Why isn't to!int() working properly?

Question

Why does this assertion fail?

import std.conv;

void main()
{
    auto y = 0.6, delta=0.1;
    auto r = to!int(y/delta);
    assert(r == 6);
}

r's value should be 6 and yet it's 5, Why?

Deepsix

Is floating point math broken? What Every Programmer Should Know About Floating-Point Arithmetic – Mohammadmohammed

after that read this thorough article What Every Computer Scientist Should Know About Floating-Point Arithmetic – Mohammadmohammed

Answer 1

This is probably because 0.6 can't be represented purely in a floating point number. You write 0.6, but that's not exactly what you get - you get something like 0.599999999. When you divide that by 0.1, you get something like 5.99999999, which converts to an integer of 5 (by rounding down).

Examples in other languages:

C#: Why is (double)0.6f > (double)(6/10f)?

Java: Can someone please explain me that in java why 0.6 is <0.6f but 0.7is >=0.7f

Hear

How can I round up then? – Deepsix

You can round to the nearest integer with std.math.round: dlang.org/library/std/math/round.html. If you know your floating point result is within 0.5 of the integer you want, this will round the result to what you want. Two answers for the price of one! – Hear

Or by adding 0.5. I.e use (y/delta+0.5) – Udella

Answer 2

Computers represent floating point numbers in binary. The decimal numbers 0.6 and 0.1 do not have an exact binary representation, while number of bits used to represent them is finite. As a result, there would be truncation, whose effect is seen during division. The result of that division is not exactly 6.00000000, but perhaps 5.99999999, which is then truncated to 5.

Benedetto