# How to use a loss function that is not differentiable?

I am trying to find a codebook at the output of a fully connected neural network which chooses points such that the minimum distance (Euclidean norm) between the so produced codebook is maximized. The input to the neural network is the points that need to be mapped into higher dimension of the output space.

For instance, if the input dimension is 2 and output dimension is 3, the following mapping (and any permutations) works best: 00 - 000, 01 - 011, 10 - 101, 11 - 110

import tensorflow as tf
import numpy as np
import itertools

input_bits = tf.placeholder(dtype=tf.float32, shape=[None, 2], name='input_bits')
code_out = tf.placeholder(dtype=tf.float32, shape=[None, 3], name='code_out')
np.random.seed(1331)

def find_code(message):
weight1 = np.random.normal(loc=0.0, scale=0.01, size=[2, 3])
init1 = tf.constant_initializer(weight1)
out = tf.layers.dense(inputs=message, units=3, activation=tf.nn.sigmoid, kernel_initializer=init1)
return out

code = find_code(input_bits)

distances = []
for i in range(0, 3):
for j in range(i+1, 3):
distances.append(tf.linalg.norm(code_out[i]-code_out[j]))
min_dist = tf.reduce_min(distances)
# avg_dist = tf.reduce_mean(distances)

loss = -min_dist

init_variables = tf.global_variables_initializer()
sess = tf.Session()
sess.run(init_variables)

saver = tf.train.Saver()

count = int(1e4)

for i in range(count):
input_bit = [list(k) for k in itertools.product([0, 1], repeat=2)]
code_preview = sess.run(code, feed_dict={input_bits: input_bit})
sess.run(opt, feed_dict={input_bits: input_bit, code_out: code_preview})


Since the loss function itself is not differentiable, I am getting the error

ValueError: No gradients provided for any variable, check your graph for ops that do not support gradients, between variables


Am I doing something silly or is there a way to circumvent this? Any help in this regard is appreciated. Thanks in advance.

UPDATE:

import tensorflow as tf
import numpy as np
import itertools
from random import shuffle

input_bits = tf.placeholder(dtype=tf.float32, shape=[None, 2], name='input_bits')
learning_rate_val = tf.placeholder(dtype=tf.float32, shape=(), name='learning_rate')

def find_code(message):
weight1 = np.random.normal(loc=0.0, scale=0.01, size=[2, 3])
init1 = tf.constant_initializer(weight1)
out1 = tf.layers.dense(inputs=message, units=3, activation=tf.nn.sigmoid, kernel_initializer=init1)
return out1

code = find_code(input_bits)

distances = []
for i in range(0, 4):
for j in range(i+1, 4):
distances.append(tf.linalg.norm(code[i]-code[j]))
min_dist = tf.reduce_min(distances)
# avg_dist = tf.reduce_mean(distances)

loss = - min_dist

init_variables = tf.global_variables_initializer()
sess = tf.Session()
sess.run(init_variables)

saver = tf.train.Saver()

count = int(9e5)
threshold = 0.5

for i in range(count):
input_bit = [list(k) for k in itertools.product([0, 1], repeat=2)]
shuffle(input_bit)
input_bit = 2 * np.array(input_bit) - 1
code_preview = sess.run(code, feed_dict={input_bits: input_bit})
sess.run(opt, feed_dict={input_bits: input_bit, learning_rate_val: initial_learning_rate})
train_loss_track.append(sess.run(loss, feed_dict={input_bits: input_bit}))
if i % 5000 == 0 or i == 0 or i == count - 1:
input_bit = [list(k) for k in itertools.product([0, 1], repeat=2)]
input_bit = 2 * np.array(input_bit) - 1
output, train_loss = sess.run([code, loss], feed_dict={input_bits: input_bit,
learning_rate_val: initial_learning_rate})
print("\nEpoch: " + str(i))
print("Code: " + str(output))
output[output > threshold] = 1
output[output <= threshold] = 0
print("Code: " + str(output))
print("Loss: " + str(train_loss) + "\n")



This seems to work fine. But the final output is

Code: [[9.9976158e-01 0.0000000e+00 1.0000000e+00]
[4.9997061e-01 0.0000000e+00 0.0000000e+00]
[5.0000829e-01 1.0000000e+00 1.0000000e+00]
[2.3837961e-04 1.0000000e+00 4.6849247e-11]]
Code: [[1. 0. 1.]
[0. 0. 0.]
[1. 1. 1.]
[0. 1. 0.]]
Loss: -1.1179142


Although it is close to the expected output, it gets stuck here. Is there anyway to reach the expected output?

• zachdj, thanks for your response. I have updated the question. Also tf.reduce_min is differentiable. The problem was with the code_out variable I was using. However I am partially successful. Can you please go through the updated question? Thanks in advance – learner Aug 20 '19 at 13:56