What does Modulus do?

[viscid heart]

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[latent flower]

it's defined such that a/b*b+a%b == a when a/b is valid

it's the part that gets lost in the rounding of a/b

for integers

[knotty anchor]

So modulus gives me the remainder instead of how many times a number can go into another number?

[latent flower]

yes, % is returning the remainder

[knotty anchor]

So 10 / 3 = 3 and 10 % 3 = 1

like this?

[latent flower]

mhm

[knotty anchor]

Thank you!

[knotty anchor]

also how do I use it when creating a RNG?

[latent flower]

wdym?

[knotty anchor]

so im trying to make a random number generator but there was a tutorial on youtube, and they put

srand (time (0));

cout << (rand () % 6);

so im trying to understand modulus being used here

[viscid heart]

Generating Random Numbers in C++

The <random> header in C++ provides (pseudo-)random number generation (PRNG):

• UniformRandomBitGenerators produce random bits (entropy)
• RandomNumberDistributions use entropy to generate random numbers
  These types are used via their overloaded call operators.

Example: Printing Ten Random Dice Rolls

#include <random>
#include <iostream>
int main() {
  std::random_device dev; // for seeding
  std::default_random_engine gen{dev()};
  std::uniform_int_distribution<int> dis{1, 6};
  for (int i = 0; i < 10; ++i)
    std::cout << dis(gen) << ' ';
} ```

Possible Output (will be different each time)

1 1 6 5 2 2 5 5 6 2 ```

Common Generators

• std::random_device: truly random
• std::default_random_engine
• std::mt19937: popular default choice

Common Distributions

• std::uniform_int_distribution
• std::uniform_real_distribution
• std::normal_distribution

See Also

:cppreference: Pseudo-random number generation :stackoverflow: Generate random numbers using C++11 random library :stackoverflow: Why is the use of rand() considered bad?

[plucky estuary]

using % for this wouldn't be correct anyways, the numbers won't be uniformly distributed

there are better tools in C++ to do that

[knotty anchor]

int main() {

srand (time (0));

for (int i = 0; i < 10; i++)
{
cout << (rand () % 6) + 1 << "\t";
}

} // end of main

this is it completed sorry i forgot the rest of the program

im just trying to understand how modlus works here when making a random number generator

So i get it is getting random numbers between 1 and 6 but what is modulus being used for here?

[plucky estuary]

rand() generates uniformly distributed ints in some range [0, RAND_MAX]

using the remainder operator just maps that onto the range [0, 5]

however, this is biased and not actually uniform, which is why it's a bad implementation

rand() is just trash in pretty much every regard

[knotty anchor]

im very new to C++ so my class taught us by using rand()

so theres a better way to make a random number generator?

[plucky estuary]

yeah, see this

[knotty anchor]

hm

i dont know what dev means

they havent taught me that

[plucky estuary]

that's the name of the variable

[knotty anchor]

so dev(100) means 100?

[plucky estuary]

dev just has a call operator which generates truly random bits

so calling dev() gives you some seed that will be different each time you run the program

but using it directly is very slow, which is why you seed a PRNG with it and use that in the rest of the code

[knotty anchor]

so dev() is the same as time()?

like it works the same?

[plucky estuary]

yeah, the parentheses are a function call here

time() is a poor-man's seed; you would get the same result if you ran the program twice within the same second

std::random_device gets you entropy from more than just the system clock

[knotty anchor]

this seems so much better

why would they teach me time() and not this other one?

[plucky estuary]

because it's more verbose I guess; it doesn't "look" that easy even though it's a much better solution

at the end of the day, rand() kinda works and it's just some regular function call; there is much less to explain

[knotty anchor]

maybe thats why

[plucky estuary]

there have been proposals for a std::randint function that would simplify this by a lot, but that never made it into the language

[knotty anchor]

wow

[plucky estuary]

you would just write

std::cout << std::randint(1, 6) << ' ';

then for example

unfortunately, we don't have this; you could make a function like that yourself relatively easily though

[knotty anchor]

so this doesnt work?

[plucky estuary]

it's going to work more or less correct, the numbers just won't be uniformly distributed

[knotty anchor]

dang

[plucky estuary]

in this case you wouldn't notice because the range is too small for the bias to be obvious

consider if rand() hypothetically generated 7 random bits, so you had a uniform range [0, 127]

and you did rand() % 100

the first 27 numbers would be twice as likely then, because 0 % 100 == 0 and 100 % 100 == 0, for example

[knotty anchor]

I kind of get it..?

[plucky estuary]

or 27 % 100 == 27 and 127 % 100 == 100

there is no 128 though, so only the first 27 numbers can appear with double likelihood

[viscid heart]

@knotty anchor Has your question been resolved? If so, run !solved :)

[knotty anchor]

I kind of get it..?

I think i need to try using these different commands myself to fully understand

@latent flower@plucky estuary Thank you for helping me

[plucky estuary]

(◠‿◠✿)

[knotty anchor]

!solved

[viscid heart]

Thank you and let us know if you have any more questions!

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@knotty anchor

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