比較在foreach語句中分配的變量的問題

Question

我一直在研究一個cgi文件，該文件將檢查用戶是否想要註冊其憑據時是否已經使用了用戶名。如果使用用戶名，它應該通知他們，如果不是，則將其憑據保存到原始平面文件。我在比較foreach語句中賦予值的變量時遇到問題。如果用戶輸入的名稱與已存儲的名稱相同，我告訴foreach將用戶名分配給變量。我有它適當地分配變量，但後話我想告訴它在foreach之外再次比較這些變量，所以操作只能進行一次。這裏是我當前的代碼

#!/usr/bin/perl 
use warnings; 
use strict; 
use CGI qw(:standard); 
use CGI::Carp qw/fatalsToBrowser warningsToBrowser/; 
use Digest::MD5 qw(md5 md5_hex md5_base64); 

#telling what variables are still to be used as global 
our ($username, ,$user, $nametaken); 

#assigning some local variables 
my $username = param("username"); 
my $password = param("password"); 
my $hashpass = md5_hex($password); 

print header, start_html(); 

#creating an array from the flatfile that usernames and passwords are stored 
my @users = do { open my $fh, "<", "password.txt" or die $!; map { chomp; split /:/ } <$fh> }; 

#comparing the values in the array to the username entered 
foreach my $user (@users) { 
if ($user eq $username) { 
    #printing here to test if it is comparing correctly which it is 
    print p("$user\n"); 
    #assigning the $user value to $nametaken so it can be compared to later 
    my $nametaken = $user; 
    #printing here to test if the variable was correctly assigned, which it is 
    print p("$nametaken\n"); 
    } 
} 

#printing here to test if the variable was correctly assigned, which it is not printing 
#so the foreach must be causing some king of issue for this variable after it is done and I don't know what that is 
print p("$nametaken\n"); 

#Here is where I am trying to check if the username already exists and then save the user credentials if it does not 
if ($nametaken eq $username) { 
print p("Username already taken, Try again"); 
} 

#As of now the else statement is running everytime and saving new user credentials even if a username is already taken 
else { 
open my $fh, ">>", "password.txt" or die $!; 
print $fh "$username:$hashpass\n"; 
print p("Your account has been created sucessfully"); 
close $fh; 

} 
print end_html(); 

Answer 1

您聲明一個新的內部你foreach循環的詞法範圍的變量$nametaken - 或者說，if {}塊內：my $nametaken = $user;

，可能與$nametaken變量共享相同的名稱你在外面，但它是一個完全不同的變量，其範圍爲if塊 - 一旦您退出if，該變量就完全被遺忘了。無論你分配給它什麼價值都會丟失。

你可以在這裏看到更多的細節有關詞法變量：

http://perldoc.perl.org/perlsub.html#Private-Variables-via-my%28%29

---

從戰術解決您的問題，您只需從內取出my聲明，如果：$nametaken=$user;

爲了正確地做到這一點，以Perl的方式，你應該重新思考你對問題的方法。您可以使用foreach循環來檢測列表中是否有值，但它絕對不是最好的（可讀性明智，有時甚至是性能明智的）Perl技術。更慣用的方法是使用哈希查找：

my %users = map { ($_ => 1) } @users; # Create a hash with users being keys 
if ($users{$username}) { 
    print "$username already taken!\n"; 
} 

Answer 2

爲什麼$nametaken不具有價值外foreach環是因爲它已被詞法範圍的原因而只內foreach定義。

---

總是有這樣做的方法不止一種：

my ($nametaken) = grep { /$username/ } @users; 

if ($nametaken) { ... } else { ... } 

或者乾脆：

if (grep { /$username/ } @users) { ... } else { ... } 

這是通常當你在Perl較少的臨時變量減少噪音。

Answer 3

my $nametaken = $user; 

創建一個名爲$nametaken一個新變量無關，與你的外循環宣佈$nametaken。