# Basic+functions+used+in+Neural+networks

sigmoid function, using math.exp() and np.exp()

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In :

import math

import numpy as np

def basic_sigmoid(x):
"""
Compute sigmoid of x.

Arguments:
x -- A scalar

Return:
s -- sigmoid(x)
"""

s = 1/(1+math.exp(-x))

return s
# it will throw error when an array is passes thats why we use np.exp(). it will apply exp elementwise
#np.exp(x)=(ex1,ex2,...,exn)np.exp(x)=(ex1,ex2,...,exn)
def sigmoid(x):
"""
Compute sigmoid of x.

Arguments:
x -- A scalar

Return:
s -- sigmoid(x)
"""

s = 1/(1+np.exp(-x))

return s

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x = np.array([1, 2, 3])
sigmoid(x)

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Out:

array([ 0.73105858,  0.88079708,  0.95257413])

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In :

#this will throw error
basic_sigmoid(x)

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---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-4ceb880775ed> in <module>()
1 #this will throw error
----> 2 basic_sigmoid(x)

<ipython-input-1-09318697b20a> in basic_sigmoid(x)
14     """
15
---> 16     s = 1/(1+math.exp(-x))
17
18     return s

TypeError: only length-1 arrays can be converted to Python scalars

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def sigmoid_derivative(x):
"""
Compute the gradient (also called the slope or derivative) of the sigmoid function with respect to its input x.
You can store the output of the sigmoid function into variables and then use it to calculate the gradient.

Arguments:
x -- A scalar or numpy array

Return:
"""

### START CODE HERE ### (≈ 2 lines of code)
s = 1/(1+np.exp(-x))
ds = np.array(s*(1-s))
### END CODE HERE ###

return ds

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In [ ]:

x = np.array([1, 2, 3])
print ("sigmoid_derivative(x) = " + str(sigmoid_derivative(x)))

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def image2vector(image):
"""
Argument:
image -- a numpy array of shape (length, height, depth)

Returns:
v -- a vector of shape (length*height*depth, 1)
"""

### START CODE HERE ### (≈ 1 line of code)
v = image.reshape(image.shape*image.shape*image.shape,1)
### END CODE HERE ###
return v

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# This is a 3 by 3 by 2 array, typically images will be (num_px_x, num_px_y,3) where 3 represents the RGB values
image = np.array([[[ 0.67826139,  0.29380381],
[ 0.90714982,  0.52835647],
[ 0.4215251 ,  0.45017551]],

[[ 0.92814219,  0.96677647],
[ 0.85304703,  0.52351845],
[ 0.19981397,  0.27417313]],

[[ 0.60659855,  0.00533165],
[ 0.10820313,  0.49978937],
[ 0.34144279,  0.94630077]]])

print ("image2vector(image) = " + str(image2vector(image)))

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def normalizeRows(x):
"""
Implement a function that normalizes each row of the matrix x (to have unit length).

Argument:
x -- A numpy matrix of shape (n, m)

Returns:
x -- The normalized (by row) numpy matrix. You are allowed to modify x.
"""

### START CODE HERE ### (≈ 2 lines of code)
# Compute x_norm as the norm 2 of x. Use np.linalg.norm(..., ord = 2, axis = ..., keepdims = True)
x_norm = np.linalg.norm(x,axis=1,keepdims=True)
print("shape of x_norm =" + str (x_norm.shape))

# Divide x by its norm.
x = x/x_norm

print("shape of x =" + str (x.shape))
### END CODE HERE ###

return x

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x = np.array([
[0, 3, 4],
[1, 6, 4]])
print("normalizeRows(x) = " + str(normalizeRows(x)))

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def softmax(x):
"""Calculates the softmax for each row of the input x.

Your code should work for a row vector and also for matrices of shape (n, m).

Argument:
x -- A numpy matrix of shape (n,m)

Returns:
s -- A numpy matrix equal to the softmax of x, of shape (n,m)
"""

### START CODE HERE ### (≈ 3 lines of code)
# Apply exp() element-wise to x. Use np.exp(...).
x_exp = np.exp(x)

# Create a vector x_sum that sums each row of x_exp. Use np.sum(..., axis = 1, keepdims = True).
x_sum = np.sum(x_exp, axis=1, keepdims=True)

# Compute softmax(x) by dividing x_exp by x_sum. It should automatically use numpy broadcasting.
s = x_exp/x_sum

### END CODE HERE ###

return s

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In [ ]:

x = np.array([
[9, 2, 5, 0, 0],
[7, 5, 0, 0 ,0]])
print("softmax(x) = " + str(softmax(x)))

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def L1(yhat, y):
"""
Arguments:
yhat -- vector of size m (predicted labels)
y -- vector of size m (true labels)

Returns:
loss -- the value of the L1 loss function defined above
"""

### START CODE HERE ### (≈ 1 line of code)
loss = np.sum(abs(y-yhat),axis=0,keepdims=True)
### END CODE HERE ###

return loss

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yhat = np.array([.9, 0.2, 0.1, .4, .9])
y = np.array([1, 0, 0, 1, 1])
print("L1 = " + str(L1(yhat,y)))

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def L2(yhat, y):
"""
Arguments:
yhat -- vector of size m (predicted labels)
y -- vector of size m (true labels)

Returns:
loss -- the value of the L2 loss function defined above
"""

### START CODE HERE ### (≈ 1 line of code)
loss = np.sum((y-yhat)**2)
### END CODE HERE ###

return loss

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In [ ]:

yhat = np.array([.9, 0.2, 0.1, .4, .9])
y = np.array([1, 0, 0, 1, 1])
print("L2 = " + str(L2(yhat,y)))

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