Double trouble

[ntg]

Hi,

I recently used something along the lines of:

Dim d As Double = 5078927777236477
Dim s As String = d.ToString("N")
Debug.WriteLine(s)

...and the resulting string isn't the correct representation of the number. I run into this when I displayed a debit card number. Anyone knows what's the problem ?

Cheers,
NTG

[taxes]

Hi,

Dim d As Double = 5078927777236477
Dim s As String = FormatNumber(d, 0, , , TriState.False)

[ntg]

Hi,

Dim d As Double = 5078927777236477
Dim s As String = FormatNumber(d, 0, , , TriState.False)

Thanks, but that doesn't work either. Did it work for you ?

Cheers,
NTG

[taxes]

Hi,

Sorry. It works almost. It rounded the number to 5078927777236480. I did not notice that so I will experiment.

[taxes]

Hi,

Got it.

Dim d As Decimal = 5078927777236477
Dim s As String = d.ToString("G")


"G" being the general format character.

[ntg]

Right on. Thanks!

Cheers,
NTG

[Mike Hildner]

I believe that when you need to have exact numbers, and not approximations, you don't want to use any floating point types.

[SuperSparks]

I agree, I would use the Decimal type for that.

[ntg]

Decimal would work fine in this situation, but I was pretty scared when it turned out that there are problems with adding decimals (lost decimal digits).

Anyway, the double doesn't look like an approximation to me, it's storing 16-digit card numbers fine - my problem was with the string representation of the number.

Cheers,
NTG

[Mike Hildner]

When I say approximations, I mean that floating point types (Single and Double in VB.NET) are subject to rounding errors. That may not apply to what you're doing, or even your original post.

Just something I wanted to throw out there.

[kleinma]

ntg, being that you are an experienced programer and all.. I gotta ask why you would be using double in any way to store a value that you won't be doing any math on like a credit card number... I would always think to use string for something like that??? or is there more to it

[ntg]

There is some validation that's running on the number but nothing I would call real math, so there's really not that much more to it. To be honest, the original representation of the proprietary internal storage of these cards is in PL/I as a number and when I saw that double could store 16-digit numbers I didn't give it much more thought and used that. But it's true that next time I'll use Strings.

Cheers,
NTG