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Octave Tutorial

5 Questions

1. Suppose I first execute the following Octave commands:

A = [1 2; 3 4; 5 6];

B = [1 2 3; 4 5 6];

Which of the following are then valid Octave commands? Check all that apply and assume all options are written in an Octave command. (Hint: A ‘sign the transpose of A.)

0.29926222492940724

C = A * B;

0.6644976132083684

C = B ‘+ A;

0.04820065735839307

C = A ‘* B;

0.18387076747603714

C = B + A;

Answer: ab (C = A * B and C = B ’+ A;)

2. Question text

Let A = ???? 16594211714310615138121 ????.

Which of the following indexing expressions gives B = ???? 16594211714 ????? Check all that apply.

0.06797481491230428

B = A (:, 1: 2);

0.46775804040953517

B = A (1: 4, 1: 2);

0.7515415323432535

B = A (0: 2, 0: 4)

0.40075311926193535

B = A (1: 2, 1: 4);

Answer: ab (B = A (:, 1: 2); and B = A (1: 4, 1: 2);)

3.Let A be a 10x10 matrix and x be a 10-element vector. Your friend wants to compute the product Ax and writes the following code:

v = zeros (10, 1);

for i = 1:10

for j = 1:10

v (i) = v (i) + A (i, j) * x (j);

end

end

How would you vectorize this code to run without any FOR loops? Check all that apply.

0.647465850925073

v = A * x;

0.9062917539849877

v = Ax;

0.990483850473538

v = x ‘* A;

0.4759985599666834

v = sum (A * x);

Answer: a. V = A * x;

v = Ax: Undefined function or variable ‘Ax’.

4. Say you have two column vectors v and w, each with 7 elements (i.e., they have dimensions 7x1). Consider the following code:

z = 0;

for i = 1: 7

z = z + v (i) * w (i)

end

Which of the following vectorizations correctly compute z? Check all that apply.

0.47586352098733187

z = sum (v. * w);

0.9338313168846071

z = w ‘* v;

0.03846661816351116

z = v * w ‘;

0.3784104809165001

z = w * v ‘;

Answer: ab (z = sum (v. * W); and z = w ‘* v;)

column vectors column vectors

5. In Octave, many functions work on single numbers, vectors, and matrices. For example, the sin function when applied to a matrix will return a new matrix with the sin of each element. But you have to be careful, as certain functions have different behavior. Suppose you have a 7x7 matrix X. You want to compute the log of every element, the square of every element, add 1 to every element, and divide every element by 4. You will store the results in four matrices, A, B, C, D. One way to do so is the following code:

for i = 1: 7

for j = 1: 7

A (i, j) = log (X (i, j));

B (i, j) = X (i, j) ^ 2;

C (i, j) = X (i, j) + 1;

D (i, j) = X (i, j) / 4;

end

end

Which of the following correctly compute A, B, C, or D? Check all that apply.

0.23870734148658812

C = X + 1;

0.7951180564705282

D = X / 4;

0.24619056400842965

B = X. ^ 2;

0.8001980590634048

B = X ^ 2;

B = X. ^ 2 instead of X ^ 2

Coursera machine learning Week 2 Quiz answer Octave / Matlab Tutorial

# Original Source:

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