cs231n作业1—

cs231n作业1——Softmax

参考文章：cs231n assignment1——softmax

Softmax

softmax其实和SVM差别不大，两者损失函数不同，softmax就是把各个类的得分转化成了概率。
损失函数：在这里插入图片描述

def softmax_loss_naive(W, X, y, reg):loss = 0.0dW = np.zeros_like(W)num_classes = W.shape[1]num_train = X.shape[0]for i in range(num_train):scores = X[i].dot(W)                # 矩阵点乘：第 i 张照片在各类别上的得分scores -= np.max(scores)            # 减去最大得分，减小计算量correct_class_score = scores[y[i]]  # 接下来三行是损失函数的计算exp_sum = np.sum(np.exp(scores))loss += -correct_class_score + np.log(exp_sum) # np.log()以e为底for j in range(num_classes):if j == y[i]:dW[:, y[i]] += (np.exp(scores[y[i]])/exp_sum-1)*X[i]else:dW[:, j] += np.exp(scores[j])/exp_sum*X[i]    loss /= num_train                      # 求平均损失loss += reg * np.sum(W * W)            # 损失加上正则化惩罚dW /= num_train                        # 求平均梯度dW += 2.0*reg*Wreturn loss, dW

用向量法实现 Softmax

def softmax_loss_vectorized(W, X, y, reg):loss = 0.0dW = np.zeros_like(W)num_classes = W.shape[1]num_train = X.shape[0]scores = X.dot(W)                                                  # N*C 的矩阵scores -= np.max(scores, axis=1, keepdims=True)                    # 减去每行（每张图片对于每一类）的最大值correct_class_score = scores[range(num_train),y]exp_sum = np.sum(np.exp(scores), axis=1, keepdims=True)            # 按行求和，并保持为二维（列向量）loss = -np.sum(correct_class_score) + np.sum(np.log(exp_sum))      # 损失函数公式并求和loss = loss/num_train + reg * np.sum(W * W)med = np.exp(scores)/exp_sum         # 对于j!=yi的情况，dw=np.exp(scores[j])/exp_sum*X[i]med[range(num_train),y] -= 1         # 对于j=yi的情况，dw=(np.exp(scores[j])/exp_sum-1)*X[i]dW = X.T.dot(med)                    # 最后同时乘以 X[i]dW /= num_traindW += 2.0*reg*Wreturn loss, dW