code2_demo


//This program is intended to demonstrate simple serial programming where a vectors element gets modified serially. 
//
#include <iostream>
#include <vector>
#include <cmath>

using std::cout;
using std::endl;
using std::vector;

void increase_magnitude(float *starting_address,unsigned int size_vec, float mag_multiplyer)
{
  //the variable *starting_address is a pointer which will point to (or contain address of), the first element of the array.
  for (int i=0;i<size_vec;i++) 
  {
	*starting_address=*starting_address * mag_multiplyer;
	starting_address+=1; 
  } 
  //in this way at the same memory location we will have modified the vector.
}

double magnitude_finder(float *starting_address_vec,unsigned int size)
{
  //This function will return the magnitude of the vector. 
  double sum=0.0;
  for (int i=0;i<size;i++) 
  {
	sum+=pow((*starting_address_vec),2); 
	starting_address_vec+=1; 
  }
 return pow(sum,0.5); 
}


int main(int argc, char *argv[])
{
  unsigned int N=1<<31; //the number implies 31 zeroes in front of 1. So it is 2^31.
  vector<float> vec; //whenever we declare an vector, it has undefined size(if we don't provide the size). Unless we declare it with some size. 
  //let us fill the vector with N natural numbers(1,2,.......N).
  for (int i=0;i<N;i++)
  {
	  vec.push_back(i);
  }
  //let us define a pointer to integer vector
  float *ptr_to_vec=&vec[0]; //giving address of first element. Or we can just write: 
  //we shall modify the vector by multiply it with some real number. 
  //Real numbers are stored in float data types(require 4byte per real number) and double data types(8bytes).
  float multiplier=4.0;
  //Let us find the magnitude before changing the vector, 
  double mag_before=magnitude_finder(ptr_to_vec,N);
  increase_magnitude(ptr_to_vec,N,multiplier);
  double mag_after=magnitude_finder(ptr_to_vec,N);
  double ratio_of_magnitudes=mag_after/mag_before;
  printf("Value of magnitude before multiplying the vector is %0.f  \n",mag_before);
  printf("Value of magnitude after multiplying the vector is %0.f   \n",mag_after);
  cout<<"the final vector has magnitude "<<ratio_of_magnitudes<<" times the earlier one"<<endl<<"which should be equal to the multiplier (which is: "<<multiplier<<")"<<endl;
  return 0;
}

Output:
```
Value of magnitude before multiplying the vector is 57455839005302
Value of magnitude after multiplying the vector is 229823356021209
the final vector has magnitude 4 times the earlier one
which should be equal to the multiplier (which is: 4)

real 3m8.202s
user 2m59.683s
sys 0m6.290s

```

Remarks: we can have a look at the time it takes for different parts of the code

Code 2