Re: (s)printf and rounding woes

If someone should be blamed for this, it is not Perl, but c, and Perl just inherits it. If you are using languages support fixed decimal, for example COBOL or PL/SQL, then this problem disappears.

Here is an example what can happen to c: (It does not give 14720)

#include <stdio.h>

void main() {
    float x = 147.2;
    printf("%20.20f", x * 100);
}
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If you really care the accuracy and precision, try Math::BigFloat.

Now here is something a little bit off topic, but interesting. If you do this in c:

float x = 147.2;
printf("%d", x * 100);
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It produces -167772160, which is ridiculous, as c tries to interprete that piece of memory as integer, as you required.

Good you are using Perl. In Perl, if you do the same thing with:

$x = 147.2;
printf("%d", $x * 100);
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It produces 14719 (not precise, but better than c), as internally Perl does:

float x = 147.2;
printf("%d", (int)(x * 100));
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Re: Re: (s)printf and rounding woes by nevyn (Monk) on Nov 05, 2003 at 11:57 UTC
Now here is something a little bit off topic, but interesting. If you do this in c: `float x = 147.2; printf("%d", x * 100);` [download] It produces -167772160, which is ridiculous, as c tries to interprete that piece of memory as integer, as you required. Well gcc has had static format checking included for only at least 10 years. Which would give a warning on the above, and so make that mistake practically impossible. In fact there are solutions for printing arbitrary objects via. printf like mechanisms in C. Which might be nice in perl (if XS wasn't so ick I'd be happier to look at doing it). In fact, to be somewhat on topic, it can sometimes be pretty annoying that perl doesn't give any warnings if you do... `my $i = 1; my $j = 2.3; printf("%f %d\n", $i, $j);` [download] ...although more annoying and probably easier to fix would be to have all the normal printf() POSIX extensions like "%'d" work. -- James Antill	[reply] [d/l] [select]

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Re: Re: (s)printf and rounding woes
by nevyn (Monk) on Nov 05, 2003 at 11:57 UTC

Now here is something a little bit off topic, but interesting. If you do this in c:
float x = 147.2;
printf("%d", x * 100);
[download]
It produces -167772160, which is ridiculous, as c tries to interprete that piece of memory as integer, as you required.

Well gcc has had static format checking included for only at least 10 years. Which would give a warning on the above, and so make that mistake practically impossible. In fact there are solutions for printing arbitrary objects via. printf like mechanisms in C. Which might be nice in perl (if XS wasn't so ick I'd be happier to look at doing it).

In fact, to be somewhat on topic, it can sometimes be pretty annoying that perl doesn't give any warnings if you do...

my $i = 1;
my $j = 2.3;

printf("%f %d\n", $i, $j);
[download]

...although more annoying and probably easier to fix would be to have all the normal printf() POSIX extensions like "%'d" work.

--
James Antill

[reply]
[d/l]
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