# whats the difference between float & double

Vas Miriyala
Ranch Hand
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
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
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 ... ...

Vas Miriyala
Ranch Hand
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
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.

Sebastian Hatt
Greenhorn
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
Posts: 15495
43
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.

Campbell Ritchie
Sheriff
Posts: 50277
80
Welcome to the Ranch Sebastian Hatt, and thank you for that useful first reply.