Strange loop!! why???

[grimbys]

Hi! we have a problem with decimal numbers.
When we have next loops, we don't get the result expected
for ($i = 0.001; $i <= 0.01 ;$i += 0.001)
{
print $i.&quot; &quot;;
}
print &quot;\n&quot;;
# OUT: 0.001 0.002 0.003 0.004 0.005 0.006 0.007 0.008 0.009 WITHOUT 0.01!!!
for ($i = 0.01; $i <= 0.1 ; $i += 0.01)
{
print $i.&quot; &quot;;
}
print &quot;\n&quot;;
# OUT: 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 WITH 0.1 ¡OK!
for ($i = 0.01; $i < 0.1 ; $i += 0.01)
{
print $i.&quot; &quot;;
}
# OUT: 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 WITH 0.1!!!!! (look at ¡¡¡¡<!!!)

Where is the problem???? We don't know
Thanks!

 

[varocho]

Weird, I tried the first for loop and got the same result but if I change the for statement to

for ($i = 0.001; $i < 0.011 ;$i += 0.001)

then 0.01 does appear at the end of the output.

I wonder if somehow 0.009 + 0.001 somehow ends up 'becoming' 0.010000001 (or something like that) which causes the for loop to terminate.

 

[grimbys]

We think that this success because of the internal representation of numbers of perl. I read about the imposibility of save 0'9 on a variable becouse its representation is 0.111000111000...

Are there any way (casting numbers how C, for example) for this loop can run?
Sorry for my poor english

 

[justice41]

This problem isn't specific to Perl, all computer languages that represent floating point numbers in binary are plagued by it. As a programmer you must be aware of it.

One work-around is to change the test statement as varocho suggested so that you are testing strictly less-than (and not equal to) an upperbound.

Another method would be to set up the list of loop values explicitly.
For example

for $i ( map { $_ / 1000 } 1..10 )
{
    print $i." ";
}

jaa

 

[icrf]

Testing says it ends up being 0.01000000000000001, but printing $i just after the loop still just prints 0.01

Lesson: always increment with integers (generally faster, too). ----------------------------------------------------------------------------------
...but I'm just a C man trying to see the light

 

[tanderso]

Yeah, integers sounds like the way to go. Try something like this...

for ($i = 1; $i <= 10; $i++)
{
    print $i/1000 . &quot; &quot;;
}

I haven't tried it, but I think it should accomplish what you need.
Sincerely,

Tom Anderson
CEO, Order amid Chaos, Inc.

http://www.oac-design.com

 

[grimbys]

Thanks you. When you are working with variables is hard know if you have to divide by 1000, 10000, 100000 or other quantity.

you have to do this:
$divisor = find_divisor ($step);
$start = find_start ($start);
$finish = find_finish ($finish);
for $i ( map { $_ / $divisor } $start..$finish )
{ ... }

and if you have a code with 3000 line-codes and you have which for loops have this problem, I don't know it's bore.
Apart, what about third loop for?
Remember (it is with < ) :
for ($i = 0.01; $i < 0.1 ; $i += 0.01)
{
print $i.&quot; &quot;;
}
# OUT: 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 WITH 0.1!!!!!


Sorry for my bad english

 

[tanderso]

I'm not sure exactly what you were saying in the last post or exactly what those functions accomplished, but try something like this:

my $divisor = find_divisor ($step);
$start = find_start ($start) / $divisor;
$finish = find_finish ($finish) / $divisor;

my $multiplier = 1 / $start;
$start = $start * $multiplier;
$finish = $finish * $multiplier;

for (my $i = $start; $i < $finish; $i++)
{
    print $i/$multiplier . &quot; &quot;;
}

Sincerely,

Tom Anderson
CEO, Order amid Chaos, Inc.

http://www.oac-design.com

 

[grimbys]

Thanks, don't worry. Now I know that this for loop too fails in C++. The thing is only use integer on loops.