String to float vs double conversion

Question

Why is fvalue1 123456792 after the following line of code has executed?

float fvalue1 = Convert.ToSingle("123456789", CultureInfo.InvariantCulture);

Converting to double works as expected:

double dvalue1 = Convert.ToDouble("123456789", CultureInfo.InvariantCulture);

also note `double d = 123456789; float f = (float) d; // f is 123456792` — Bala R, Mar 31 '11 at 17:58

score 4 · Accepted Answer · edited May 23 '17 at 10:33

4

You've just discovered for yourself the reason double type exists - its precision is better then float's.

Check out following on the differences between these types: Difference between decimal, float and double in .NET?

edited May 23 '17 at 10:33

Community

answered Mar 31 '11 at 17:53

Alexei Levenkov

score 3 · Answer 2 · answered Mar 31 '11 at 17:51

3

float doesn't have the required precision to represent 123456789, whereas double does.

answered Mar 31 '11 at 17:51

dlev

[Are you sure?](http://msdn.microsoft.com/en-us/library/system.single.maxvalue.aspx) – Uwe Keim Mar 31 '11 at 17:54
@Uwe I am. Still need to learn to not couch my language with unnecessary qualifiers :) – dlev Mar 31 '11 at 17:55
@dlev Can you explain, why the precision is relevant here, since the number contains no decimal places, but just an integer? – Uwe Keim Mar 31 '11 at 18:20
2

@Uwe: **The number certainly does contain decimal places.** Remember how a float is stored. A float is a binary number 1.0101010101010 multiplied by 2^x for some integer x. A float is a sign bit, an integer representing the exponent, and the digits after the "1." – Eric Lippert Mar 31 '11 at 20:36
Thanks, @Eric - I really never got into the storage of floating points and all that epsilon stuff. Have do to some homework, I guess. – Uwe Keim Mar 31 '11 at 20:45
2

@Uwe: See my series on floating point arithmetic if you want the basics and some fun facts about Benford's Law. http://blogs.msdn.com/b/ericlippert/archive/tags/floating+point+arithmetic/ – Eric Lippert Mar 31 '11 at 22:30

2 Answers2