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whats the difference between float & double

Vas Miriyala
Ranch Hand

Joined: Sep 14, 2009
Posts: 114

Dear all
I have got some question about float & double , my answer is due to precision because float is 32 bit integer and double is 64 bit integer, However I am not sure...please help me


Output :
different
same

In first case why it is not equal and in case the value is same,why is it so?
Ian Lubelsky
Ranch Hand

Joined: Feb 10, 2010
Posts: 49
I think you are getting "different" for the 3.2 because a computer's floating point unit works with base 2 binary and 0.2 can't be represented precisely in binary, it is called a repeater fraction. In base-2 only numbers with denominators that are powers of 2 are terminating, which I think is only .25, .50, and .75, which is why 6.5 shows up as "same".

Hope this helps, but there may be better minded people out there that can give you a better explanation perhaps.
Jim Hoglund
Ranch Hand

Joined: Jan 09, 2008
Posts: 525
Float and double values are not integers, but rather floating point numbers. The Java
integer types are: byte, short, int, long (and char - 16 bits unsigned). In both cases
your code is comparing a float value to a double, as the compiler sees 3.2f as a float
and 3.2 (without the f) as a double. I suspect the precision of the compare logic has
something to do with the different results. Someone else may comment further.

Jim ... ...


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Vas Miriyala
Ranch Hand

Joined: Sep 14, 2009
Posts: 114

The float data type is a single-precision 32-bit
The double data type is a double-precision 64-bit
marc weber
Sheriff

Joined: Aug 31, 2004
Posts: 11343

According to JLS - 3.10.2 Floating-Point Literals...
A floating-point literal is of type float if it is suffixed with an ASCII letter F or f; otherwise its type is double...

So in line 6, f1 is assigned the value 3.2f, which is a float.

But in line 9, this value is compared to the value 3.2, which is a double.

These are not equal because the value 3.2 cannot be precisely stored (in binary) within the range of a float.


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Sebastian Hatt
Greenhorn

Joined: May 30, 2010
Posts: 1
As Ian pointed out, 6.5 can be represented exactly in binary, whereas 3.2 can't. That's why the difference in precision doesn't matter for 6.5, so 6.5 == 6.5f.

To quickly refresh how binary numbers work:

100 -> 4
10 -> 2
1 -> 1
0.1 -> 0.5 (or 1/2)
0.01 -> 0.25 (or 1/4)
etc.

6.5 in binary: 110.1 (exact result, the rest of the digits are just zeroes)
3.2 in binary: 11.001100110011001100110011001100110011001100110011001101... (here precision matters!)

A float only has 24 bits precision (the rest is used for sign and exponent), so:

3.2f in binary: 11.0011001100110011001100 (not equal to the double precision approximation)


There's a handy calculator for things like this at http://www.digitconvert.com/


Basically it's the same as when you're writing 1/5 and 1/7 in decimal numbers:

1/5 = 0,2
1,7 = 0,14285714285714285714285714285714...
Jesper de Jong
Java Cowboy
Saloon Keeper

Joined: Aug 16, 2005
Posts: 14266
    
  21

A float is a 32-bit, single-precision IEEE 754 floating point number.

A double is a 64-bit, double-precision IEEE 754 floating point number.


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Campbell Ritchie
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Joined: Oct 13, 2005
Posts: 39393
    
  28
Welcome to the Ranch Sebastian Hatt, and thank you for that useful first reply.
 
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