Keras ValueError: Shapes (32, 2) and (32, 4) are incompatible

Question

I just posted about another problem with the same code, but progress is extremely slow due to the fact that I know very little about what I'm doing. The link to the previous problem is here: Keras ValueError: No gradients provided for any variable

I'm currently trying to get my model to run in order to classify 5000 different events which are 2D numpy arrays of 29x29 values

I define my NN like so:

inputs = keras.Input(shape=(29,29,1))

x=inputs

x = keras.layers.Conv2D(16, kernel_size=(3,3), name='Conv_1')(x)
x = keras.layers.LeakyReLU(0.1)(x)      
x = keras.layers.MaxPool2D((2,2), name='MaxPool_1')(x)

x = keras.layers.Conv2D(16, kernel_size=(3,3), name='Conv_2')(x)
x = keras.layers.LeakyReLU(0.1)(x)
x = keras.layers.MaxPool2D((2,2), name='MaxPool_2')(x)

x = keras.layers.Conv2D(32, kernel_size=(3,3), name='Conv_3')(x)
x = keras.layers.LeakyReLU(0.1)(x)
x = keras.layers.MaxPool2D((2,2), name='MaxPool_3')(x)
x = keras.layers.Flatten(name='Flatten')(x)

x = keras.layers.Dense(64, name='Dense_1')(x)
x = keras.layers.ReLU(name='ReLU_dense_1')(x)
x = keras.layers.Dense(64, name='Dense_2')(x)
x = keras.layers.ReLU(name='ReLU_dense_2')(x)

outputs = keras.layers.Dense(4, activation='softmax', name='Output')(x)

model = keras.Model(inputs=inputs, outputs=outputs, name='VGGlike_CNN')
model.summary()

keras.utils.plot_model(model, show_shapes=True)

OPTIMIZER = tf.keras.optimizers.Adam(learning_rate=LR_ST)

model.compile(optimizer=OPTIMIZER,
              loss='categorical_crossentropy',
              metrics=['accuracy'],
              run_eagerly=False)

def lr_decay(epoch):
  if epoch < 10:
    return LR_ST
  else:
    return LR_ST * tf.math.exp(0.2 * (10 - epoch))

lr_scheduler = keras.callbacks.LearningRateScheduler(lr_decay)


model_checkpoint = keras.callbacks.ModelCheckpoint(
        filepath='mycnn_best',
        monitor='val_accuracy',
        save_weights_only=True, 
        save_best_only=True,
        save_freq='epoch')

callbacks = [ lr_scheduler, model_checkpoint ]    

print('X_train.shape = ',X_train.shape)

history = model.fit(X_train, Y_train epochs=50,
                    validation_data=X_test, shuffle=True, verbose=1,
                    callbacks=callbacks)

It now gives me the error: ValueError: Shapes (32, 2) and (32, 4) are incompatible.

I want to classify each of the events has having 1,2,3 or 4 clusters, but before working on something complex, I'm using events which I know only have 1 cluster, so the label for each event is 1.

All of this gives me the idea that the problem is to do with my output being 4 neurons, but I really don't know if that's true, nor do I know how to go about debugging the code.

If anyone could help me I'd be really grateful.

Answer 1

The issue comes from the difference between the shape of your labels and the output shape of your model. Since you are using categorical_crossentropy and there are 4 units for your output layer, your model expects labels in one hot encoded form and as a vector of length 4. However, your labels are vectors of length 2. Therefore, if your labels are integers, you can do

Y_train = tf.one_hot(Y_train, 4)

and the resulting shape will be (5000, 4).