Compare C++ and Fortran in run time for calculating the sum of a large array.

comp_time.cpp

#include <iostream>
#include <iomanip>
#include <sstream>
#include <time.h>
using namespace std;

int main(int argc, char *argv[] ) {
  int i, max_itr;
  double *val, sum, t1, t2, a, x0;
  stringstream ss;

  if (argc > 1) {
    ss << argv[1];
    ss >> max_itr;
  } else {
    max_itr = 100;
  }
  
  cout << "----------------------------------------" << endl;
  cout << "C++" << endl;
  cout << "----------" << endl;
  cout << "num of itr:" << max_itr << endl;

  t1 = clock();
  val = new double[max_itr];

  a = 3.5;
  x0 = 0.2;
  val[0] = a * x0 * (1-x0);
  for (i=1; i<max_itr ;i++) {
    val[i] = a * val[i-1] * (1-val[i-1]);
  }
  sum = 0.0;
  for (i=0; i<max_itr; i++) {
    sum = sum + val[i];
  }
  delete val;
  t2 = clock();
  
  cout << "sum:" << scientific << setprecision(16) << sum << endl;
  cout << "Ellapsed time:" << fixed << setprecision(5) << (t2-t1)/CLOCKS_PER_SEC << endl;
  cout << "----------------------------------------" << endl;
  return 0;
}

comp_time.f90

program main
  integer i, max_itr
  integer,parameter :: buf_size = 100
  double precision, allocatable :: val(:)
  double precision sum, t1, t2, a, x0
  character(buf_size) line_buf

  if (iargc() >= 1) then
     call getarg(1, line_buf)
     read(line_buf, *) max_itr
  else
     max_itr = 100
  end if

  print '(a)', '----------------------------------------'
  print '(a)', 'Fortran'
  print '(a)', '----------'
  print '("num of itr:"i0)', max_itr

  call cpu_time(t1)
  allocate( val(max_Itr) )
  

  a = 3.5d0
  x0 = 0.2d0
  val(1) = a * x0 * (1-x0)
  do i=2,max_itr
     val(i) = a * val(i-1) * (1-val(i-1))
  end do
  sum = 0d0
  do i=1,max_itr
     sum = sum + val(i)
  end do
  deallocate(val)
  call cpu_time(t2)
  
  print '("sum:"e22.17)', sum
  print '("Ellapsed time:"f10.5)', t2-t1
  print '(a)', '----------------------------------------'

end program main

comp_time.mak

all:
  g++ comp_time.cpp -o comp_time_c;
	gfortran comp_time.f90 -o comp_time_f;
run:
	./comp_time_c
	./comp_time_f
run_E6:
	./comp_time_c 1000000
	./comp_time_f 1000000
run_E9:
	./comp_time_c 1000000000
	./comp_time_f 1000000000
clean:
	rm -rf comp_time_c
	rm -rf comp_time_f

To compile:

$ make -f comp_time.mak
++ comp_time.cpp -o comp_time_c;
gfortran comp_time.f90 -o comp_time_f;

Compare for 100 elements:

$ make -f comp_time.mak run
./comp_time_c
----------------------------------------
C++
----------
num of itr:100
sum:6.4667728017033156e+01
Ellapsed time:0.00000
----------------------------------------
./comp_time_f
----------------------------------------
Fortran
----------
num of itr:100
sum:.64667728017033156E+02
Ellapsed time:   0.00001
----------------------------------------

Compare for million(1E6) elements:

$ make -f comp_time.mak run_E6
./comp_time_c 1000000
----------------------------------------
C++
----------
num of itr:1000000
sum:6.4641049256157817e+05
Ellapsed time:0.01287
----------------------------------------
./comp_time_f 1000000
----------------------------------------
Fortran
----------
num of itr:1000000
sum:.64641049256157817E+06
Ellapsed time:   0.01925
----------------------------------------    

Compare for billion(1E9) elements:

$ make -f comp_time.mak run_E9
./comp_time_c 1000000000
----------------------------------------
C++
----------
num of itr:1000000000
sum:6.4641046613093603e+08
Ellapsed time:13.47483
----------------------------------------
./comp_time_f 1000000000
----------------------------------------
Fortran
----------
num of itr:1000000000
sum:.64641046613093603E+09
Ellapsed time:  19.36442
----------------------------------------