Modulo and integer division in C++ and C languages.

Question

From the point of view of C++, C:

-17 % 5 = -2
-17 / 5 = -3

But from the point of view of mathematics (and python😁):

-17 % 5 = 3
-17 / 5 = -4

Explain inner logic of compiler (e. g. gcc) to me. How it calculates? Why designers chose such logic in compiler, instead of mathematical approach?

Answer 1

Maybe this approach suits for old languages(they didn't notice difference and didn't want to aim to design the language more consistently and of course can't solve and notice all features related with mathematics)? But the in modern (don't argue🙃) languages they decided to make it more correctly from the point of view of mathematics🤔
Honestly, I dislike these implicit things in languages zoo, but nevertheless we have what we have.
For example, in JS we have strange thing as **null**. And they can't fix it because of backward compatibility in legacy codes😁

A surprising, but very interesting world of programming languages💃

Answer 2

See:
https://stackoverflow.com/questions/24074869/why-is-the-behavior-of-the-modulo-operator-different-between-c-and-ruby-for

Answer 3

-- found this which maybe of interest.

Originally, MOD(n, m) was defined only for positive values of both m and n, and leaves the result to be implementation-dependent when either of m or n is negative.

Answer 4

In Mathematics we have :

Dividend = Divisor * Quotient + Remainder

so according to your words :) :

In C++ we have:

(-17) = (+5) * (-3) + (-2)
=> (-17) = (-15) + (-2)
=>(-17) = (-17) => So its correct :)

In Python we have :

(-17) = (+5) * (-4) + (+3)
=> (-17) = (-20) + (+3)
=>(-17) = (-17) => So its correct :)

So don't say Mathematics and Python says other answer :) because both of them (C++ & Python) are correct just having different processes :)

Answer 5

Non euclidean division is used in c and it's variants.

You can use euclidean division, but you have to do so manually.

https://code.sololearn.com/ct8gPZL2Qj8R/?ref=app

Answer 6

At first glance, I'd attempt to implement what is referred to as 'typecasting'.
Perhaps try something *similar* to the following (toy around with it some) ...

*C++ :
int var1 = (int)(-17 % 5);
int var2 = (int)(-17 / 5);

*Python :
var1, var2 = int(-17 % 5), int(-17 / 5)

Answer 7

it's interesting

Answer 8

mod operation for any negative operand is platform/language (and person 😀) dependent
https://en.m.wikipedia.org/wiki/Modulo_operation

Answer 9

it's interesting

Answer 10

-17/5 gives -3.4 and int(-17/5) gives -3 in python idle 3.5.4

Answer 11

it isn't necessary to use an integral.

Answer 12

https://www.cprogramming.com/tutorial/modulus.html

it is converted into integer numbers then calculate

using boxing unboxing concepts......

Answer 13

What is the difference between languages C# and C++?

Answer 14

you are the question
you are the answer. @4rontender

Answer 15

I think what 4rontender is pointing out clearly is that

-n%m in .cpp != -n%m in .py

&

-n/m in .cpp != -n/m in .py

Answer 16

Это Точно что интересно

Answer 17

c & c++ are hardware related languages. it is searched for the fastest and most direct method to compute modulo. IEEE allows you to: IEEE 754 defines a remainder function where the quotient is a/n rounded according to the round to nearest convention. Thus, the sign of the remainder is chosen to be nearest to zero.

Answer 18

because of bigger precedence

Answer 19

Привет

Answer 20

36