比较在foreach语句中分配的变量的问题

Question

我一直在研究一个cgi文件，该文件将检查用户是否想要注册其凭据时是否已经使用了用户名。如果使用用户名，它应该通知他们，如果不是，则将其凭据保存到原始平面文件。我在比较foreach语句中赋予值的变量时遇到问题。如果用户输入的名称与已存储的名称相同，我告诉foreach将用户名分配给变量。我有它适当地分配变量，但后话我想告诉它在foreach之外再次比较这些变量，所以操作只能进行一次。这里是我当前的代码

#!/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

您声明一个新的内部你foreach循环的词法范围的变量$nametaken - 或者说，if {}块内：my $nametaken = $user;

，可能与$nametaken变量共享相同的名称你在外面，但它是一个完全不同的变量，其范围为if块 - 一旦您退出if，该变量就完全被遗忘了。无论你分配给它什么价值都会丢失。

你可以在这里看到更多的细节有关词法变量：

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

---

从战术解决您的问题，您只需从内取出my声明，如果：$nametaken=$user;

为了正确地做到这一点，以Perl的方式，你应该重新思考你对问题的方法。您可以使用foreach循环来检测列表中是否有值，但它绝对不是最好的（可读性明智，有时甚至是性能明智的）Perl技术。更惯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

为什么$nametaken不具有价值外foreach环是因为它已被词法范围的原因而只内foreach定义。

---

总是有这样做的方法不止一种：

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

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

或者干脆：

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

这是通常当你在Perl较少的临时变量减少噪音。

Answer 3

my $nametaken = $user; 

创建一个名为$nametaken一个新变量无关，与你的外循环宣布$nametaken。