http://www.perlmonks.org?node_id=989020

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I want to create a simple output ranging from 0 to zmax in distinct steps. Those steps shall get larger towards the end. Here's my simple script: 
my $zmax = 60;    # Maximum depth in your model
my $dz = 0.05;    # Grid spacing in z-direction

for (my $i=0; $i <= $zmax; $i = $i+$dz) {
   print $i."\n";
}

[download]
And here's the output at a later stage: 
3.6
3.65
3.69999999999999
3.74999999999999

[download]
That's not the correct value !! This prevents me from making a direct comparison with an integer number (which I can find ways around), but the result is simply wrong!! What's happening there?

Replies are listed 'Best First'.
Re: Wrong calculations in for loop
by toolic (Bishop) on Aug 22, 2012 at 13:02 UTC
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Re: Wrong calculations in for loop
by MidLifeXis (Monsignor) on Aug 22, 2012 at 13:01 UTC
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Re: Wrong calculations in for loop
by BrowserUk (Patriarch) on Aug 22, 2012 at 13:50 UTC

    Simple solution:

    for( my $i = 0; $i < 6000; $i += 5 ) { 
    say $i / 100 
};;
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
...
3.5
3.55
3.6
3.65
3.7
3.75
3.8
3.85
3.9
3.95
4
...

    [download]
    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.

    The start of some sanity?
[reply]
[d/l]
Re: Wrong calculations in for loop
by moritz (Cardinal) on Aug 22, 2012 at 13:10 UTC

    Luckily you can run the script with only very small changes under Perl 6, and it produces the output you expect: 

    use v6;
my $zmax = 60;    # Maximum depth in your model
my $dz = 0.05;    # Grid spacing in z-direction

loop (my $i=0; $i <= $zmax; $i = $i+$dz) {
    say $i;
}

    [download]

    produces 

    ...
3.6
3.65
3.7
3.75
...
59.9
59.95
60

    [download]

    It accomplishes that magic by doing math with rationals (stored as integer numerator and denominator) by default.

    Perl 6 - the future is here, just unevenly distributed
[reply]
[d/l]
[select]

      OK, but getting the same result in Perl 5 is almost as easy:

      #! perl
use strict;
use warnings;

my $zmax = 60;      # Maximum depth in your model
my $dz   =  0.05;   # Grid spacing in z-direction

print +($_ * $dz), "\n" for (0 .. int(($zmax / 0.05) + 0.5));

      [download]

      :-)

      Athanasius <°(((>< contra mundum

[reply]
[d/l]

        Or even simpler, just change the print to printf 

        my $zmax = 60;    # Maximum depth in your model
my $dz = 0.05;    # Grid spacing in z-direction

for (my $i=0; $i <= $zmax; $i = $i+$dz) {
   printf("%.2f\n", $i);
}

        [download]

[reply]
[d/l]
[select]
Re: Wrong calculations in for loop
by Anonymous Monk on Aug 22, 2012 at 13:08 UTC
[reply]

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