I am learning c++ and in trying to code a little program i found something strange. It is about modulus : c++ code
cout << -325 - (-325 % 100) << endl;
// -300
cout << 325 - (325 % 100);
// 300
and python code
print(-325 - (-325 % 100))
# -400
print(325 - (325 % 100))
# 300
Can someone explain me that pls ? Thq
This is a bit out of my expertise but its likely python follows the Donald Knuth remainder calculation which is: a + b*[a/b] being [] the floor operation. As so:
a = -325
b = 100
print(a - b*math.floor(a/b))
print(math.floor(-3.25))
print(-325 -(a - b*math.floor(a/b)))
results in:
75
-4
-400
I'm a bit surprised C++ does not return the same (but I'm also not knowledgeable enough about the subject).
EDIT: Made a quick search for this topic and learncpp mentions the reason why C++, specifically C++ 11, standardized the floor operation always towards 0. I quote:
Prior to C++11, if either of the operands of integer division are negative, the compiler is free to round up or down! For example, -5 / 2 can evaluate to either -3 or -2, depending on which way the compiler rounds. However, most modern compilers truncate towards 0 (so -5 / 2 would equal -2). The C++11 specification changed this to explicitly define that integer division should always truncate towards 0 (or put more simply, the fractional component is dropped).
Also prior to C++11, if either operand of the modulus operator is negative, the results of the modulus can be either negative or positive! For example, -5 % 2 can evaluate to either 1 or -1. The C++11 specification tightens this up so that a % b always resolves to the sign of a.
Regarding Python you can take a look at the documentation for divmod which states:
Take two (non complex) numbers as arguments and return a pair of numbers consisting of their quotient and remainder when using integer division. With mixed operand types, the rules for binary arithmetic operators apply. For integers, the result is the same as (a // b, a % b). For floating point numbers the result is (q, a % b), where q is usually math.floor(a / b) but may be 1 less than that. In any case q * b + a % b is very close to a, if a % b is non-zero it has the same sign as b, and 0 <= abs(a % b) < abs(b)
EDIT2: Even more information on the Python Programming FAQ regarding this subject.
Why does -22 // 10 return -3?
It’s primarily driven by the desire that
i % jhave the same sign asj. If you want that, and also want:
i == (i // j) * j + (i % j)then integer division has to return the floor. C also requires that identity to hold, and then compilers that truncate
i // jneed to makei % jhave the same sign asi.There are few real use cases for
i % jwhenjis negative. Whenjis positive, there are many, and in virtually all of them it’s more useful fori % jto be>= 0. If the clock says 10 now, what did it say 200 hours ago?-190 % 12 == 2is useful;-190 % 12 == -10is a bug waiting to bite.
It looks phyton doesn't accept negative modulus results, so adds 100 to -25.
print(-325 - (-325 % 100))
a % b always has the same sign as b.I don't really know much about it, but since you are learning C++,i would like to tell you something from my personal experience, that if you are making a program in C++ and you're using modulus operator, and you want +ve mod always, you should do something like this -
a = b%m;
if(a<0) a += m;
I am not so sure on when and why, but a can take a negative value for any b and m (not only when b is -ve)
-325 % 100separately to see what it does. Pay close attention to the notes at the end of the section.