scientific & setprecision (iomanip)

[PengYu.UT]

Hi,

I setprecision to be 100 for both cases. I'm wondering why the number
of digits are different.

Also, for a double number, I think any digits that are longer than 15
(or 16) are not meaningful, because it exceed the double number's
precision limit. Even if I setprecision to be 100, shall it truncate
the number to be of 15(or 16) digits?

Thanks,
Peng

$ cat main.cc
#include <iostream>
#include <iomanip>

int main() {
double a = .01;
std::cout << std::setprecisi on(100) << a << std::endl;
std::cout << std::scientific << std::setprecisi on(100) << a <<
std::endl;
}
$ ./main-g.exe
0.0100000000000 000002081668171 172168513294309 377670288085937 5
1.0000000000000 000208166817117 216851329430937 767028808593750 000000000000000 000000000000000 000000000000e-02

[Victor Bazarov]

Pe*******@gmail .com wrote:

I setprecision to be 100 for both cases. I'm wondering why the number
of digits are different.

Also, for a double number, I think any digits that are longer than 15
(or 16) are not meaningful, because it exceed the double number's
precision limit. Even if I setprecision to be 100, shall it truncate
the number to be of 15(or 16) digits?

Thanks,
Peng

$ cat main.cc
#include <iostream>
#include <iomanip>

int main() {
double a = .01;
std::cout << std::setprecisi on(100) << a << std::endl;
std::cout << std::scientific << std::setprecisi on(100) << a <<
std::endl;
}
$ ./main-g.exe
0.0100000000000 000002081668171 172168513294309 377670288085937 5
1.0000000000000 000208166817117 216851329430937 767028808593750 000000000000000 000000000000000 000000000000e-02

I believe 'fixed' (the default, when 'scientific' is not used), always
skips the trailing zeros.

V
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[=?ISO-8859-1?Q?Erik_Wikstr=F6m?=]

On 2007-09-03 18:03, Victor Bazarov wrote:

Pe*******@gmail .com wrote:

>I setprecision to be 100 for both cases. I'm wondering why the number
of digits are different.

Also, for a double number, I think any digits that are longer than 15
(or 16) are not meaningful, because it exceed the double number's
precision limit. Even if I setprecision to be 100, shall it truncate
the number to be of 15(or 16) digits?

Thanks,
Peng

$ cat main.cc
#include <iostream>
#include <iomanip>

int main() {
double a = .01;
std::cout << std::setprecisi on(100) << a << std::endl;
std::cout << std::scientific << std::setprecisi on(100) << a <<
std::endl;
}
$ ./main-g.exe
0.010000000000 000000208166817 117216851329430 937767028808593 75
1.000000000000 000020816681711 721685132943093 776702880859375 000000000000000 000000000000000 0000000000000e-02

I believe 'fixed' (the default, when 'scientific' is not used), always
skips the trailing zeros.

Running the same code under Windows using VC++ 2005 I get the same
number of digits printed, but it's all zeroes after the 1. I find that a
bit troubling, since I thought that 0.01 was a number that couldn't be
exactly represented.

--
Erik Wikström

[Victor Bazarov]

Erik Wikström wrote:

On 2007-09-03 18:03, Victor Bazarov wrote:

>Pe*******@gmail .com wrote:

>>I setprecision to be 100 for both cases. I'm wondering why the
number of digits are different.
[..]
$ ./main-g.exe
0.01000000000 000000020816681 711721685132943 093776702880859 375
1.00000000000 000002081668171 172168513294309 377670288085937 500000000000000 000000000000000 00000000000000e-02

I believe 'fixed' (the default, when 'scientific' is not used),
always skips the trailing zeros.

Running the same code under Windows using VC++ 2005 I get the same
number of digits printed, but it's all zeroes after the 1. I find
that a bit troubling, since I thought that 0.01 was a number that
couldn't be exactly represented.

What you see is the external equivalent of the imprecise internal
representation. A decent library should be able to successfully
round the number when outputting is so that the round-trip (no pun
indended) from external rep to internal and back to external does
not change the appearance of the number. "0.01" in--0xB1AB1AAA (or
whatever) out--"0.01"

V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask