Strange Logic in Power of arithmetic

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

I came across a funny thing when the input to a calculation involving the exponential operator turned negativ.

To demonstrate - here is some code:

print "Power of calculation test\n";
print("line 1: ", -4**1.5, "\n");
print("line 2: ", (-4)**1.5, "\n");
print("line 3: ", 1/(-4)**1.5, "\n");
print("line 4: ", 3 + (-4)**1.5, "\n");
[download]

The output looked like this:

Power of calculation test
line 1: -8
line 2: 0
line 3: 0
line 4: 0
[download]

So, the line 1 output is probably explained by precedence.

However, if line 2 is a permissible operation - how does it get to zero?

And why (line 3) can I use the result as a divisor without Perl complaining about division by zero or add 3 and still get zero?

It cost me a few hours to find why my numbers did not add up - shouldn't (-4)**1.5 be an illegal statement?

Appreciate your wisdom...

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Re: Strange Logic in Power of arithmetic
by Athanasius (Archbishop) on Jan 12, 2016 at 13:11 UTC

Hello stephanm,

What version of Perl are you using? On v5.14.4 and v5.20.2 I get this:

23:07 >perl 1512_SoPW.pl
Power of calculation test
line 1: -8
line 2: -1.#IND
line 3: -1.#IND
line 4: -1.#IND

23:07 >
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On v5.22.0 and v5.22.1 I get:

23:06 >perl 1512_SoPW.pl
Power of calculation test
line 1: -8
line 2: NaN
line 3: NaN
line 4: NaN

23:06 >
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— as you expected.

Update: I’m using Strawberry Perl on Windows 8.1, 64-bit.

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^2: Strange Logic in Power of arithmetic

by choroba (Cardinal) on Jan 12, 2016 at 13:18 UTC

line 1: -8
line 2: nan
line 3: nan
line 4: nan
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line 1: -8
line 2: -nan
line 3: -nan
line 4: -nan
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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Re^2: Strange Logic in Power of arithmetic

by hotchiwawa (Scribe) on Jan 12, 2016 at 13:22 UTC

Power of calculation test
line 1: -8
line 2: -1.#IND
line 3: -1.#IND
line 4: -1.#IND
[download]

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Re: Strange Logic in Power of arithmetic
by Your Mother (Archbishop) on Jan 12, 2016 at 13:50 UTC

I want to be in the pageant! Perl 6.c on Darwin.

moo@cow[1102]~>perl6
> print("line 1: ", -4**1.5, "\n");
print("line 2: ", (-4)**1.5, "\n");
print("line 3: ", 1/(-4)**1.5, "\n");
print("line 4: ", 3 + (-4)**1.5, "\n");
> line 1: -8
> line 2: NaN
> line 3: NaN
> line 4: NaN
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Re: Strange Logic in Power of arithmetic
by ExReg (Priest) on Jan 12, 2016 at 15:59 UTC

Active State 5.8.8 on Win64

line 1: -8
line 2: -1.#IND
line 3: Illegal division by zero at -e line 1
line 4: -2147483645
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Had to do each one line by line as it failed as a script on line 3 otherwise

version 5.6.1 on Solars

line 1: -8
line 2: NaN
line 3: NaN
line 4: NaN
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If I type them in on Windows as perl -e "print -4**1.5;"..., I get

-8
-4
Illegal division by zero at -e line 1
-2147483645
[download]

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Re^2: Strange Logic in Power of arithmetic

by 1nickt (Canon) on Jan 12, 2016 at 16:03 UTC

Active State 5.8.8 on Win64 line 1: -8 line 2: -1.#IND line 3: Illegal division by zero at -e line 1 line 4: -2147483645
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$ perlbrew exec perl 1152551.pl 
perl-5.8.8
==========
Power of calculation test
line 1: -8
line 2: nan
line 3: nan
line 4: nan
[download]

The way forward always starts with a minimal test.

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Re^3: Strange Logic in Power of arithmetic

by ExReg (Priest) on Jan 12, 2016 at 16:18 UTC

Win 7 64 bit, ActiveState 5.8.8 built for MSWin32-x86-multithread Binary build 817

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Re: Strange Logic in Power of arithmetic
by VinsWorldcom (Prior) on Jan 12, 2016 at 14:50 UTC

Windows 7 x64 with Strawberry 5.18.1 64-bit.

VinsWorldcom@C:\Users\VinsWorldcom\tmp> perl -v

This is perl 5, version 18, subversion 1 (v5.18.1) built for MSWin32-x
+64-multi-thread

[...]

VinsWorldcom@C:\Users\VinsWorldcom\tmp> test.pl
Power of calculation test
line 1: -8
line 2: -1.#IND
line 3: -1.#IND
line 4: -1.#IND
[download]

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Re^2: Strange Logic in Power of arithmetic

by stephanm (Sexton) on Jan 27, 2016 at 12:08 UTC


This is perl 5, version 16, subversion 3 (v5.16.3) built for MSWin32-x
+86-multi-thread
(with 1 registered patch, see perl -V for more detail)

Copyright 1987-2012, Larry Wall

Binary build 1604 [298023] provided by ActiveState http://www.ActiveSt
+ate.com
Built Apr 14 2014 14:32:20
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Vista Home Premium, Service Pack 2, 32 bit architecture.
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