Why does my random number program output garbage?

Question

I'm trying to display a random number in the array of 5 ints. I have an int distribution from 0 to the size of the array. But whenever I go to output the numbers, sometimes I get a garbage value. Why is this happening?

#include <iostream>
#include <random>

int main()
{
    int a[5] = { 1, 2, 3, 4, 5 };
    std::random_device seed;
    std::default_random_engine rand(seed());
    std::uniform_int_distribution<int> dist(0, sizeof(a)/sizeof(*a));
    for (int i = 0; i < 100; ++i)
        std::cout << a[dist(rand)] << "\n";
}

Answer 1

Array indexes are zero-based . For an array of 5 elements the valid indexes are 0..4 . Your distribution returns values in the range 0..5 and is the source of the problem.

Answer 2

Because of undefined behavior - your are accessing an element outside the bounds of array a . Since the distribution goes from 0 to X and not 0 to one before X you should construct it like so.

std::uniform_int_distribution<int> dist(0, sizeof(a)/sizeof(*a) - 1);