Lecture 3

Question 1

a = [‘hi’]

a += ‘hello’

Answer 1

a == [‘hi’, ‘h’, ‘e’, ‘l’, ‘l’, ‘o’]

should do a += [‘hello’]

Question 2

a.append()
a += []
a = a + []
Difference?

Answer 2

append and += are the same, modifying a. However, the 3rd one creates a new object.

Question 3

a = [0,1,2,3,4,5,6,7,8,9]
a[1::2][::-1] = ?

Answer 3

[9,7,5,3,1] (successive filters)

Question 4

A list passed in a function has “a.append(b)”. An integer passed in a function has “a+=1”. Which variable is modified?

Answer 4

Only mutable variables (if not redefined as a local variable inside the function i.e. a=[‘ok’]). Int won’t be modified!

Question 5

Transpose of a matrix

Answer 5

np.transpose(A)
A.T
A.transpose()
A.swapaxes(0,1)
Be careful, A.reshape() won't give the transpose!

Question 6

Broadcasting, when are 2 matrices compatibles?

Answer 6

Must have a dimension in common, it’ll be stretched along the other axis which must be of dimension 1.
ex: (7,5,9) and (1,5,1)
A (5,1) matrix would be interpreted as (1,5,1)

Question 7

Get unique elements of an array

Answer 7

np.unique(x)

Question 8

Get a list of elements in a matrix

Answer 8

x.flatten()

Question 9

Modify numpy type

Answer 9

x.astype(‘int’)

Question 10

Resize a matrix

Answer 10

Modify: x.resize(x,y)
Copy: x.reshape(x,y)

Question 11

Generate a numpy sequence of numbers

Answer 11

arange()

Question 12

Inner product

Answer 12

inner(a,b) = a1b1+a2b2+a3b3 (Same as dot product), A@B

Question 13

Outer product

Answer 13

outer(u,v) = v u^T