Why does this random hash value selector always return the same value the first time it's called?

Question

While trying to understand how refp's solution for random hash value selection works, I noticed something strange.

With repeated calls to the following Perl script, I consistently found that the first result returned was the same. The subsequent values returned were random:

use strict;
use warnings;
use 5.010;

my %hash = map { $_ => ord $_ } 'a' .. 'z';

say( (@_=%hash)[1|rand@_] ) for 1 .. 10;       # First value always 119

Interestingly, the following does not suffer from this issue:

sub random_value { ( @_ )[ 1 | rand @_ ] }

say random_value %hash for 1 .. 10;            # First value is random

Removing the references to @_ also remedies the problem:

say( (%hash)[1|rand keys %hash] ) for 1 .. 10; # First value is random

This has been tested on Windows (ActivePerl 5.14.2).

On the surface, it looks like setting @_ has something to do with it, but I'm not sure. Can anyone shed some light on what's happening here?

---

EDIT

I thought this question was answered until refp provided an update. Why does the arrayref form not suffer from the same issue discussed above? :

[@_=%hash]->[1|rand@_] for 1 .. 10;            # First value is random

Answer 1

I suspect there is a race condition where @_ is not defined in the first loop iteration.

say( (@_=%hash)[1|rand@_] ) for 1 .. 10; 

Will become

say( (@_=%hash)[1|rand ()] ) for 1 .. 10; 

It escapes warnings because @_ is a predeclared variable. As you will notice:

say( (my @a=%hash)[1|rand@a] ) for 1 .. 10;   

Will crash and burn because @a is not defined in the postscript.

Update:

[@_=%hash]->[1|rand@_] for 1 .. 10;    

Is not any different. It is still bad practice to use a variable in the same statement that you assign it. The difference, I am guessing, is that the precedence is somewhat altered, so that the assignment is evaluated first.

Answer 2

No race condition, or anything to do with whether @_ is "defined" or not, just an order of operations issue.

The indexes for a list slice are evaluated before the list being sliced. (The documentation doesn't guarantee this one way or the other.) So on the first iteration, @_ is empty and the argument to rand is 1|0 (= 0). Historically, rand(0) has behaved like rand(1), though this is now documented as subject to change. So the index on the first iteration is >= 0 and < 1, and taken to be 0 by the implicit int of indexing.

The array element fetch ([@_=%hash]->[1|rand@_]) doesn't suffer a similar problem because it evaluates the index after the array operand. An array slice (@{[@_=%hash]}[1|rand@_]), on the other hand, behaves as the list slice does.

Compare:

List slice:

$ perl -MO=Concise,-exec -e'(@_=%hash)[1|rand@_]'
1  <0> enter 
2  <;> nextstate(main 1 -e:1) v:{
3  <0> pushmark s
4  <$> const[IV 1] s
5  <#> gv[*_] s
6  <1> rv2av[t7] sK/1
7  <1> rand[t8] sK/1
8  <2> bit_or[t9] sK
9  <0> pushmark s
a  <0> pushmark s
b  <#> gv[*hash] s
c  <1> rv2hv[t4] lK/1
d  <0> pushmark s
e  <#> gv[*_] s
f  <1> rv2av[t2] lKRM*/1
g  <2> aassign[t5] lKS/COMMON
h  <2> lslice vK/2
i  <@> leave[1 ref] vKP/REFC
-e syntax OK

Array slice:

$ perl -MO=Concise,-exec -e'@{[@_=%hash]}[1|rand@_]'
1  <0> enter 
2  <;> nextstate(main 2 -e:1) v:{
3  <0> pushmark s
4  <$> const[IV 1] s
5  <#> gv[*_] s
6  <1> rv2av[t8] sK/1
7  <1> rand[t9] sK/1
8  <2> bit_or[t10] sK
9  <0> pushmark s
a  <0> pushmark s
b  <#> gv[*hash] s
c  <1> rv2hv[t4] lK/1
d  <0> pushmark s
e  <#> gv[*_] s
f  <1> rv2av[t2] lKRM*/1
g  <2> aassign[t5] lKS/COMMON
h  <@> anonlist sK*/1
i  <1> rv2av[t6] sKR/1
j  <@> aslice vK
k  <@> leave[1 ref] vKP/REFC
-e syntax OK

Array element:

$ perl -MO=Concise,-exec -e'[@_=%hash]->[1|rand@_]'
1  <0> enter 
2  <;> nextstate(main 1 -e:1) v:{
3  <0> pushmark s
4  <0> pushmark s
5  <#> gv[*hash] s
6  <1> rv2hv[t4] lK/1
7  <0> pushmark s
8  <#> gv[*_] s
9  <1> rv2av[t2] lKRM*/1
a  <2> aassign[t5] lKS/COMMON
b  <@> anonlist sK*/1
c  <1> rv2av[t10] sKR/1
d  <$> const[IV 1] s
e  <#> gv[*_] s
f  <1> rv2av[t7] sK/1
g  <1> rand[t8] sK/1
h  <2> bit_or[t9] sK
i  <2> aelem vK/2
j  <@> leave[1 ref] vKP/REFC
-e syntax OK