}
http://ideone.com/2jastJ
You can place a vector inside a vector with the syntax:
std::vector<std::vector<int>>
but you will need to make the outer vector large enough for num_variables.
#include <vector>
#include <cstdlib>
using Individual = std::vector<int>;
using Population = std::vector<Individual>;
// short for std::vector<std::vector<int>>;
const size_t number_of_variables = 8;
int main() {
Population population(10);
for (auto& individual : population) {
individual.resize(number_of_variables);
for (size_t j = 0; j < number_of_variables; ++j) {
individual[j] = j; // replace with random number
}
}
}
Live demo: http://ideone.com/pfufGt

Is using a vector of boolean values slower than a dynamic bitset?

Is using a vector of boolean values slower than a dynamic bitset?
I just heard about boost's dynamic bitset, and I was wondering is it worth
the trouble. Can I just use vector of boolean values instead?
A great deal here depends on how many Boolean values you're working with.
Both bitset and vector<bool> normally use a packed representation where a Boolean is stored as only a single bit.
On one hand, that imposes some overhead in the form of bit manipulation to access a single value.
On the other hand, that also means many more of your Booleans will fit in your cache.
If you're using a lot of Booleans (e.g., implementing a sieve of Eratosthenes) fitting more of them in the cache will almost always end up a net gain. The reduction in memory use will gain you a lot more than the bit manipulation loses.
Most of the arguments against std::vector<bool> come back to the fact that it is not a standard container (i.e., it does not meet the requirements for a container). IMO, this is mostly a question of expectations -- since it says vector, many people expect it to be a container (other types of vectors are), and they often react negatively to the fact that vector<bool> isn't a container.
If you're using the vector in a way that really requires it to be a container, then you probably want to use some other combination -- either deque<bool> or vector<char> can work fine. Think before you do that though -- there's a lot of (lousy, IMO) advice that vector<bool> should be avoided in general, with little or no explanation of why it should be avoided at all, or under what circumstances it makes a real difference to you.
Yes, there are situations where something else will work better. If you're in one of those situations, using something else is clearly a good idea. But, be sure you're really in one of those situations first. Anybody who tells you (for example) that "Herb says you should use vector<char>" without a lot of explanation about the tradeoffs involved should not be trusted.
Let's give a real example. Since it was mentioned in the comments, let's consider the Sieve of Eratosthenes:
#include <vector>
#include <iostream>
#include <iterator>
#include <chrono>
unsigned long primes = 0;
template <class bool_t>
unsigned long sieve(unsigned max) {
std::vector<bool_t> sieve(max, false);
sieve[0] = sieve[1] = true;
for (int i = 2; i < max; i++) {
if (!sieve[i]) {
++primes;
for (int temp = 2 * i; temp < max; temp += i)
sieve[temp] = true;
}
}
return primes;
}
// Warning: auto return type will fail with older compilers
// Fine with g++ 5.1 and VC++ 2015 though.
//
template <class F>
auto timer(F f, int max) {
auto start = std::chrono::high_resolution_clock::now();
primes += f(max);
auto stop = std::chrono::high_resolution_clock::now();
return stop - start;
}
int main() {
using namespace std::chrono;
unsigned number = 100000000;
auto using_bool = timer(sieve<bool>, number);
auto using_char = timer(sieve<char>, number);
std::cout << "ignore: " << primes << "\n";
std::cout << "Time using bool: " << duration_cast<milliseconds>(using_bool).count() << "\n";
std::cout << "Time using char: " << duration_cast<milliseconds>(using_char).count() << "\n";
}
We've used a large enough array that we can expect a large portion of it to occupy main memory. I've also gone to a little pain to ensure that the only thing that changes between one invocation and the other is the use of a vector<char> vs. vector<bool>. Here are some results. First with VC++ 2015:
ignore: 34568730
Time using bool: 2623
Time using char: 3108
...then the time using g++ 5.1:
ignore: 34568730
Time using bool: 2359
Time using char: 3116
Obviously, the vector<bool> wins in both cases--by around 15% with VC++, and over 30% with gcc. Also note that in this case, I've chosen the size to show vector<char> in quite favorable light. If, for example, I reduce number from 100000000 to 10000000, the time differential becomes much larger:
ignore: 3987474
Time using bool: 72
Time using char: 249
Although I haven't done a lot of work to confirm, I'd guess that in this case, the version using vector<bool> is saving enough space that the array fits entirely in the cache, while the vector<char> is large enough to overflow the cache, and involve a great deal of main memory access.
You should usually avoid std::vector<bool> because it is not a standard container. It's a packed version, so it breaks some valuable guarantees usually given by a vector. A valid alternative would be to use std::vector<char> which is what Herb Sutter recommends.
You can read more about it in his GotW on the subject.
Update:
As has been pointed out, vector<bool> can be used to good effect, as a packed representation improves locality on large data sets. It may very well be the fastest alternative depending on circumstances. However, I would still not recommend it by default since it breaks many of the promises established by std::vector and the packing is a speed/memory tradeoff which may be beneficial in both speed and memory.
If you choose to use it, I would do so after measuring it against vector<char> for your application. Even then, I'd recommend using a typedef to refer to it via a name which does not seem to make the guarantees which it does not hold.
#include "boost/dynamic_bitset.hpp"
#include <chrono>
#include <iostream>
#include <random>
#include <vector>
int main(int, char*[])
{
auto gen = std::bind(std::uniform_int_distribution<>(0, 1), std::default_random_engine());
std::vector<char> randomValues(1000000);
for (char & randomValue : randomValues)
{
randomValue = static_cast<char>(gen());
}
// many accesses, few initializations
auto start = std::chrono::high_resolution_clock::now();
for (int i = 0; i < 500; ++i)
{
std::vector<bool> test(1000000, false);
for (int j = 0; j < test.size(); ++j)
{
test[j] = static_cast<bool>(randomValues[j]);
}
}
auto end = std::chrono::high_resolution_clock::now();
std::cout << "Time taken1: " << std::chrono::duration_cast<std::chrono::milliseconds>(end - start).count()
<< " milliseconds" << std::endl;
auto start2 = std::chrono::high_resolution_clock::now();
for (int i = 0; i < 500; ++i)
{
boost::dynamic_bitset<> test(1000000, false);
for (int j = 0; j < test.size(); ++j)
{
test[j] = static_cast<bool>(randomValues[j]);
}
}
auto end2 = std::chrono::high_resolution_clock::now();
std::cout << "Time taken2: " << std::chrono::duration_cast<std::chrono::milliseconds>(end2 - start2).count()
<< " milliseconds" << std::endl;
auto start3 = std::chrono::high_resolution_clock::now();
for (int i = 0; i < 500; ++i)
{
std::vector<char> test(1000000, false);
for (int j = 0; j < test.size(); ++j)
{
test[j] = static_cast<bool>(randomValues[j]);
}
}
auto end3 = std::chrono::high_resolution_clock::now();
std::cout << "Time taken3: " << std::chrono::duration_cast<std::chrono::milliseconds>(end3 - start3).count()
<< " milliseconds" << std::endl;
// few accesses, many initializations
auto start4 = std::chrono::high_resolution_clock::now();
for (int i = 0; i < 1000000; ++i)
{
std::vector<bool> test(1000000, false);
for (int j = 0; j < 500; ++j)
{
test[j] = static_cast<bool>(randomValues[j]);
}
}
auto end4 = std::chrono::high_resolution_clock::now();
std::cout << "Time taken4: " << std::chrono::duration_cast<std::chrono::milliseconds>(end4 - start4).count()
<< " milliseconds" << std::endl;
auto start5 = std::chrono::high_resolution_clock::now();
for (int i = 0; i < 1000000; ++i)
{
boost::dynamic_bitset<> test(1000000, false);
for (int j = 0; j < 500; ++j)
{
test[j] = static_cast<bool>(randomValues[j]);
}
}
auto end5 = std::chrono::high_resolution_clock::now();
std::cout << "Time taken5: " << std::chrono::duration_cast<std::chrono::milliseconds>(end5 - start5).count()
<< " milliseconds" << std::endl;
auto start6 = std::chrono::high_resolution_clock::now();
for (int i = 0; i < 1000000; ++i)
{
std::vector<char> test(1000000, false);
for (int j = 0; j < 500; ++j)
{
test[j] = static_cast<bool>(randomValues[j]);
}
}
auto end6 = std::chrono::high_resolution_clock::now();
std::cout << "Time taken6: " << std::chrono::duration_cast<std::chrono::milliseconds>(end6 - start6).count()
<< " milliseconds" << std::endl;
return EXIT_SUCCESS;
}
Time taken1: 1821 milliseconds
Time taken2: 1722 milliseconds
Time taken3: 25 milliseconds
Time taken4: 1987 milliseconds
Time taken5: 1993 milliseconds
Time taken6: 10970 milliseconds
dynamic_bitset = std::vector<bool>
if you allocate many times but you only access the array that you created few times, go for std::vector<bool> because it has lower allocation/initialization time.
if you allocate once and access many times, go for std::vector<char>, because of faster access
Also keep in mind that std::vector<bool> is NOT safe to be used is in multithreading because you might write to different bits but it might be the same byte.
It appears that the size of a dynamic bitset cannot be changed:
"The dynamic_bitset class is nearly identical to the std::bitset class. The difference is that the size of the dynamic_bitset (the number of bits) is specified at run-time during the construction of a dynamic_bitset object, whereas the size of a std::bitset is specified at compile-time through an integer template parameter." (from http://www.boost.org/doc/libs/1_36_0/libs/dynamic_bitset/dynamic_bitset.html)
As such, it should be slightly faster since it will have slightly less overhead than a vector, but you lose the ability to insert elements.
UPDATE: I just realize that OP was asking about vector<bool> vs bitset, and my answer does not answer the question, but I think I should leave it, if you search for c++ vector bool slow, you end up here.
vector<bool> is terribly slow. At least on my Arch Linux system (you can probably get a better implementation or something... but I was really surprised). If anybody has any suggestions why this is so slow, I'm all ears! (Sorry for the blunt beginning, here's the more professional part.)
I've written two implementations of the SOE, and the 'close to metal' C implementation is 10 times faster. sievec.c is the C implementation, and sievestl.cpp is the C++ implementation. I just compiled with make (implicit rules only, no makefile): and the results were 1.4 sec for the C version, and 12 sec for the C++/STL version:
sievecmp % make -B sievec && time ./sievec 27
cc sievec.c -o sievec
aa 1056282
./sievec 27 1.44s user 0.01s system 100% cpu 1.455 total
and
sievecmp % make -B sievestl && time ./sievestl 27
g++ sievestl.cpp -o sievestl
1056282./sievestl 27 12.12s user 0.01s system 100% cpu 12.114 total
sievec.c is as follows:
#include <stdio.h>
#include <stdlib.h>
typedef unsigned long prime_t;
typedef unsigned long word_t;
#define LOG_WORD_SIZE 6
#define INDEX(i) ((i)>>(LOG_WORD_SIZE))
#define MASK(i) ((word_t)(1) << ((i)&(((word_t)(1)<<LOG_WORD_SIZE)-1)))
#define GET(p,i) (p[INDEX(i)]&MASK(i))
#define SET(p,i) (p[INDEX(i)]|=MASK(i))
#define RESET(p,i) (p[INDEX(i)]&=~MASK(i))
#define p2i(p) ((p)>>1) // (((p-2)>>1))
#define i2p(i) (((i)<<1)+1) // ((i)*2+3)
unsigned long find_next_zero(unsigned long from,
unsigned long *v,
size_t N){
size_t i;
for (i = from+1; i < N; i++) {
if(GET(v,i)==0) return i;
}
return -1;
}
int main(int argc, char *argv[])
{
size_t N = atoi(argv[1]);
N = 1lu<<N;
// printf("%u\n",N);
unsigned long *v = malloc(N/8);
for(size_t i = 0; i < N/64; i++) v[i]=0;
unsigned long p = 3;
unsigned long pp = p2i(p * p);
while( pp <= N){
for(unsigned long q = pp; q < N; q += p ){
SET(v,q);
}
p = p2i(p);
p = find_next_zero(p,v,N);
p = i2p(p);
pp = p2i(p * p);
}
unsigned long sum = 0;
for(unsigned long i = 0; i+2 < N; i++)
if(GET(v,i)==0 && GET(v,i+1)==0) {
unsigned long p = i2p(i);
// cout << p << ", " << p+2 << endl;
sum++;
}
printf("aa %lu\n",sum);
// free(v);
return 0;
}
sievestl.cpp is as follows:
#include <iostream>
#include <vector>
#include <sstream>
using namespace std;
inline unsigned long i2p(unsigned long i){return (i<<1)+1; }
inline unsigned long p2i(unsigned long p){return (p>>1); }
inline unsigned long find_next_zero(unsigned long from, vector<bool> v){
size_t N = v.size();
for (size_t i = from+1; i < N; i++) {
if(v[i]==0) return i;
}
return -1;
}
int main(int argc, char *argv[])
{
stringstream ss;
ss << argv[1];
size_t N;
ss >> N;
N = 1lu<<N;
// cout << N << endl;
vector<bool> v(N);
unsigned long p = 3;
unsigned long pp = p2i(p * p);
while( pp <= N){
for(unsigned long q = pp; q < N; q += p ){
v[q] = 1;
}
p = p2i(p);
p = find_next_zero(p,v);
p = i2p(p);
pp = p2i(p * p);
}
unsigned sum = 0;
for(unsigned long i = 0; i+2 < N; i++)
if(v[i]==0 and v[i+1]==0) {
unsigned long p = i2p(i);
// cout << p << ", " << p+2 << endl;
sum++;
}
cout << sum;
return 0;
}

Why is accumulate faster than a simple for cycle?

I was testing algorithms and run into this weird behavior, when std::accumulate is faster than a simple for cycle.
Looking at the generated assembler I'm not much wiser :-) It seems that the for cycle is optimized into MMX instructions, while accumulate expands into a loop.
This is the code. The behavior manifests with -O3 optimization level, gcc 4.7.1
#include <vector>
#include <chrono>
#include <iostream>
#include <random>
#include <algorithm>
using namespace std;
int main()
{
const size_t vsize = 100*1000*1000;
vector<int> x;
x.reserve(vsize);
mt19937 rng;
rng.seed(chrono::system_clock::to_time_t(chrono::system_clock::now()));
uniform_int_distribution<uint32_t> dist(0,10);
for (size_t i = 0; i < vsize; i++)
{
x.push_back(dist(rng));
}
long long tmp = 0;
for (size_t i = 0; i < vsize; i++)
{
tmp += x[i];
}
cout << "dry run " << tmp << endl;
auto start = chrono::high_resolution_clock::now();
long long suma = accumulate(x.begin(),x.end(),0);
auto end = chrono::high_resolution_clock::now();
cout << "Accumulate runtime " << chrono::duration_cast<chrono::nanoseconds>(end-start).count() << " - " << suma << endl;
start = chrono::high_resolution_clock::now();
suma = 0;
for (size_t i = 0; i < vsize; i++)
{
suma += x[i];
}
end = chrono::high_resolution_clock::now();
cout << "Manual sum runtime " << chrono::duration_cast<chrono::nanoseconds>(end-start).count() << " - " << suma << endl;
return 0;
}
When you pass the 0 to accumulate, you are making it accumulate using an int instead of a long long.
If you code your manual loop like this, it will be equivalent:
int sumb = 0;
for (size_t i = 0; i < vsize; i++)
{
sumb += x[i];
}
suma = sumb;
or you can call accumulate like this:
long long suma = accumulate(x.begin(),x.end(),0LL);
I have some different results using Visual Studio 2012
// original code
Accumulate runtime 93600 ms
Manual sum runtime 140400 ms
Note that the original std::accumulate code isn't equivalent to the for loop because the third parameter to std::accumulate is an int 0 value. It performs the summation using an int and only at the end stores the result in a long long. Changing the third parameter to 0LL forces the algorithm to use a long long accumulator and results in the following times.
// change std::accumulate initial value -> 0LL
Accumulate runtime 265200 ms
Manual sum runtime 140400 ms
Since the final result fits in an int I changed suma and std::accumulate back to using only int values. After this change the MSVC 2012 compiler was able to auto-vectorize the for loop and resulted in the following times.
// change suma from long long to int
Accumulate runtime 93600 ms
Manual sum runtime 46800 ms
After fixing the accumulate issue others noted I tested with both Visual Studio 2008 & 2010 and accumulate was indeed faster than the manual loop.
Looking at the disassembly I saw some additional iterator checking being done in the manual loop so I switched to just a raw array to eliminate it.
Here's what I ended up testing with:
#include <Windows.h>
#include <iostream>
#include <numeric>
#include <stdlib.h>
int main()
{
const size_t vsize = 100*1000*1000;
int* x = new int[vsize];
for (size_t i = 0; i < vsize; i++) x[i] = rand() % 1000;
LARGE_INTEGER start,stop;
long long suma = 0, sumb = 0, timea = 0, timeb = 0;
QueryPerformanceCounter( &start );
suma = std::accumulate(x, x + vsize, 0LL);
QueryPerformanceCounter( &stop );
timea = stop.QuadPart - start.QuadPart;
QueryPerformanceCounter( &start );
for (size_t i = 0; i < vsize; ++i) sumb += x[i];
QueryPerformanceCounter( &stop );
timeb = stop.QuadPart - start.QuadPart;
std::cout << "Accumulate: " << timea << " - " << suma << std::endl;
std::cout << " Loop: " << timeb << " - " << sumb << std::endl;
delete [] x;
return 0;
}
Accumulate: 633942 - 49678806711
Loop: 292642 - 49678806711
Using this code, the manual loop easily beats accumulate. The big difference is the compiler unrolled the manual loop 4 times, otherwise the generated code is almost identical.