Example 7 - Primal tape evaluation

Goal: Reevaluate a tape at a different position.

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::RealReversePrimal;  // Step 1: Use a primal value taping approach
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 2: Do a normal recording and evaluation
  tape.setActive();
 
  tape.registerInput(x);
  Real y = func(x);
  tape.registerOutput(y);
 
  tape.setPassive();
 
  y.setGradient(1.0);
  tape.evaluate();
 
  std::cout << "f(4.0) = " << y << std::endl;
  std::cout << "df/dx(4.0) = " << x.getGradient() << std::endl;
 
  tape.clearAdjoints();
 
  tape.setPrimal(x.getIdentifier(), 10.0);
  tape.evaluatePrimal();
 
  y.setGradient(1.0);
  tape.evaluate();
 
  std::cout << "f(3.0) = " << tape.getPrimal(y.getIdentifier()) << std::endl;
  std::cout << "df/dx(3.0) = " << x.getGradient() << std::endl;
 
  tape.reset();
 
  return 0;
}

A primal reevaluation follows the same rules as an evaluation of the tape. There are a few things to consider:

• A primal value taping approach needs to be used, e.g. codi::RealReversePrimal
• The primal value needs to be changed on the tape with setPrimal.
• New primal values of the outputs have to be received with getPrimal.
• CoDiPack does not record branching statements. These changes are not considered in a primal reevaluation.