From the Udacity’s deep learning class, the softmax of y_i is simply the exponential divided by the sum of exponential of the whole Y vector:

Where S(y_i) is the softmax function of y_i and e is the exponential and j is the no. of columns in the input vector Y.

I’ve tried the following:

import numpy as np

def softmax(x):
    """Compute softmax values for each sets of scores in x."""
    e_x = np.exp(x - np.max(x))
    return e_x / e_x.sum()

scores = [3.0, 1.0, 0.2]
print(softmax(scores))

which returns:

[ 0.8360188   0.11314284  0.05083836]

But the suggested solution was:

def softmax(x):
    """Compute softmax values for each sets of scores in x."""
    return np.exp(x) / np.sum(np.exp(x), axis=0)

which produces the same output as the first implementation, even though the first implementation explicitly takes the difference of each column and the max and then divides by the sum.

Can someone show mathematically why? Is one correct and the other one wrong?

Are the implementation similar in terms of code and time complexity? Which is more efficient?

They’re both correct, but yours is preferred from the point of view of numerical stability.

You start with

e ^ (x - max(x)) / sum(e^(x - max(x))

By using the fact that a^(b – c) = (a^b)/(a^c) we have

= e ^ x / (e ^ max(x) * sum(e ^ x / e ^ max(x)))

= e ^ x / sum(e ^ x)

Which is what the other answer says. You could replace max(x) with any variable and it would cancel out.

(Well… much confusion here, both in the question and in the answers…)

To start with, the two solutions (i.e. yours and the suggested one) are not equivalent; they happen to be equivalent only for the special case of 1-D score arrays. You would have discovered it if you had tried also the 2-D score array in the Udacity quiz provided example.

Results-wise, the only actual difference between the two solutions is the axis=0 argument. To see that this is the case, let’s try your solution (your_softmax) and one where the only difference is the axis argument:

import numpy as np

# your solution:
def your_softmax(x):
    """Compute softmax values for each sets of scores in x."""
    e_x = np.exp(x - np.max(x))
    return e_x / e_x.sum()

# correct solution:
def softmax(x):
    """Compute softmax values for each sets of scores in x."""
    e_x = np.exp(x - np.max(x))
    return e_x / e_x.sum(axis=0) # only difference

As I said, for a 1-D score array, the results are indeed identical:

scores = [3.0, 1.0, 0.2]
print(your_softmax(scores))
# [ 0.8360188   0.11314284  0.05083836]
print(softmax(scores))
# [ 0.8360188   0.11314284  0.05083836]
your_softmax(scores) == softmax(scores)
# array([ True,  True,  True], dtype=bool)

Nevertheless, here are the results for the 2-D score array given in the Udacity quiz as a test example:

scores2D = np.array([[1, 2, 3, 6],
                     [2, 4, 5, 6],
                     [3, 8, 7, 6]])

print(your_softmax(scores2D))
# [[  4.89907947e-04   1.33170787e-03   3.61995731e-03   7.27087861e-02]
#  [  1.33170787e-03   9.84006416e-03   2.67480676e-02   7.27087861e-02]
#  [  3.61995731e-03   5.37249300e-01   1.97642972e-01   7.27087861e-02]]

print(softmax(scores2D))
# [[ 0.09003057  0.00242826  0.01587624  0.33333333]
#  [ 0.24472847  0.01794253  0.11731043  0.33333333]
#  [ 0.66524096  0.97962921  0.86681333  0.33333333]]

The results are different – the second one is indeed identical with the one expected in the Udacity quiz, where all columns indeed sum to 1, which is not the case with the first (wrong) result.

So, all the fuss was actually for an implementation detail – the axis argument. According to the numpy.sum documentation:

The default, axis=None, will sum all of the elements of the input array

while here we want to sum row-wise, hence axis=0. For a 1-D array, the sum of the (only) row and the sum of all the elements happen to be identical, hence your identical results in that case…

The axis issue aside, your implementation (i.e. your choice to subtract the max first) is actually better than the suggested solution! In fact, it is the recommended way of implementing the softmax function – see here for the justification (numeric stability, also pointed out by some other answers here).

So, this is really a comment to desertnaut’s answer but I can’t comment on it yet due to my reputation. As he pointed out, your version is only correct if your input consists of a single sample. If your input consists of several samples, it is wrong. However, desertnaut’s solution is also wrong. The problem is that once he takes a 1-dimensional input and then he takes a 2-dimensional input. Let me show this to you.

import numpy as np

# your solution:
def your_softmax(x):
    """Compute softmax values for each sets of scores in x."""
    e_x = np.exp(x - np.max(x))
    return e_x / e_x.sum()

# desertnaut solution (copied from his answer): 
def desertnaut_softmax(x):
    """Compute softmax values for each sets of scores in x."""
    e_x = np.exp(x - np.max(x))
    return e_x / e_x.sum(axis=0) # only difference

# my (correct) solution:
def softmax(z):
    assert len(z.shape) == 2
    s = np.max(z, axis=1)
    s = s[:, np.newaxis] # necessary step to do broadcasting
    e_x = np.exp(z - s)
    div = np.sum(e_x, axis=1)
    div = div[:, np.newaxis] # dito
    return e_x / div

Lets take desertnauts example:

x1 = np.array([[1, 2, 3, 6]]) # notice that we put the data into 2 dimensions(!)

This is the output:

your_softmax(x1)
array([[ 0.00626879,  0.01704033,  0.04632042,  0.93037047]])

desertnaut_softmax(x1)
array([[ 1.,  1.,  1.,  1.]])

softmax(x1)
array([[ 0.00626879,  0.01704033,  0.04632042,  0.93037047]])

You can see that desernauts version would fail in this situation. (It would not if the input was just one dimensional like np.array([1, 2, 3, 6]).

Lets now use 3 samples since thats the reason why we use a 2 dimensional input. The following x2 is not the same as the one from desernauts example.

x2 = np.array([[1, 2, 3, 6],  # sample 1
               [2, 4, 5, 6],  # sample 2
               [1, 2, 3, 6]]) # sample 1 again(!)

This input consists of a batch with 3 samples. But sample one and three are essentially the same. We now expect 3 rows of softmax activations where the first should be the same as the third and also the same as our activation of x1!

your_softmax(x2)
array([[ 0.00183535,  0.00498899,  0.01356148,  0.27238963],
       [ 0.00498899,  0.03686393,  0.10020655,  0.27238963],
       [ 0.00183535,  0.00498899,  0.01356148,  0.27238963]])


desertnaut_softmax(x2)
array([[ 0.21194156,  0.10650698,  0.10650698,  0.33333333],
       [ 0.57611688,  0.78698604,  0.78698604,  0.33333333],
       [ 0.21194156,  0.10650698,  0.10650698,  0.33333333]])

softmax(x2)
array([[ 0.00626879,  0.01704033,  0.04632042,  0.93037047],
       [ 0.01203764,  0.08894682,  0.24178252,  0.65723302],
       [ 0.00626879,  0.01704033,  0.04632042,  0.93037047]])

I hope you can see that this is only the case with my solution.

softmax(x1) == softmax(x2)[0]
array([[ True,  True,  True,  True]], dtype=bool)

softmax(x1) == softmax(x2)[2]
array([[ True,  True,  True,  True]], dtype=bool)

Additionally, here is the results of TensorFlows softmax implementation:

import tensorflow as tf
import numpy as np
batch = np.asarray([[1,2,3,6],[2,4,5,6],[1,2,3,6]])
x = tf.placeholder(tf.float32, shape=[None, 4])
y = tf.nn.softmax(x)
init = tf.initialize_all_variables()
sess = tf.Session()
sess.run(y, feed_dict={x: batch})

And the result:

array([[ 0.00626879,  0.01704033,  0.04632042,  0.93037045],
       [ 0.01203764,  0.08894681,  0.24178252,  0.657233  ],
       [ 0.00626879,  0.01704033,  0.04632042,  0.93037045]], dtype=float32)

I would say that while both are correct mathematically, implementation-wise, first one is better. When computing softmax, the intermediate values may become very large. Dividing two large numbers can be numerically unstable. These notes (from Stanford) mention a normalization trick which is essentially what you are doing.

sklearn also offers implementation of softmax

from sklearn.utils.extmath import softmax
import numpy as np

x = np.array([[ 0.50839931,  0.49767588,  0.51260159]])
softmax(x)

# output
array([[ 0.3340521 ,  0.33048906,  0.33545884]]) 

From mathematical point of view both sides are equal.

And you can easily prove this. Let’s m=max(x). Now your function softmax returns a vector, whose i-th coordinate is equal to

notice that this works for any m, because for all (even complex) numbers e^m != 0

• from computational complexity point of view they are also equivalent and both run in O(n) time, where n is the size of a vector.

• from numerical stability point of view, the first solution is preferred, because e^x grows very fast and even for pretty small values of x it will overflow. Subtracting the maximum value allows to get rid of this overflow. To practically experience the stuff I was talking about try to feed x = np.array([1000, 5]) into both of your functions. One will return correct probability, the second will overflow with nan

• your solution works only for vectors (Udacity quiz wants you to calculate it for matrices as well). In order to fix it you need to use sum(axis=0)

EDIT. As of version 1.2.0, scipy includes softmax as a special function:

https://scipy.github.io/devdocs/generated/scipy.special.softmax.html

I wrote a function applying the softmax over any axis:

def softmax(X, theta = 1.0, axis = None):
    """
    Compute the softmax of each element along an axis of X.

    Parameters
    ----------
    X: ND-Array. Probably should be floats. 
    theta (optional): float parameter, used as a multiplier
        prior to exponentiation. Default = 1.0
    axis (optional): axis to compute values along. Default is the 
        first non-singleton axis.

    Returns an array the same size as X. The result will sum to 1
    along the specified axis.
    """

    # make X at least 2d
    y = np.atleast_2d(X)

    # find axis
    if axis is None:
        axis = next(j[0] for j in enumerate(y.shape) if j[1] > 1)

    # multiply y against the theta parameter, 
    y = y * float(theta)

    # subtract the max for numerical stability
    y = y - np.expand_dims(np.max(y, axis = axis), axis)

    # exponentiate y
    y = np.exp(y)

    # take the sum along the specified axis
    ax_sum = np.expand_dims(np.sum(y, axis = axis), axis)

    # finally: divide elementwise
    p = y / ax_sum

    # flatten if X was 1D
    if len(X.shape) == 1: p = p.flatten()

    return p

Subtracting the max, as other users described, is good practice. I wrote a detailed post about it here.

Here you can find out why they used - max.

From there:

“When you’re writing code for computing the Softmax function in practice, the intermediate terms may be very large due to the exponentials. Dividing large numbers can be numerically unstable, so it is important to use a normalization trick.”

A more concise version is:

def softmax(x):
    return np.exp(x) / np.exp(x).sum(axis=0)

To offer an alternative solution, consider the cases where your arguments are extremely large in magnitude such that exp(x) would underflow (in the negative case) or overflow (in the positive case). Here you want to remain in log space as long as possible, exponentiating only at the end where you can trust the result will be well-behaved.

import scipy.special as sc
import numpy as np

def softmax(x: np.ndarray) -> np.ndarray:
    return np.exp(x - sc.logsumexp(x))

I needed something compatible with the output of a dense layer from Tensorflow.

The solution from @desertnaut does not work in this case because I have batches of data. Therefore, I came with another solution that should work in both cases:

def softmax(x, axis=-1):
    e_x = np.exp(x - np.max(x)) # same code
    return e_x / e_x.sum(axis=axis, keepdims=True)

Results:

logits = np.asarray([
    [-0.0052024,  -0.00770216,  0.01360943, -0.008921], # 1
    [-0.0052024,  -0.00770216,  0.01360943, -0.008921]  # 2
])

print(softmax(logits))

#[[0.2492037  0.24858153 0.25393605 0.24827873]
# [0.2492037  0.24858153 0.25393605 0.24827873]]

Ref: Tensorflow softmax

I would suggest this:

def softmax(z):
    z_norm=np.exp(z-np.max(z,axis=0,keepdims=True))
    return(np.divide(z_norm,np.sum(z_norm,axis=0,keepdims=True)))

It will work for stochastic as well as the batch.
For more detail see : https://medium.com/@ravish1729/analysis-of-softmax-function-ad058d6a564d

In order to maintain for numerical stability, max(x) should be subtracted. The following is the code for softmax function;

def softmax(x):

if len(x.shape) > 1:
    tmp = np.max(x, axis = 1)
    x -= tmp.reshape((x.shape[0], 1))
    x = np.exp(x)
    tmp = np.sum(x, axis = 1)
    x /= tmp.reshape((x.shape[0], 1))
else:
    tmp = np.max(x)
    x -= tmp
    x = np.exp(x)
    tmp = np.sum(x)
    x /= tmp


return x

Already answered in much detail in above answers. max is subtracted to avoid overflow. I am adding here one more implementation in python3.

import numpy as np
def softmax(x):
    mx = np.amax(x,axis=1,keepdims = True)
    x_exp = np.exp(x - mx)
    x_sum = np.sum(x_exp, axis = 1, keepdims = True)
    res = x_exp / x_sum
    return res

x = np.array([[3,2,4],[4,5,6]])
print(softmax(x))

Everybody seems to post their solution so I’ll post mine:

def softmax(x):
    e_x = np.exp(x.T - np.max(x, axis = -1))
    return (e_x / e_x.sum(axis=0)).T

I get the exact same results as the imported from sklearn:

from sklearn.utils.extmath import softmax

import tensorflow as tf
import numpy as np

def softmax(x):
    return (np.exp(x).T / np.exp(x).sum(axis=-1)).T

logits = np.array([[1, 2, 3], [3, 10, 1], [1, 2, 5], [4, 6.5, 1.2], [3, 6, 1]])

sess = tf.Session()
print(softmax(logits))
print(sess.run(tf.nn.softmax(logits)))
sess.close()

Based on all the responses and CS231n notes, allow me to summarise:

def softmax(x, axis):
    x -= np.max(x, axis=axis, keepdims=True)
    return np.exp(x) / np.exp(x).sum(axis=axis, keepdims=True)

Usage:

x = np.array([[1, 0, 2,-1],
              [2, 4, 6, 8], 
              [3, 2, 1, 0]])
softmax(x, axis=1).round(2)

Output:

array([[0.24, 0.09, 0.64, 0.03],
       [0.  , 0.02, 0.12, 0.86],
       [0.64, 0.24, 0.09, 0.03]])

I would like to supplement a little bit more understanding of the problem. Here it is correct of subtracting max of the array. But if you run the code in the other post, you would find it is not giving you right answer when the array is 2D or higher dimensions.

Here I give you some suggestions:

1. To get max, try to do it along x-axis, you will get an 1D array.
2. Reshape your max array to original shape.
3. Do np.exp get exponential value.
4. Do np.sum along axis.
5. Get the final results.

Follow the result you will get the correct answer by doing vectorization. Since it is related to the college homework, I cannot post the exact code here, but I would like to give more suggestions if you don’t understand.

The purpose of the softmax function is to preserve the ratio of the vectors as opposed to squashing the end-points with a sigmoid as the values saturate (i.e. tend to +/- 1 (tanh) or from 0 to 1 (logistical)). This is because it preserves more information about the rate of change at the end-points and thus is more applicable to neural nets with 1-of-N Output Encoding (i.e. if we squashed the end-points it would be harder to differentiate the 1-of-N output class because we can’t tell which one is the “biggest” or “smallest” because they got squished.); also it makes the total output sum to 1, and the clear winner will be closer to 1 while other numbers that are close to each other will sum to 1/p, where p is the number of output neurons with similar values.

The purpose of subtracting the max value from the vector is that when you do e^y exponents you may get very high value that clips the float at the max value leading to a tie, which is not the case in this example. This becomes a BIG problem if you subtract the max value to make a negative number, then you have a negative exponent that rapidly shrinks the values altering the ratio, which is what occurred in poster’s question and yielded the incorrect answer.

The answer supplied by Udacity is HORRIBLY inefficient. The first thing we need to do is calculate e^y_j for all vector components, KEEP THOSE VALUES, then sum them up, and divide. Where Udacity messed up is they calculate e^y_j TWICE!!! Here is the correct answer:

def softmax(y):
    e_to_the_y_j = np.exp(y)
    return e_to_the_y_j / np.sum(e_to_the_y_j, axis=0)

Goal was to achieve similar results using Numpy and Tensorflow. The only change from original answer is axis parameter for np.sum api.

Initial approach : axis=0 – This however does not provide intended results when dimensions are N.

Modified approach: axis=len(e_x.shape)-1 – Always sum on the last dimension. This provides similar results as tensorflow’s softmax function.

def softmax_fn(input_array):
    """
    | **@author**: Prathyush SP
    |
    | Calculate Softmax for a given array
    :param input_array: Input Array
    :return: Softmax Score
    """
    e_x = np.exp(input_array - np.max(input_array))
    return e_x / e_x.sum(axis=len(e_x.shape)-1)