What am I doing wrong here - canFind with iota is not working

[John Colvin via Digitalmars-d]

On Thursday, 26 February 2015 at 11:00:05 UTC, John Colvin wrote:
> On Thursday, 26 February 2015 at 10:55:43 UTC, Kingsley wrote:
>> float oneDegree = (PI / 180.0);
>> float first = -(oneDegree * 10.0);
>> float second = (oneDegree * 10.0);
>> float step = 0.000001;
>> float[] r = iota(first,second,step).array;
>>
>> writeln(r);
>>
>> float item = 0.174531;
>> writeln(r.canFind(item));
>>
>>
>> // returns false for canFind - even though that float is in 
>> the array ???
>
> Never check floating point numbers for equality, unless you've 
> really thought about how the floating point arithmetic for this 
> specific problem will go. Pretty much the only place it's 
> useful to do is in unit tests for maths libraries.

This is the classic reference on the topic: 
http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html

but there are loads of other less rigorous explanations around 
the web.

To cut a long story short: floating point maths is not the same 
as maths with real numbers. It's mostly roughly the same, 
sometimes catastrophically different and very, very rarely 
exactly the same.