# Numerically Stable Softmax and Cross Entropy

December 15, 2022

In this post, we'll take a look at softmax and cross entropy loss. We'll see that naive implementations result in numerically instability, and then we'll derive implementations that are numerically stable.

## Symbols

• $$x$$: Input vector of dimensionality $$d$$.
• $$y$$: Correct class, an integer on the range $$y \in [1\ldots K]$$.
• $$\hat{y}$$: Raw outputs (i.e. logits) of our neural network, vector of dimensionality $$K$$.
• We use $$\log$$ to denote the natural logarithm.

## Softmax

The softmax function is defined as:
$\text{softmax}(x)_i = \frac{e^{x_i}}{\sum_j e^{x_j}}$

In code:

def softmax(x):
# assumes x is a vector
return np.exp(x) / np.sum(np.exp(x))

x = np.array([1.2, 2, -4, 0.0])
softmax(x)
# outputs: [0.28310553, 0.63006295, 0.00156177, 0.08526975]


For large inputs we start seeing some numerical instability:

x = np.array([1.2, 2000, -4000, 0.0])
softmax(x)
# outputs: [0., nan, 0.,  0.]


Floating point numbers aren't magic, they have limits:

np.finfo(np.float64).max
# 1.7976931348623157e+308, largest positive number

np.finfo(np.float64).tiny
# 2.2250738585072014e-308, smallest positive number at full precision

np.finfo(np.float64).smallest_subnormal
# 5e-324, smallest positive number


When we go beyond these limits, we start seeing funky behaviour:

np.finfo(np.float64).max * 2
# inf, overflow error

np.inf - np.inf
# nan, not a number error

np.finfo(np.float64).smallest_subnormal / 2
# 0.0, underflow error


Looking back at our softmax example that resulted in [0., nan, 0., 0.], we can see that the overflow of np.exp(2000) = np.inf is causing the nan, since we end up with np.inf / np.inf = nan. If we want to avoid nans, we need to avoid infs. To avoid infs, we need to avoid overflows. To avoid overflows, we need to prevent our numbers from growing too large.

Underflows on the other hand don't seem quite as detrimental. Worst case scenario, we get a result 0 and lose all precision (i.e. np.exp(-4000) = 0). While this is not ideal, this is a lot better than running into inf and nan.

Given the relative stability of floating point underflows vs overflows, how can we fix softmax?

\begin{align} \text{softmax}(x)_i &= \frac{e^{x_i}}{\sum_j e^{x_j}} \\ &= 1\cdot \frac{e^{x_i}}{\sum_j e^{x_j}} \\ &= \frac{C}{C}\frac{e^{x_i}}{\sum_j e^{x_j}} \\ &= \frac{Ce^{x_i}}{\sum_j Ce^{x_j}} \\ &= \frac{e^{x_i + \log C}}{\sum_j e^{x_j + \log C}} \\ \end{align}
Here, we're taking advantage of the rule $$a\cdot b^x = b^{x + \log_b a}$$. As a result, we are given the ability to offset our inputs by any constant we choose. So, if we want to prevent large numbers, we can simply set $$\log C = -\max(x)$$:
$\text{softmax}(x)_i = \frac{e^{x_i - \max(x)}}{\sum_j e^{x_j - \max(x)}}$

This version is numerically stable:

• All exponentiated values will be between 0 and 1: $$0 \leq e^{x_i - \max(x)} \leq 1$$
• This prevents overflow errors (but we are still prone to underflows)
• At least one of the exponentiated values is 1: $$e^{x_i - \max(x)} = e^{ \max(x)- \max(x)} = e^0 = 1$$
• i.e. at least one value is guarenteed not to underflow
• Our denominator will always be $$>= 1$$, preventing division by zero errors.
• We have at least one non-zero numerator, so softmax can't result in a zero vector

In code:

def softmax(x):
# assumes x is a vector
x = x - np.max(x)
return np.exp(x) / np.sum(np.exp(x))

x = np.array([1.2, 2, -4, 0])
softmax(x)
# outputs: [0.28310553, 0.63006295, 0.00156177, 0.08526975]

# works for large numbers!!!
x = np.array([1.2, 2, -4, 0]) * 1000
softmax(x)
# outputs: [0., 1., 0., 0.]


## Cross Entropy and Log Softmax

The cross entropy between two probability distributions is defined as.
$H(p, q) = -\sum_i p_i\log(q_i)$
where $$p$$ and $$q$$ are our probability distributions represented as probability vectors (that is $$p_i$$ and $$q_i$$are the probabilities of event $$i$$ occuring for $$p$$ and $$q$$ respectively).

Roughly speaking, cross entropy measures the similarity between two probability distributions, so in the context of machine learning, we use cross entropy as a loss function where:

• $$q$$ is our predicted probabilities vector (i.e. the softmax of our network outputs): $$q = \text{softmax}(\hat{y})$$
• $$p$$ is a one-hot encoded vector of our label, that is a probability vector that assigns 100% probability to the position $$y$$ (our label for the correct class): $$p_i = \begin{cases} 1 & i = y \\ 0 & i \neq y \end{cases}$$

In this setup, our cross entropy simplifies to:
\begin{align} H(p, q) &= -\sum_i p_i\log(q_i) \\ &= -p_y\cdot\log(q_y) -\sum_{i \neq y} p_i\log(q_i) \\ &= -1\cdot\log(q_y) -\sum_{i \neq y} 0\cdot\log(q_i) \\ &= -\log(q_y) - 0 \sum_{i \neq y} \log(q_i) \\ &= -\log(q_y) \\ &= -\log(\text{softmax}(\hat{y})_y) \end{align}

In code:

def cross_entropy(y_hat, y_true):
# assume y_hat is a vector and y_true is an integer
return -np.log(softmax(y_hat)[y_true])

cross_entropy(
y_hat=np.random.normal(size=(10)),
y_true=3,
)
# 2.580982279204241


For large numbers in y_hat, we start seeing inf:

cross_entropy(
y_hat = np.array([-1000, 1000]),
y_true = 0,
)
# inf


In this case softmax(y_hat) = [0, 1], with y_true = 0, we get -log(0) = inf. So to make log(softmax(x)) stable, we need to prevent log(0). Let's see if we can employ some math tricks to make this happen:
\begin{align} \log(\text{softmax}(x)_i) & = \log(\frac{e^{x_i - \max(x)}}{\sum_j e^{x_j - \max(x)}}) \\ &= \log(e^{x_i - \max(x)}) - \log(\sum_j e^{x_j - \max(x)}) \\ &= (x_i - \max(x))\log(e) - \log(\sum_j e^{x_j - \max(x)}) \\ &= (x_i - \max(x))\cdot 1 - \log(\sum_j e^{x_j - \max(x)}) \\ &= x_i - \max(x) - \log(\sum_j e^{x_j - \max(x)}) \\ \end{align}
This new equation guarentees that the sum inside the log will always be $$\geq 1$$, so we no longer need to worry about log(0) errors.

In code:

def log_softmax(x):
# assumes x is a vector
x_max = np.max(x)
return x - x_max - np.log(np.sum(np.exp(x - x_max)))

def cross_entropy(y_hat, y_true):
return -log_softmax(y_hat)[y_true]

cross_entropy(
y_hat=np.random.normal(size=(10)),
y_true=3,
)
# 2.580982279204241

# works for large inputs!!!!
cross_entropy(
y_hat = np.array([-1000, 1000]),
y_true = 3,
)
# 705.3963550098291