Errata

The statement "For example, Thor (id 4) has no friends in common with Devin (id 7) , . . ." is incorrect. They share the friend Clive (id 5).

Note from the Author or Editor:
good point, change that sentence to

For example, Hero (id 0) has no friends in common with Klein (id 9), but they share interests in Java and big data.

In the histogram code, please add the following to resolve Counter():

from collections import Counter

Note from the Author or Editor:
I don't care that much either way, I sort of assumed importing Counter was implied, but I don't mind adding a

from collections import Counter

to the start of the example

The list x contains second value 1, whereas it should contain second value -1.

Note from the Author or Editor:
yes, this is a mistake, x should be [-2, -1, 0, 1, 2]

Values are assigned to low_p and hi_p, but these are never used. Statements that refer to low_p and hi_p should be simplified.

Note from the Author or Editor:
agree with this, revised version at

https://gist.github.com/joelgrus/71c1ba8f96b6422a12adf10d04783512

In:
" X should be distributed approximately normally with mean 50 and
standard deviation 15.8:"

mean should be 500 not 50

Note from the Author or Editor:
confirmed, the mean should be 500

both 50 should be 500

Note from the Author or Editor:
agreed, change both 50 to 500

If beta_pdf is called with [x=0 and alpha<1] or with [x=1 and beta<1], the function crashes, because python does not permit 0 to be raised to a negative power. An easy fix is to change the 1st statement to
if x <= 0 or x >= 1:

Note from the Author or Editor:
I agree, change the first line of the beta_pdf function to

if x <= 0 or x >= 1:

for line in file:
should be
for line in f:

Note from the Author or Editor:
agree, should be

for line in f: # look at each line in the file

The script in lines 1 through 9 on page 108 is incorrect, because it does not produce the results printed on lines 11 through 14. Instead, the lines of text in bad_csv.txt get merged into a single line of text - as if the f.write("\n") were missing. One way to correct the error is to change the 6th line to "with open('bad_csv.txt', 'w') as f:" (omitting 'b' from open's 2nd argument). This script does not need 'wb', because it does not use the CSV module. Another (less elegant) resolution is to replace line 9 with f.write("\r\n").

Note from the Author or Editor:
I cannot reproduce this (the code as written works for me); however, I agree that the 'b' parameter does not need to be there, and I am ok with getting rid of the b and changing the line of code to

`with open('bad_csv.txt', 'w') as f:`

```
# back-propagate errors to hidden layer
hidden_deltas = [hidden_output * (1 - hidden_output) * dot(output_deltas, [n[i] for n in output_layer])
for i, hidden_output in enumerate(hidden_outputs)]
```

`output_layer` is not defined in the `backpropagate` function, and not passed in, hence running the example produces `NameError: global name 'output_layer' is not defined`

Note from the Author or Editor:
the line of code

dot(output_deltas, [n[i] for n in output_layer])

should be replaced with

dot(output_deltas, [n[i] for n in network[-1]])

For the function of matrix_multiply_mapper, two matrix indexes should be passed: the row number of A and column number of B

For any nonzero A_ij, all C_ik may be affected, with k being any column index of B

Similarly, for any nonzero B_ij, all C_kj may be affected, with k being any row index of A

In the text, the common dimension was used, which is wrong.

Also, for the function of matrix_multiply_reducer, m is not used.

Note from the Author or Editor:
yes, the code is wrong. here is a fixed version of the functions

https://gist.github.com/joelgrus/cd0558f2fc6eeaea22ba8d286775e6a1

and then at the very bottom of the page you need to change the definition of mapper

mapper = partial(matrix_multiply_mapper, 2, 3)

and at the top of the next page change the definition of reducer

reducer = matrix_multiply_reducer