Mysterious "uninitialized value in an array" in algorithm for Perl challenge_问答_开发者

Currently learning Perl, and trying to solve a little challenge to find the sum of even terms from the first 4,000,000 Fibonacci terms. I have created a Fibonacci array that seems to work, and then tried different methods to throw out the odd-valued terms, and continually run into an error when trying to sum my resulting array, getting reports of:

Use of uninitialized value in addition (+) at prob2_3.plx line 23

Here is what I have:

#!/usr/bin/perl -w
# prob2_2.plx
use warnings;
use strict;

my @fib; my $i; my $t; my $n;
@fib = (1, 2);



for ($i=2; $i<4000000; $i++)  {
  my $new= ( $f开发者_运维问答ib[$i-1] + $fib[$i-2] );
  push @fib, $new;}


for ($t=3; $t<4000000; $t++)  {
  if (($fib[$t] % 2) != 0 ) {
  delete $fib[$t];  }  }

my $total = 0;

for ($n=1; $n<$#fib; $n++) {
$total += $fib[($n+1)];}  

print  $total;

The warning means you are adding undef to something. delete $fib[$t]; is a bad way of doing $fib[$t] = undef;, which you later add to $total.

You have at least one other error:

The first two Fibonacci numbers are 0 and 1, not 1 and 2.

You have a major problem:

The 4,000,000th Fib number is going to be extremely large, much too large to fit in a double.

For reference purposes,

10,000th has 2090 digits: 20793608237133498072112648988642836825087036094015903119682945866528501423455686648927456034305226515591757343297190158010624794267250973176133810179902738038231789748346235556483191431591924532394420028067810320408724414693462849062668387083308048250920654493340878733226377580847446324873797603734794648258113858631550404081017260381202919943892370942852601647398213554479081823593715429566945149312993664846779090437799284773675379284270660175134664833266377698642012106891355791141872776934080803504956794094648292880566056364718187662668970758537383352677420835574155945658542003634765324541006121012446785689171494803262408602693091211601973938229446636049901531963286159699077880427720289235539329671877182915643419079186525118678856821600897520171070499437657067342400871083908811800976259727431820539554256869460815355918458253398234382360435762759823179896116748424269545924633204614137992850814352018738480923581553988990897151469406131695614497783720743461373756218685106856826090696339815490921253714537241866911604250597353747823733268178182198509240226955826416016690084749816072843582488613184829905383150180047844353751554201573833105521980998123833253261228689824051777846588461079790807828367132384798451794011076569057522158680378961532160858387223882974380483931929541222100800313580688585002598879566463221427820448492565073106595808837401648996423563386109782045634122467872921845606409174360635618216883812562321664442822952537577492715365321134204530686742435454505103269768144370118494906390254934942358904031509877369722437053383165360388595116980245927935225901537634925654872380877183008301074569444002426436414756905094535072804764684492105680024739914490555904391369218696387092918189246157103450387050229300603241611410707453960080170928277951834763216705242485820801423866526633816082921442883095463259080471819329201710147828025221385656340207489796317663278872207607791034431700112753558813478888727503825389066823098683355695718137867882982111710796422706778536913192342733364556727928018953989153106047379741280794091639429908796650294603536651238230626

20,000th has 4180 digits: 1564344347109763849734765364072743458162050946855915883181245417404580803852433819127477934504143316103671237797087184052487157589846395314335101792632666883301188491698850377253383735812017943059782268835280360618754466932406192674904182868594738499500415166599602737300793712012046275485369495600019495004126039595217556097603510836899682827827626851274417838565958464881549888154511565687715162081527027421167926710592169405764372872023265791851279526521097739802047796738013885512616267273220024096214780132567479711643567372517808245262560562426651659391013837988476506124649092538307827326285964637268328029765707984607120961599796336714632362497169952413163370558311283612961033588836334352432860332222878648950508154331165678617373097939647648015552782638392654938551724289386017566932982065441392025369213734676739845068956966278536757235977421127565055467060906533383001625925978595472181091151062798507286798754728450358266089744616465914255799764431508559485853637841082521780322710748029546001980460990695999087046617731317608498316428164179967150350939374702201821818895349621858954893061034598954341939850973673870946183079728029105624782161827626661367017673681922257604178810154438462080217794489109678386881153826838075832058191153133704042628156419344516917867369755345135618986917642004521509538436204298618130363401395547933177643760161135638357088649014469358006518300404036431113143777969391584246934245800739809135619744598808977628245309941537928439431608665523308894967310600529498446943933665468406306292762942409786097847875240014036353917928156220446650579514092031254308059314931618726692376640987446459276331196950780063664171751110087644649773058213117640640085100552927878404516279461437503857017398937097042607258059612257878307007002086913210922626760728342901272768408974906007921227446242552261362505471751722906558235533709070548109789519920405521647836164156675304784097782435865165640401897107828859121831521126567446611716077075769257072773697947064329836969249852382976202348037425889031090020976240691949742160088733357875561841760194799534815496104106903184713919847662253483806138312440578732122855388348848736018217032877013531004653902335692761900988709302797685265501972628217528866551995479526195626503247164073793787381643388365618488630255600890924552511767690989186316859159306438477097458585889829326938198129884953178437411315486719927412151054551726325421747462698125767761987300812744880048122138953746796038485281452086680809803469350470844184375258620810652745992631459076192613797545486775651410699327289089628593588395142531659083933746399666161863597357735290387376161440280731398703030590410957840047591721635117677190494658658256770952605314604687704388833897300447300322491720569722311756874534871145435101596346787454258165870310592717473670917638475152605474446188958081898150393481484970581519902582271877141251593259282483539345792009117894084860435326938689664322383123823631494470354941767039585133484331342468806167901166928052638999423311570618981137348891538818027216596300491989181231598151123614651043205656474490923109982595235880446420678700336717534914381729578113169753046083981752465156933790288020841880688083888166659362896648911608716373579944854235997384986302902608821566689026676371268703303207406827737925274781301986480762462594420398637607893961010824979395439225300832931626540179218558345947558472159906873998923767432504278838419479068093778976997276416592421223235719653905071392295735398272851826350645605643470417155719500185143594804374322010189545136205568856276559806316789533450612097900180399440915139647060459321993254566103255011590902408116018722996267956826555434955409390951728022815209412027248353062982911544674007147249326697275010788100666958314965810320432736615962898175585320993128871046552842068867557341007383399180807449030159797672605530835244157256109268527578172314358255179589605335375414082046575557122636364391407861922824529441261003866098066404526541912783214030236752423547997110159548536582622929575859635210831021463323632502412193578592457118234067116894159316798758933206918936334540039454055299101076302263831614132510576874528929742319396129011617501

Even the 10,000th is too large for a double, and the 20,000th is double the size of the 10,000th, so imagine how large the 4,000,000th will be!

Stylistic issues:

my $i; for ($i=2; $i<4000000; $i++)

is much harder to read than

for my $i (2..$N-1)

with the following at the top to avoid having to repeat the number everywhere:

my $N = 4_000_000;

As if the fact that the 4 millionth Fibonacci number is more than 10^835950 isn't a big enough problem, this is not very good:

for ($t=3; $t<4000000; $t++)  {
  if (($fib[$t] % 2) != 0 ) {
  delete $fib[$t];  }  }

my $total = 0;

for ($n=1; $n<$#fib; $n++) {
$total += $fib[($n+1)];}

Why are you walking through the list twice here? Much better would be to combine the two loops into one. You want the sum of the odd terms, so sum the odd terms. Don't delete the odd terms (stylistically very bad) and then walk over the list again, relying on the fact that undef has a numerical value of 0 (but only with a warning).

And mn, the formatting of that code is very, very ugly. Eventually you will write code that someone else needs to read or maintain. My motto: Imagine that the person who will maintain your code is a psychopath who knows where you live.

As ikegami points out, your uninitialized problem is assuming delete removes elements from an array, when in fact it just sets them to undef (unless they are at the end of the array).

Given the storage requirements of the larger Fibonacci numbers, you don't want them in an array at all; fortunately, there's no need to keep them around for this problem. I would do it like this (takes many minutes to run):

use strict;
use warnings;
use Math::BigInt 'lib' => 'GMP';

my $fib_A = Math::BigInt->new(0);
my $fib_B = Math::BigInt->new(1);
my $sum = Math::BigInt->new(0);

# get the next 3999998
for (1..(4000000-2)) {
    my $next = $fib_A + $fib_B;
    $sum += $next if $next % 2 == 0;
    ($fib_A, $fib_B) = ($fib_B, $next);
}

print "The sum is $sum\n";