acos()

Returns the inverse cosine of the argument in radians.

acos(n)

Use acos() to return the inverse cosine of 0.0:

#include <iostream>
#include <cmath>

int main() {
  double x = 0.0;
  double result;

  result = std::acos(x);

  std::cout << result << " radians" << "\n";
  // Output: 1.5708 radians
}