Example Programs
****************
Hello World - Simple Bell State
-------------------------------

.. code-block:: cpp

    #include <cudaq.h>

    struct bell {
      int operator()(int num_iters) __qpu__ {
        cudaq::qarray<2> q;
        int nCorrect = 0;
        for (int i = 0; i < num_iters; i++) {
          h(q[0]);
          x<cudaq::ctrl>(q[0], q[1]);
          auto results = mz(q);
          if (results[0] == results[1]) 
            nCorrect++;
          
          reset(q[0]);
          reset(q[1]);
        }
        return nCorrect;
      }
    };

    int main() { printf("N Correct = %d\n", bell{}(100)); }

GHZ State Preparation and Sampling
----------------------------------

.. code-block:: cpp 
 
    #include <cudaq.h>

    struct ghz {
      void operator()(const int n_qubits) __qpu__ {
        cudaq::qvector q(n_qubits);
        h(q[0]);
        for (int i = 0; i < n_qubits - 1; ++i) 
          // note use of ctrl modifier
          x<cudaq::ctrl>(q[i], q[i+1]); 
        
        mz(q);
      }
    };

    int main() {
      // Sample the state produced by the ghz kernel
      auto counts = cudaq::sample(ghz{}, 10);
      for (auto [bits, count] : counts) {
        printf("Observed %s %lu times.\n", bits.c_str(), count);
      }
      return 0;
    }

Quantum Phase Estimation
------------------------

.. literalinclude:: ../../examples/cpp/algorithms/phase_estimation.cpp
    :language: cpp

Deuteron Binding Energy Parameter Sweep
---------------------------------------

.. code-block:: cpp 

    #include <cudaq.h>
    #include <cudaq/algorithm.h>

    struct deuteron_n2_ansatz {
      void operator()(double theta) __qpu__ {
        cudaq::qarray<2> q;
        x(q[0]);
        ry(theta, q[1]);
        x<cudaq::ctrl>(q[1], q[0]);
      }
    };

    int main() {
      using namespace cudaq::spin;
      cudaq::spin_op h = 5.907 - 2.1433 * x(0) * x(1) - 2.1433 * y(0) * y(1) +
                  .21829 * z(0) - 6.125 * z(1);
  
      // Perform parameter sweep for deuteron N=2 Hamiltonian
      const auto param_space = cudaq::linspace(-M_PI, M_PI, 25);
      for (const auto& param : param_space) {
        // KERNEL::observe(...) <==> 
        // E(params...) = <psi(params...) | H | psi(params...)>
        double energy_at_param = cudaq::observe(deuteron_n2_ansatz{}, h, param);
        printf("<H>(%lf) = %lf\n", param, energy_at_param);
      }
    }


Grover's Algorithm
------------------

.. literalinclude:: ../../examples/cpp/algorithms/grover.cpp
    :language: cpp

Iterative Phase Estimation
--------------------------

.. literalinclude:: ../../examples/cpp/other/iterative_qpe.cpp
    :language: cpp
    :start-after: [Begin Documentation]
    :end-before: [End Documentation]