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algorithms.cc
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Wed, Jul 10, 04:28

algorithms.cc
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#include <vector>
#include <iostream>
#include <algorithm>
#include <numeric>
#include <functional>
#include <cmath>
void print_vector(std::vector<double>& v) {
for (double x : v)
std::cout << x << " ";
std::cout << std::endl;
}
int main() {
unsigned int n = 20; // size of vector
std::vector<double> values(n); // create vector with n numbers
// Filling with zeros
std::fill(values.begin(), values.end(), 0);
print_vector(values);
// Applying std::iota
std::iota(values.begin(), values.end(), 1);
print_vector(values); // should print "1 2 3 4 5 ..."
// Squaring a value
auto square_it = [](double & x) { x *= x; };
double y = 3.;
square_it(y);
std::cout << "3*3 = " << y << std::endl; // should print 9.
// Squaring values of the vector
for (double & y : values)
square_it(y);
print_vector(values); // should print "1 4 9 16 25 ..."
// [resetting values]
std::iota(values.begin(), values.end(), 1);
// Squaring values using std::for_each
std::for_each(values.begin(), values.end(), square_it);
print_vector(values); // should print "1 4 9 16 25 ..."
// Summing the squares
double answer = n*(n+1)*(2*n+1)/6.;
double result = 0; // will contain the result
// capture the result variable by-reference
auto summation = [&result] (double & x) { result += x; };
// apply the summation function
std::for_each(values.begin(), values.end(), summation);
std::cout << result << " = " << answer << std::endl;
// Do the same with std::reduce
result = std::accumulate(values.begin(), values.end(), 0.);
std::cout << result << " = " << answer << std::endl;
// We need to invert the squares
auto inverse = [](double & x) { x = 1. / x; };
std::for_each(values.begin(), values.end(), inverse); // apply inverse
// Sum all squares in reverse order to avoid roundoff errors
result = std::accumulate(values.rbegin(), values.rend(), 0.);
double pi_estimate = std::sqrt(6. * result);
std::cout << pi_estimate << " ~ " << M_PI << std::endl;
return 0;
}

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