layers.py

import numpy as np

def affine_forward(x, w, b):
  """
  Computes the forward pass for an affine (fully-connected) layer.

  The input x has shape (N, d_1, ..., d_k) where x[i] is the ith input.
  We multiply this against a weight matrix of shape (D, M) where
  D = \prod_i d_i

  Inputs:
  x - Input data, of shape (N, d_1, ..., d_k)
  w - Weights, of shape (D, M)
  b - Biases, of shape (M,)
  
  Returns a tuple of:
  - out: output, of shape (N, M)
  - cache: (x, w, b)
  """
  out = None
  #############################################################################
  # TODO: Implement the affine forward pass. Store the result in out. You     #
  #might need to reshape the input into rows.                                 #
  #############################################################################

  out = ((np.dot(w.T,x.T)).T)
  out = out+b

  #############################################################################
  #                             END OF YOUR CODE                              #
  #############################################################################
  cache = (x, w, b)
  return out, cache


def affine_backward(dout, cache):
  """
  Computes the backward pass for an affine layer.

  Inputs:
  - dout: Upstream derivative, of shape (N, M)
  - cache: Tuple of:
    - x: Input data, of shape (N, d_1, ... d_k)
    - w: Weights, of shape (D, M)

  Returns a tuple of:
  - dx: Gradient with respect to x, of shape (N, d1, ..., d_k)
  - dw: Gradient with respect to w, of shape (D, M)
  - db: Gradient with respect to b, of shape (M,)
  """
  x, w, b = cache
  dx, dw, db = None, None, None
  #############################################################################
  # TODO: Implement the affine backward pass.                                 #
  #############################################################################

  dx = np.dot(dout,w.T);
  dw =  np.dot(x.T, dout) #(np.dot(dout.T,x)).T;
  db = np.sum(dout, axis=0)
  

  #print "dxShape: ",dx.shape
  #print "dwShape: ",dw.shape
  #print "dbShape: ",db.shape

  #############################################################################
  #                             END OF YOUR CODE                              #
  #############################################################################
  return dx, dw, db

def relu_forward(x):
  """
  Computes the forward pass for a layer of rectified linear units (ReLUs).

  Input:
  - x: Inputs, of any shape

  Returns a tuple of:
  - out: Output, of the same shape as x
  - cache: x
  """
  out = None
  #############################################################################
  # TODO: Implement the ReLU forward pass.                                    #
  #############################################################################

  out = np.maximum(0,x)

  #############################################################################
  #                             END OF YOUR CODE                              #
  #############################################################################
  cache = x
  return out, cache


def relu_backward(dout, cache):
  """
  Computes the backward pass for a layer of rectified linear units (ReLUs).

  Input:
  - dout: Upstream derivatives, of any shape
  - cache: Input x, of same shape as dout

  Returns:
  - dx: Gradient with respect to x
  """
  dx, x = None, cache
  
  
  #############################################################################
  # TODO: Implement the ReLU backward pass.                                   #
  #############################################################################

  #x[x<0]=0 
  #x[x>0]=1
   
  dout[x <= 0] = 0
  dx = dout
    
  #print dx.shape
  #############################################################################
  #                             END OF YOUR CODE                              #
  #############################################################################
  return dx


def svm_loss(x, y):
  """
  Computes the loss and gradient using for multiclass SVM classification.

  Inputs:
  - x: Input data, of shape (N, C) where x[i, j] is the score for the jth class
    for the ith input.
  - y: Vector of labels, of shape (N,) where y[i] is the label for x[i] and
    0 <= y[i] < C

  Returns a tuple of:
  - loss: Scalar giving the loss
  - dx: Gradient of the loss with respect to x
  """
  N = x.shape[0]
  correct_class_scores = x[np.arange(N), y]
  margins = np.maximum(0, x - correct_class_scores[:, np.newaxis] + 1.0)
  margins[np.arange(N), y] = 0
  loss = np.sum(margins) / N
  num_pos = np.sum(margins > 0, axis=1)
  dx = np.zeros_like(x)
  dx[margins > 0] = 1
  dx[np.arange(N), y] -= num_pos
  dx /= N
  return loss, dx


def softmax_loss(x, y):
  """
  Computes the loss and gradient for softmax classification.

  Inputs:
  - x: Input data, of shape (N, C) where x[i, j] is the score for the jth class
    for the ith input.
  - y: Vector of labels, of shape (N,) where y[i] is the label for x[i] and
    0 <= y[i] < C

  Returns a tuple of:
  - loss: Scalar giving the loss
  - dx: Gradient of the loss with respect to x
  """
  probs = np.exp(x - np.max(x, axis=1, keepdims=True))
  probs /= np.sum(probs, axis=1, keepdims=True)
  N = x.shape[0]
  loss = -np.sum(np.log(probs[np.arange(N), y])) / N

  dx = probs.copy()
  dx[np.arange(N), y] -= 1
  dx /= N
  return loss, dx

def softmax_loss_for_scores(x, y):
  """
  Computes the loss and gradient for softmax classification.

  Inputs:
  - x: Input data, of shape (N, C) where x[i, j] is the score for the jth class
    for the ith input.
  - y: Vector of labels, of shape (N,) where y[i] is the label for x[i] and
    0 <= y[i] < C

  Returns a tuple of:
  - loss: Scalar giving the loss
  - dx: Gradient of the loss with respect to x
  """
  probs = np.exp(x - np.max(x, axis=1, keepdims=True))
  probs /= np.sum(probs, axis=1, keepdims=True)
  N = x.shape[0]
  loss = -np.sum(np.log(probs[np.arange(N), y])) / N
  dx = probs.copy()
  dx[np.arange(N), y] -= 1
  dx /= N
  return loss, dx, probs