Errata for Practical Programming (2nd edition)

#50997: Thank you for the quality of your cours in Cousera.
Excuse my english it is not my mother tong.

I want to add some explanation to get a better understanding
about the relation // and % in the page 13, starting at
"Be careful about using % and // with negative operands.

My suggestion:
Be careful about using % and // with negative operands. It
could look more natural to think about this functions like
this:

Dividend Divisor Quotient Remainder
17 10 1 7
-17 -10 1 -7
17 -10 -1 7
-17 10 -1 -7

Freepascal and Delphi work like this. But this cause a lot
of bug. Example:

If you have a clock at 9.00h, what is the time 24 hours
before?

The answer is: -15 % 12
With Freepascal and Delphi you will get 2.00h, and it is
wrong.

To solve this kind of bug, the majority of the programming
language like Python use an other interpretation of //
and %:

Dividend Divisor Quotient Remainder
17 10 1 7
-17 -10 1 -7
17 -10 -2 -3
-17 10 -2 3

And with Python you will get the good answer:
-15 % 12 is equal to 9

What is important to remember:
--Python takes the floor of the result of an integer
division, the result is one smaller in negative answer
then positive answer:
>>> -17 // 10
-2
--When using modulo,the sign of the result matches the
sign of the divisor (The second operand):
>>> -17 % 10
3
>>> 17 % -10
-3

Remark:
Both interpretation of // and % work with the relation:
(b *(a // b) + a % b) is equal to a

My referance is: fr.wikipedia.org/wiki/Modulo_(op%C3%A9ration)

Thank you, Michel Lemay.

I do some corrections but the blog print differently then i do.--Michel Lemay

#50998: Second version.
Thank you for the quality of your cours in Cousera.
Excuse my english it is not my mother tong.

I want to add some explanation to get a better understanding
about the relation // and % in the page 13, starting at
"Be careful about using % and // with negative operands.

My suggestion:
Be careful about using % and // with negative operands. It
could look more natural to think about this functions like
this:

Dividend Divisor Quotient Remainder
_______ 17________ 10_______ 1_________ 7
_______-17________-10_______ 1_________-7
_______ 17________-10_______-1_________ 7
_______-17________ 10_______-1_________-7

Freepascal and Delphi work like this. But this cause a lot
of bug. Example:

If you have a clock at 9.00h, what is the time 24 hours
before?

The answer is: -15 % 12
With Freepascal and Delphi you will get 2.00h, and it is
wrong.

To solve this kind of bug, the majority of the programming
language like Python use an other interpretation of //
and %:

Dividend Divisor Quotient Remainder
_______ 17________ 10________ 1_________ 7
_______-17________-10________ 1_________-7
_______ 17________-10________-2_________-3
_______-17________ 10________-2_________ 3

_______ 17________ 17________ 1_________ 0
_______-17________-17________ 1_________ 0
_______ 17________-17________-1_________ 0
_______-17________ 17________-1_________ 0

And with Python you will get the good answer:
-15 % 12 is equal to 9

What is important to remember:

--Python takes the floor of the result of an integer
division, the result is one smaller in negative answer
then positive answer:
__>>> -17 // 10
__-2

--Exception if your remainder is 0, the result is not
one smaller in negative answer but it still negative:
__>>> 17 // -17
__-1

--When using modulo,the sign of the result matches the
sign of the divisor (The second operand):
__>>> -17 % 10
__3
__>>> 17 % -10
__-3

Remark:
Both interpretation of // and % work with the relation:
(b *(a // b) + a % b) is equal to a

My referance is: fr.wikipedia.org/wiki/Modulo_(op%C3%A9ration)

Thank you, Michel Lemay.

PS: The line in the tables and other places is only to get the
good position.--Michel Lemay

#50999: Second version.
Thank you for the quality of your cours in Cousera.
Excuse my english it is not my mother tong.

I want to add some explanation to get a better understanding
about the relation // and % in the page 13, starting at
"Be careful about using % and // with negative operands.

My suggestion:
Be careful about using % and // with negative operands. It
could look more natural to think about this functions like
this:

____ Dividend___Divisor___Quotient___Remainder
_______ 17________ 10_______ 1_________ 7
_______-17________-10_______ 1_________-7
_______ 17________-10_______-1_________ 7
_______-17________ 10_______-1_________-7

Freepascal and Delphi work like this. But this cause a lot
of bug. Example:

If you have a clock at 9.00h, what is the time 24 hours
before?

The answer is: -15 % 12
With Freepascal and Delphi you will get 2.00h, and it is
wrong.

To solve this kind of bug, the majority of the programming
language like Python use an other interpretation of //
and %:

_____Dividend___Divisor___Quotient___Remainder
_______ 17________ 10________ 1_________ 7
_______-17________-10________ 1_________-7
_______ 17________-10________-2_________-3
_______-17________ 10________-2_________ 3

_______ 17________ 17________ 1_________ 0
_______-17________-17________ 1_________ 0
_______ 17________-17________-1_________ 0
_______-17________ 17________-1_________ 0

And with Python you will get the good answer:
-15 % 12 is equal to 9

What is important to remember:

--Python takes the floor of the result of an integer
division, the result is one smaller in negative answer
then positive answer:
__>>> -17 // 10
__-2

--Exception if your remainder is 0, the result is not
one smaller in negative answer but it still negative:
__>>> 17 // -17
__-1

--When using modulo,the sign of the result matches the
sign of the divisor (The second operand):
__>>> -17 % 10
__3
__>>> 17 % -10
__-3

Remark:
Both interpretation of // and % work with the relation:
(b *(a // b) + a % b) is equal to a

My referance is: fr.wikipedia.org/wiki/Modulo_(op%C3%A9ration)

Thank you, Michel Lemay.

PS: The line in the tables and other places is only to get the
good position.--Michel Lemay