How can I ask the user to re-enter the input when they enter the wrong input in Perl?

Question

I have a Perl routine that requests input from a user. I am checking inside the subroutine itself whether the input I entered is valid.

If this is not the case, I want to call the subroutine again to allow the user to enter valid input this time.

My routine is as follows:

sub some_routine {    
    print "Enter a number to select   (1) Apple (2) Mango (3) grapes:"
    $value=STDIN;
    if($value =~ /[^1-3]/ ) {
        print "The input is not valid!";
        print "Do you want to continue selecting a fruit again (Y or N)?";
        $choice = STDIN;
        if( $choice eq "y") {
            ### I want to call the subroutine again to enter input ###
          } else {
            exit;
        }
    }
}

So how to adjust the subroutine in this?

Answer 1

To call a subroutine recursively in Perl, you simply call sub from yourself, just like in any other language:

sub factorial {
  my $num = shift;
  return 1 if $num < 2;
  return $num * factorial($num - 1);
}

" ".
, while :

my $valid;
while (!$valid) {
  print "Enter something: ";
  my $data = <STDIN>;
  $valid = validate($data);
  print "Bzzt!  Invalid - try again!\n" unless $valid;
}

Answer 2

. while .

my $input_valid = 0;
while( !$input_valid ) { 
    print "Enter some input: ";
    my $input = <STDIN>;
    $input_valid = validate_input( $input );
}

validate_input 0, .

Answer 3

goto &some_routine;

... - , . , , . , . , , ,

return some_routine(@_);

.

, , while, ( , , ).

Answer 4

sub select_fruit {    
    print "Enter a number to select   (1) Apple (2) Mango (3) grapes:"
    $value=<STDIN>;
    if($value =~ /[^1-3]/ ) {
        print "The input is not valid!";
        print "Do you want to continue selecting a fruit again (Y or N)?";
        $choice = <STDIN>;
        if( $choice eq "y") {
            $value = select_fruit();
          } else {
            exit;
        }
    }
    return $value;
}

goto - (TCO)

sub select_fruit {
    print "Enter a number to select   (1) Apple (2) Mango (3) grapes:"
    $value=<STDIN>;
    if($value =~ /[^1-3]/ ) {
        print "The input is not valid!";
        print "Do you want to continue selecting a fruit again (Y or N)?";
        $choice = <STDIN>;
        if( $choice eq "y") {
            goto &select_fruit;
          } else {
            exit;
        }
    }
    return $value;
}

sub select_fruit {
SELECT_FRUIT: {
       print "Enter a number to select   (1) Apple (2) Mango (3) grapes:"
       $value=<STDIN>;
       if($value =~ /[^1-3]/ ) {
           print "The input is not valid!";
           print "Do you want to continue selecting a fruit again (Y or N)?";
           $choice = <STDIN>;
           if( $choice eq "y") {
               redo SELECT_FRUIT; # same as goto SELECT_FRUIT;
             } else {
               exit;
            }
        }
        return $value;
    }
}

...

Answer 5

: (, ..), , , , while loop.

[ ] " , , , while, , ".

, , :

sub get_positive_number {
    print "Please enter a positive number: ";
    my $number = <STDIN>;
    chomp $number;
    if ($number > 0) {
        return $number;
    }
    else {
        return get_positive_number();
    }
}

my $result = get_positive_number();
print "result: $result\n";

Answer 6

my $value;
until(defined $value = get_value()) {
  print"you didn't enter a valid value\n";
}

sub get_value {
 print "Enter a number to select   (1) Apple (2) Mango (3) grapes:"
    $value=<STDIN>;
    if($value =~ /[1-3]/ ) {
        return $value;
    } else {
        return undef;     
    }
}

Answer 7

IO::Prompt.

:

use IO::Prompt;
my @choices = qw( Apple Mango Grapes );
my $answer = prompt("Select :", "-menu" => \@choices);
print $answer;

Answer 8

eq "y", "y\n".

if ($choice =~ /^[Yy]/), Y y Y .

<STDIN> STDIN. use strict; use warnings; . , $value $choice, :

my $value = '';
my $choice = '';

. , , , .

#!/usr/bin/env perl
use strict;
use warnings;

some_routine();

sub some_routine {    
    my $value = '';
    my $choice = '';
    print "Enter a number to select   (1) Apple (2) Mango (3) grapes:";
    $value = <STDIN>;
    if($value !~ /[1-3]/ ) {
        print "The input is not valid!";
        print "Do you want to continue selecting a fruit again (Y or N)?";
        $choice = <STDIN>;
        if( $choice =~ /[Yy]/) {
            some_routine();
          } else {
            exit;
        }
    }
}

Answer 9

What I'm using is a simple transition:

START:
print "\n Name of the country (any one out of: china, japan or tokyo): ";
my $country_name = <>;
chomp($country_name);

unless ($country_name eq "china" || $country_name eq "japan" ||
    $country_name eq "tokyo") {
    print "\n Invalid country name entered. Please try again. \n";
    goto START;
}

This is a very naive way, but works for beginners.