Random

[kunz]

i have a code like this but it does not seem to work

int randomin(void){
const int MAXINT = 10;
const int MININT = -10;
int a = 0;
int b = 0;
a=rand() % MININT + MAXINT;
b=rand() % MININT + MAXINT;
}






but when i call the function it does not randomize it
what m i doing wrong i dont seem to understand

[Disch]

In C++, the % operator only has defined results if both sides of the % operator are positive.

rand() % MININT

rand() will always give you a positive number, but in your example, MININT is negative, therefore this has undefined behavior and is very bad.

FWIW, you shouldn't be using rand() anyway. You should be using C++11 <random> header:

#include <random>
#include <ctime>

// The random number generator.  Made global for simplicity
std::mt19937        rng( time(nullptr)  );  // <- use the current time to seed it

// function to generate a random number between [-10, 10]
int randomin()  // <- (don't put void in the parenthesis -- that's a goofy C thing and isn't necessary in C++)
{
    return std::uniform_int_distribution<int>(-10, 10)(rng);
}

[JLBorges]

> In C++, the % operator only has defined results if both sides of the % operator are positive.

The result of a%b is undefined if and only if the second operand is zero. Otherwise,

if the quotient a/b is representable in the type of the result, (a/b)*b + a%b is equal to a

.
Truncation towards zero during division and (a/b)*b + a%b == a together mean that the result of a%b has the same sign as the dividend - that is positive for positive a and negative for negative a.

In legacy C++, if one or both operands were negative, the sign of the result of a%b was implementation-defined.

[Disch]

JLBorges: Thanks for the clarification. I was thinking in legacy C++ (didn't realize they changed it), and I get implementation-defined and undefined mixed up because they're both to be avoided.

[JLBorges]

We can't really avoid implementation-defined behaviour; for instance, sizeof(int) and the mapping of source file characters to the execution character set are implementation-defined.

Undefined behaviour and unspecified behavior should be avoided, though.