# reading float point from file

Discussion in 'C++' started by Affan Syed, Dec 3, 2004.

1. ### Affan SyedGuest

Hi,
the file that i read has the following column as input:

1.000005
1.00001
1.000015
1.00002
1.000025

Now I have float variable skew.
When i read in from the file as inFile >>skew;

So it rounds up to the 5th decimal position . Why? I know it might have
something to do with the insuffereable precision of float points. But is
there some easy way of gettin to read the correct value from the file and
not go to BCD?
Similarly when i do
float x= skew-1,
I get something which is not nearly like 0.00001, but a close approximation.
Again how can i get the correct value?

Thanks
Affan

Affan Syed, Dec 3, 2004

2. ### Affan SyedGuest

actually the reading from file was okay... it read in the right value... but
when i debug it .. it just showed uptil a precision of 7 decimal points.. so
when i changed the precision of cout to 8, it printed the correctly rounded
values... So that is solved... however.. how do i do the maths with it...
as simple as subtracting one form it!!!?

regards
Affan
"Affan Syed" <> wrote in message
news:coojee\$h4p\$...
> Hi,
> the file that i read has the following column as input:
>
> 1.000005
> 1.00001
> 1.000015
> 1.00002
> 1.000025
>
> Now I have float variable skew.
> When i read in from the file as inFile >>skew;
> I read in following values:
>
> So it rounds up to the 5th decimal position . Why? I know it might have
> something to do with the insuffereable precision of float points. But is
> there some easy way of gettin to read the correct value from the file and
> not go to BCD?
> Similarly when i do
> float x= skew-1,
> I get something which is not nearly like 0.00001, but a close
> approximation. Again how can i get the correct value?
>
> Thanks
> Affan
>

Affan Syed, Dec 3, 2004

3. ### Jacek DziedzicGuest

U¿ytkownik Affan Syed napisa³:
> Hi,
> the file that i read has the following column as input:
>
> 1.000005
> 1.00001
> 1.000015
> 1.00002
> 1.000025
>
> Now I have float variable skew.
> When i read in from the file as inFile >>skew;
> I read in following values:
>
> So it rounds up to the 5th decimal position . Why? I know it might have
> something to do with the insuffereable precision of float points. But is
> there some easy way of gettin to read the correct value from the file and
> not go to BCD?
> Similarly when i do
> float x= skew-1,
> I get something which is not nearly like 0.00001, but a close approximation.
> Again how can i get the correct value?
>
> Thanks
> Affan

but only rounded on output. What is the precision of your
output?

HTH,
- J.

Jacek Dziedzic, Dec 3, 2004
4. ### Guest

Affan Syed <> wrote:
| Hi,
| the file that i read has the following column as input:

| 1.000005
| 1.00001
| 1.000015
| 1.00002
| 1.000025

| Similarly when i do
| float x= skew-1,
| I get something which is not nearly like 0.00001, but a close approximation.

Floats are inaccurate in nature, and operations on them will usually
generate round off errors. There are certain tricks though:

* Use integers. Integer operations are accurate. It's just a matter of
placing the decimal separator correctly on output. However useful, this
requires very low limits on the values.

* Write 'abs(a - b) < eps' in tests rather than 'a == b' if you want to
check for equality between a and b. eps must be a sufficiently small
floating point value.

* Don't subtract quantities of almost equal size as part of a larger
calculation. Rather try to rewrite the expression to avoid such things.

* Use types with more precision. The double type should often be
preferred over float.

For your example: If skew is generally "1.000...x", modify the string to
0.000...x before converting to float.

| Again how can i get the correct value?

You can't.
--
Robert Bauck Hamar

, Dec 3, 2004
5. ### Affan SyedGuest

Just as you said and i have already posted.. I can read in the value
correctly.
Affan
"Jacek Dziedzic" <> wrote in message
news:cookcu\$rq2\$...
U¿ytkownik Affan Syed napisa³:
> Hi,
> the file that i read has the following column as input:
>
> 1.000005
> 1.00001
> 1.000015
> 1.00002
> 1.000025
>
> Now I have float variable skew.
> When i read in from the file as inFile >>skew;
> I read in following values:
>
> So it rounds up to the 5th decimal position . Why? I know it might have
> something to do with the insuffereable precision of float points. But is
> there some easy way of gettin to read the correct value from the file and
> not go to BCD?
> Similarly when i do
> float x= skew-1,
> I get something which is not nearly like 0.00001, but a close
> approximation. Again how can i get the correct value?
>
> Thanks
> Affan

but only rounded on output. What is the precision of your
output?

HTH,
- J.

Affan Syed, Dec 3, 2004
6. ### Karl Heinz BucheggerGuest

Affan Syed wrote:
>
> actually the reading from file was okay... it read in the right value... but
> when i debug it .. it just showed uptil a precision of 7 decimal points.. so
> when i changed the precision of cout to 8, it printed the correctly rounded
> values... So that is solved... however.. how do i do the maths with it...
> as simple as subtracting one form it!!!?

the text you are replying to. Thank you).

skew = skew - 1.0;
or
skew -= 1.0;

And also note: Use 'float' only if you know what you do, have the knowledge
the fight that beast, are willing to actually fight that beast and have
a very, very, very good reason to use 'float'. In all other cases, use 'double'.

--
Karl Heinz Buchegger

Karl Heinz Buchegger, Dec 3, 2004
7. ### ArijitGuest

Affan Syed wrote:
> actually the reading from file was okay... it read in the right value... but
> when i debug it .. it just showed uptil a precision of 7 decimal points.. so
> when i changed the precision of cout to 8, it printed the correctly rounded
> values... So that is solved... however.. how do i do the maths with it...
> as simple as subtracting one form it!!!?
>
> regards
> Affan

Try this: http://docs.sun.com/source/806-3568/ncg_goldberg.html
It has everything you probably ever will want to know about floating
point (and more )

Arijit, Dec 3, 2004