double or float error

[Kelvin]

Hi:
I have
double d = 0.33

when I try to display this using:
cout << d;
it gives me 0.3329999999999999
i know it's caused by the way that computer stores float number...
but how can i get exactly 0.33 without using printf like arguement ??

thank you
--
{ Kelvin@!!! }

[Prawit Chaivong]

Kelvin wrote:

Hi:
I have
double d = 0.33

when I try to display this using:
cout << d;
it gives me 0.3329999999999999
i know it's caused by the way that computer stores float number...
but how can i get exactly 0.33 without using printf like arguement ??

thank you
--
{ Kelvin@!!! }

Hi Kelvin,
What's your compiler?
I've tested by using gcc3.2.3, the result of both cout and printf
are the same.

[velthuijsen]

try
int OldWidth = cout.width(2);
cout << d;
cout. width(OldWidth);

[velthuijsen]

GAH, must have been dreaming or something
This is wrong:

int OldWidth = cout.width(2);
cout << d;
cout. width(OldWidth);

should be:

int OldPrecision = cout.precision(3);
cout << d;
cout.precision(OldPrecision);

Note that this limits the total number of digits shown of a floating
point value. so if you get a value like 123.456 you get 123 not 123.456

[Kelvin]

"Prawit Chaivong" <pr******@gmail.com> wrote in message
news:11**********************@g49g2000cwa.googlegr oups.com...

Kelvin wrote:

Hi:
I have
double d = 0.33

when I try to display this using:
cout << d;
it gives me 0.3329999999999999
i know it's caused by the way that computer stores float number...
but how can i get exactly 0.33 without using printf like arguement ??

thank you
--
{ Kelvin@!!! }

Hi Kelvin,
What's your compiler?
I've tested by using gcc3.2.3, the result of both cout and printf
are the same.

Hi Prawit:
I'm using the VC 2005 express...
cout << d; is just an example that would show my problem in the simplist
way...

i'm lookin for precision control in C++... which i just couldn't find
anywhere...

to round off to a certain digit... i tried:
double RoundOff( double d, int precision )
{
int p = precision;
while( p-- )
d *= 10;

d = (int)d;
p = precision;
while( p++ )
d /= 10;

return d;
}

this function seems fine... but it produces the problem i stated...

--
{ Kelvin@!!! }

[Chris Theis]

"Kelvin" <ch***************@y.a.h.o.o.c.o.m.h.k.donot.spa m> wrote in message
news:l1ype.51998$tt5.50100@edtnps90...

Hi:
I have
double d = 0.33

when I try to display this using:
cout << d;
it gives me 0.3329999999999999
i know it's caused by the way that computer stores float number...
but how can i get exactly 0.33 without using printf like arguement ??

thank you

You can use the std::setprecision stream manipulator.

cout << std::setprecision(2) << d << endl; // use 2 or whatever
precision you want

HTH
Chris

[velthuijsen]

to round off to a certain digit... i tried:
double RoundOff( double d, int precision )
{
int p = precision;
while( p-- )
d *= 10;

d = (int)d;
p = precision;
while( p++ )
d /= 10;

return d;
}

this function seems fine... but it produces the problem i stated...

Uh no this function is something I'm going to hand the guy who tought
me C++ when I went to school so that he has a prime example of how not
work with doubles to show to his students.

If you want to work extensively with floating point types go and read:
http://www.physics.ohio-state.edu/~d...point_math.pdf
Note that this was written before the standarisation of C++ so you
might need to adapt the code to current standards.

A few things that just pop in why this function is wrong.
What happens if d is larger then 2^31-1 (after multiplication)
What happens if you try a precision greater then 16.
What happens if you feed in a double like 1.345e-5 with a precision
value of 4
How big will roundoff errors be.
And the biggest of all the function doesn't do what you think it should
do. It doesn't hand you a nicely cut to precision length double, it
hands you a (most likely) mangled representation of the double you put
in.

[Michal Idziorek]

maybe you should swap the
d/=10 and d*=10

"Kelvin" <ch***************@y.a.h.o.o.c.o.m.h.k.donot.spa m> schrieb im
Newsbeitrag news:l1ype.51998$tt5.50100@edtnps90...

Hi:
I have
double d = 0.33

when I try to display this using:
cout << d;
it gives me 0.3329999999999999
i know it's caused by the way that computer stores float number...
but how can i get exactly 0.33 without using printf like arguement ??

thank you
--
{ Kelvin@!!! }