Example 6 - Forward mode tape evaluation

Goal: Evaluate a tape with forward AD mode.

Prerequisite: Tutorial 2 - Reverse mode AD

Function:

Real func(const Real& x) {
  return x * x * x;
}

Full code:

 
#include <codi.hpp>
#include <iostream>
 
using Real = codi::RealReverse;
using Tape = typename Real::Tape;
 
Real func(const Real& x) {
  return x * x * x;
}
 
int main(int nargs, char** args) {
  Real x = 4.0;
 
  Tape& tape = Real::getTape();
  // Step 1: Do a normal recording
  tape.setActive();
 
  tape.registerInput(x);
  Real y = func(x);
  tape.registerOutput(y);
 
  tape.setPassive();
 
  x.setGradient(1.0);      // Step 2: Seed the input values
  tape.evaluateForward();  // Step 3: Perform forward evaluation
 
  // Step 4: Access gradients on the output values
  std::cout << "f(4.0) = " << y << std::endl;
  std::cout << "df/dx(4.0) = " << y.getGradient() << std::endl;
 
  tape.reset();
 
  return 0;
}

Forward mode tape evaluation works the same as a reverse mode tape evaluation. Only the inputs need to be seeded and the derivatives are obtained from the outputs.