I think your implementation is correct and the answer provided is just wrong.
Just for reference, the below figure represents the theory / math we are using here to implement Logistic Regression with Gradient Descent:
Here, we have the learnable parameter vector $\theta = [b,\;a]^T$ and $m=1$ (since a singe data point), with $X=[1,\; x]$, where $1$ corresponds to the intercept (bias) term.
Just making your implementation a little modular and increasing the number of epochs to 10 (instead of 1):
def update_params(a, b, x, y, z, lr):
a = a + lr * x * (z-y)
a = np.round(a, decimals=3)
b = b + lr * (z-y)
b = np.round(b, decimals=3)
return a, b
def LogitRegression(arr):
x, y, a, b = arr
lr = 1.0
num_epochs = 10
#losses, preds = [], []
for _ in range(num_epochs):
z = 1.0 / (1.0 + np.exp(-a * x - b))
bce = -y*np.log(z) -(1-y)*np.log(1-z)
#losses.append(bce)
#preds.append(z)
print(bce, y, z, a, b)
a, b = update_params(a, b, x, y, z, lr)
return ", ".join([str(a), str(b)])
LogitRegression([1,1,1,1])
# 0.12692801104297263 1 0.8807970779778823 1 1
# 0.10135698320837692 1 0.9036104015620354 1.119 1.119 # values after 1 epoch
# 0.08437500133718023 1 0.9190865327845347 1.215 1.215
# 0.0721998635352405 1 0.9303449352007099 1.296 1.296
# 0.06305834631954188 1 0.9388886913913739 1.366 1.366
# 0.05601486564909184 1 0.9455250799418752 1.427 1.427
# 0.05042252914331105 1 0.9508275872468411 1.481 1.481
# 0.04582166273506799 1 0.9552122969502131 1.53 1.53
# 0.041959389233941616 1 0.958908721799535 1.575 1.575
# 0.03871910934525996 1 0.962020893877162 1.616 1.616
If you plot the BCE loss and the predicted y (i.e., z) over iterations, you get the following figure (as expected, BCE loss is monotonically decreasing and z is getting closer to ground truth y with increasing iterations, leading to convergence):
Now, if you change your update_params() to the following:
def update_params(a, b, x, y, z, lr):
a = a + lr * x * (z-y)
a = np.round(a, decimals=3)
b = b + lr * (z-y)
b = np.round(b, decimals=3)
return a, b
and call LogitRegression() with the same set of inputs:
LogitRegression([1,1,1,1])
# 0.12692801104297263 1 0.8807970779778823 1 1
# 0.15845663982299638 1 0.8534599691639768 0.881 0.881 # provided in the answer
# 0.2073277757451888 1 0.8127532055353431 0.734 0.734
# 0.28883714051459425 1 0.7491341990786479 0.547 0.547
# 0.4403300268044629 1 0.6438239068707556 0.296 0.296
# 0.7549461015956136 1 0.4700359482354282 -0.06 -0.06
# 1.4479476778575628 1 0.2350521962362353 -0.59 -0.59
# 2.774416770021533 1 0.0623858513799944 -1.355 -1.355
# 4.596141947283801 1 0.010090691161759239 -2.293 -2.293
# 6.56740642634977 1 0.0014054377957286094 -3.283 -3.283
and you will end up with the following figure if you plot (clearly this is wrong, since the loss function increases with every epoch and z goes further away from ground-truth y, leading to divergence):
Also, the above implementation can easily be extended to multi-dimensional data containing many data points like the following:
def VanillaLogisticRegression(x, y): # LR without regularization
m, n = x.shape
w = np.zeros((n+1, 1))
X = np.hstack((np.ones(m)[:,None],x)) # include the feature corresponding to the bias term
num_epochs = 1000 # number of epochs to run gradient descent, tune this hyperparametrer
lr = 0.5 # learning rate, tune this hyperparameter
losses = []
for _ in range(num_epochs):
y_hat = 1. / (1. + np.exp(-np.dot(X, w))) # predicted y by the LR model
J = np.mean(-y*np.log2(y_hat) - (1-y)*np.log2(1-y_hat)) # the binary cross entropy loss function
grad_J = np.mean((y_hat - y)*X, axis=0) # the gradient of the loss function
w -= lr * grad_J[:, None] # the gradient descent step, update the parameter vector w
losses.append(J)
# test corretness of the implementation
# loss J should monotonically decrease & y_hat should be closer to y, with increasing iterations
# print(J)
return w
m, n = 1000, 5 # 1000 rows, 5 columns
# randomly generate dataset, note that y can have values as 0 and 1 only
x, y = np.random.random(m*n).reshape(m,n), np.random.randint(0,2,m).reshape(-1,1)
w = VanillaLogisticRegression(x, y)
w # learnt parameters
# array([[-0.0749518 ],
# [ 0.28592107],
# [ 0.15202566],
# [-0.15020757],
# [ 0.08147078],
# [-0.18823631]])
If you plot the loss function value over iterations, you will get a plot like the following one, showing how it converges.
Finally, let's compare the above implementation with sklearn's implementation, which uses a more advanced optimization algorithm lbfgs by default, hence likely to converge much faster, but if our implementation is correct both of then should converge to the same global minima, since the loss function is convex (note that sklearn by default uses regularization, in order to have almost no regularization, we need to have the value of the input hyper-parameter $C$ very high):
from sklearn.linear_model import LogisticRegression
clf = LogisticRegression(random_state=0, C=10**12).fit(x, y)
print(clf.coef_, clf.intercept_)
# [[ 0.28633262 0.15256914 -0.14975667 0.08192404 -0.18780851]] [-0.07612282]
Compare the parameter values obtained from the above implementation and the one obtained with sklearn's implementation: they are almost equal.
Also, let's compare the predicted probabilities obtained using these two different implementations of LR (one from scratch, another one from sklearn's library function), as can be seen from the following scatterplot, they are almost identical:
pred_probs = 1 / (1 + np.exp(-X@w))
plt.scatter(pred_probs, clf.predict_proba(x)[:,1])
plt.grid()
plt.xlabel('pred prob', size=20)
plt.ylabel('pred prob (sklearn)', size=20)
plt.show()
Finally, let's compute the accuracies obtained, they are identical too:
print(sum((pred_probs > 0.5) == y) / len(y))
# [0.527]
clf.score(x, y)
# 0.527
This also shows the correctness of the implementation.
