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+'''
+GAN implementation, just a demo.
+'''
+# pd for short, should be more concise.
+from paddle.v2 as pd
+import numpy as np
+import logging
+
+X = pd.data(pd.float_vector(784))
+
+# Conditional-GAN should be a class. 
+### Class member function: the initializer.
+class DCGAN(object):
+  def __init__(self, y_dim=None):
+  
+    # hyper parameters  
+    self.y_dim = y_dim # conditional gan or not
+    self.batch_size = 100
+    self.z_dim = z_dim # input noise dimension
+
+    # define parameters of discriminators
+    self.D_W1 = pd.Variable(shape=[784, 128], data=pd.gaussian_normal_randomizer())
+    self.D_b1 = pd.Variable(np.zeros(128)) # variable also support initialization using a  numpy data
+    self.D_W2 = pd.Varialble(np.random.rand(128, 1))
+    self.D_b2 = pd.Variable(np.zeros(128))
+    self.theta_D = [D_W1, D_b1, D_W2, D_b2]
+
+    # define parameters of generators
+    self.G_W1 = pd.Variable(shape=[784, 128], data=pd.gaussian_normal_randomizer())
+    self.G_b1 = pd.Variable(np.zeros(128)) # variable also support initialization using a  numpy data
+    self.G_W2 = pd.Varialble(np.random.rand(128, 1))
+    self.G_b2 = pd.Variable(np.zeros(128))
+    self.theta_G = [D_W1, D_b1, D_W2, D_b2]
+    
+    self.build_model()
+
+### Class member function: Generator Net
+def generator(self, z, y = None):
+
+    # Generator Net
+    if not self.y_dim:
+      z = pd.concat(1, [z, y])
+      
+    G_h0 = pd.fc(z, self.G_w0, self.G_b0)
+    G_h0_bn = pd.batch_norm(G_h0)
+    G_h0_relu = pd.relu(G_h0_bn)
+    
+    G_h1 = pd.fc(G_h0_relu, self.G_w1, self.G_b1)
+    G_h1_bn = pd.batch_norm(G_h1)
+    G_h1_relu = pd.relu(G_h1_bn)
+    
+    G_h2 = pd.deconv(G_h1_relu, self.G_W2, self.G_b2))
+    G_im = pd.tanh(G_im)
+    return G_im
+    
+### Class member function: Discriminator Net
+def discriminator(self, image):
+
+    # Discriminator Net
+    D_h0 = pd.conv2d(image, self.D_w0, self.D_b0)
+    D_h0_bn = pd.batchnorm(h0)
+    D_h0_relu = pd.lrelu(h0_bn)
+    
+    D_h1 = pd.conv2d(D_h0_relu, self.D_w1, self.D_b1)
+    D_h1_bn = pd.batchnorm(D_h1)
+    D_h1_relu = pd.lrelu(D_h1_bn)
+    
+    D_h2 = pd.fc(D_h1_relu, self.D_w2, self.D_b2)
+    return D_h2
+
+### Class member function: Build the model
+def build_model(self):
+
+    # input data
+    if self.y_dim:
+        self.y = pd.data(pd.float32, [self.batch_size, self.y_dim])
+    self.images = pd.data(pd.float32, [self.batch_size, self.im_size, self.im_size])
+    self.faked_images = pd.data(pd.float32, [self.batch_size, self.im_size, self.im_size])
+    self.z = pd.data(tf.float32, [None, self.z_size])
+    
+    # if conditional GAN
+    if self.y_dim:
+      self.G = self.generator(self.z, self.y)
+      self.D_t = self.discriminator(self.images)
+      # generated fake images
+      self.sampled = self.sampler(self.z, self.y)
+      self.D_f = self.discriminator(self.images)
+    else: # original version of GAN
+      self.G = self.generator(self.z)
+      self.D_t = self.discriminator(self.images)
+      # generate fake images
+      self.sampled = self.sampler(self.z)
+      self.D_f = self.discriminator(self.images)
+    
+    self.d_loss_real = pd.reduce_mean(pd.cross_entropy(self.D_t, np.ones(self.batch_size))
+    self.d_loss_fake = pd.reduce_mean(pd.cross_entropy(self.D_f, np.zeros(self.batch_size))
+    self.d_loss = self.d_loss_real + self.d_loss_fake
+    
+    self.g_loss = pd.reduce_mean(pd.cross_entropy(self.D_f, np.ones(self.batch_szie))
+
+# Main function for the demo:
+if __name__ == "__main__":
+
+    # dcgan
+    dcgan = DCGAN()
+    dcgan.build_model()
+
+    # load mnist data
+    data_X, data_y = self.load_mnist()
+    
+    # Two subgraphs required!!!
+    d_optim = pd.train.Adam(lr = .001, beta= .1).minimize(self.d_loss)
+    g_optim = pd.train.Adam(lr = .001, beta= .1).minimize(self.g_loss)
+
+    # executor
+    sess = pd.executor()
+    
+    # training
+    for epoch in xrange(10000):
+      for batch_id in range(N / batch_size):
+        idx = ...
+        # sample a batch
+        batch_im, batch_label = data_X[idx:idx+batch_size], data_y[idx:idx+batch_size]
+        # sample z
+        batch_z = np.random.uniform(-1., 1., [batch_size, z_dim])
+
+        if batch_id % 2 == 0:
+          sess.run(d_optim, 
+                   feed_dict = {dcgan.images: batch_im,
+                                dcgan.y: batch_label,
+                                dcgan.z: batch_z})
+        else:
+          sess.run(g_optim,
+                   feed_dict = {dcgan.z: batch_z})