WeBWorK Problems

array shuffle woes

array shuffle woes

by Zak Zarychta -
Number of replies: 1
I have an array shuffle sub routine

# shuffle of @array in place
sub fisher_yates_shuffle {
$array = shift;
for ($i = @$array; --$i; ) {
$j = int random(0,$i+1);
next if $i == $j;
@$array[$i,$j] = @$array[$j,$i];

that shuffles the following array of values

$ohmD = "{\rm \Omega}";
$ohmU = "ohm";
$ampD = "{\rm A}";
$ampU = "amp";
$voltD = "{\rm V}";
$voltU = "volt";

@valArr = (#

I shuffle the array then pop off the last three sub arrays and push onto another array. This so called unknown array @valArr_unkn contains the values the students are asked to calculate. This is done with the following code

fisher_yates_shuffle( ~~@valArr ); # permutes array in place
# pop of last three values of known and push onto unknown array
@valArr_unkn = ();
$size = scalar @valArr_unkn;
while ($size < 3) {
@popped = pop(@valArr);
push(@valArr_unkn, @popped);
$size = scalar @valArr_unkn;

whilst this works "most" of the time, for one particular random seed (20109 and I'm sure others), $valArr_unkn[0] and $valArr_unkn[2] contain the four values under the sub-indices 0-3, but the four values of $valArr_unkn[1] are empty leading to errors.

What might be wrong? How might I fix it?

Thanks in advance for any help with this
In reply to Zak Zarychta

Re: array shuffle woes

by Zak Zarychta -
Thankfully, I finally scratched this itch.

It came down to a difference in how the perlrand function and the WeBWorK random function treated the right limit. When (from the original perl subroutine) I translated the line

$j = int(rand(0,$i+1)); #random number between 0 and $i


$j = random(0,$i+1); #random number between 0 and $i+1

this occasionally (when $i+1 was selected) and would shuffle an empty value onto the array.