How to check if a string is an integer value.

[MonsterAar]

How do I check that a particular string, being a number, is an integer?

For example, in a program I am wriing i need to do the following two things that involve this:

unless($input is an integer) { blah }

and

if($value /2 is an integer) { blah }

Can anyone help me?

Thankyou,
Luke

 

[rharsh]

See if this helps:

my $num = 5.05;
print "$num is an INT" if $num == int($num);
print "$num is not an INT" if $num != int($num);

 

[MonsterAar]

Thanks rharsh, that works perfectly.

My code now looks like this:

unless($input eq int($input)) {
print "\nYou must enter an integer.\n";
next;
}

and

if($value / 2 eq int($value / 2)) { $value = $value / 2; }
else { $value = ( $value * 3 ) + 1 }

Thanks again,
Luke

 

[MonsterAar]

Sorry to reply again, but I have found that this doesn't work for really large integers.

For example, if I input 99999999999999999999, i get the statement "You must enter an integer.".

I'm using bignum, so I don't know what the problem is.

The highest integer i can put in is 999999999999999, and when I do this, the if statement later in the program doesnt work properly.

This is my program so far. Maybe seeing the whole program will help.

#!/usr/bin/perl -w
#perl2exe_include Math::BigInt
#perl2exe_include Math::BigInt::Calc

#################################################################
# __________                   __               __              #
# \______   \_______  ____    |__| ____   _____/  |_            #
#  |     ___/\_  __ \/  _ \   |  |/ __ \_/ ___\   __\           #
#  |    |     |  | \(  <_> )  |  \  ___/\  \___|  |             #
#  |____|     |__|   \____/\__|  |\___  >\___  >__|             #
#                         \______|    \/     \/                 #
#   ___ ___        .__.__            __                         #
#  /   |   \_____  |__|  |   _______/  |_  ____   ____   ____   #
# /    ~    \__  \ |  |  |  /  ___/\   __\/  _ \ /    \_/ __ \  #
# \    Y    // __ \|  |  |__\___ \  |  | (  <_> )   |  \  ___/  #
#  \___|_  /(____  /__|____/____  > |__|  \____/|___|  /\___  > #
#        \/      \/             \/                   \/     \/  #
#                                                               #
#################################################################

#hailstone.pl : Checks truth of Hailstone Sequences
#Written by Luke Aaron
#Version B 14/08/06

use bignum;

$loop = 1;
$delq = 0;

while($loop eq 1) {
  print "\nStart run (R) or quit(Q): ";
  $cmd = <STDIN>;
  chomp($cmd);
  $cmd =~ tr/A-Z/a-z/;
  if($cmd eq 'q') { last }
  unless($cmd eq 'r') {
    print "\nYou must enter either R or Q.\n";
    next;
  }
  $loop = 2;
}

while($loop eq 2) {
  print "\nInput starting integer: ";
  $input = <STDIN>;
  chomp($input);
  unless($input eq int($input)) {
    print "\nYou must enter an integer.\n";
    next;
  }
  $loop = 3;
}

while($loop eq 3) {
  $value = $input;
  chdir('C:\\');
  if($delq eq '1') {
    unlink('hailstone.txt');
  }

  $delq = 1;
  $frun = 1;

  while($value ne 1) {
    unless($frun eq 1) {
      if($value / 2 eq int($value / 2)) { $value = $value / 2; }
      else { $value = ( $value * 3 ) + 1 }

      $file = '<hailstone.txt';
      open(FILE, $file);

      #Search file for $value.
      #If found, print  "False for $input\n", stop program, but do not delete file.

      close(FILE);
    }

    $file = '>>hailstone.txt';
    open(FILE, $file);

    print FILE "$value\n";

    close(FILE);

    $frun = 0;
  }

  print "True for $input\n";
  $input = $input + 1;
}

print  "Program terminated.\n"

 

[KevinADC]

'ne' and 'eq' are string operators, not numerical operators. If you are using numbers use != and == instead of 'ne' and 'eq'.

If you want to make sure only whole numbers are entered use a regexp to check:

while($loop == 2) {
  print "\nInput starting integer: ";
  $input = <STDIN>;
  chomp($input);
  [b]unless($input =~ /^\d+$/) {[/b]
    print "\nYou must enter an integer.\n";
    next;
  }
  $loop = 3;
}

if it's possible to have negative numbers use:

unless($input =~ /^-?\d+$/) {

 

[MonsterAar]

Thanks KevinADC, that works for the unless part, but not in the if statement later.

if($value / 2 =~ /^\d+$/) { $value = $value / 2; }
      else { $value = ( $value * 3 ) + 1 }

is what I have. Is that the correct use of the regexp? if not how do I implement it there? If it is not possible to use it in this way, what is a better way of achieving what I want? ie when the number is even, divide it by two and if it is odd, multiply it by three and then add 1?

Thanks,
Luke

 

[KevinADC]

use the modulus operator '%' to check for eveness or oddness of numbers:

if ($value % 2 == 0) { 
   even number
}
else {
   odd number
}

modulus returns the remainder of division, and since perl treats 0 as false you can do something like this:

if ($value % 2) {
   odd number
}
else {
   even number
}

if your script is only interested in using integers, you may also be interested in the 'integer' pragma:

http://perldoc.perl.org/integer.html

 

[MonsterAar]

Thanks Kevin,
I forgot about the Modulo divide operator.
My program now works exactly how I'd like it, just one bit of code to work out now!

Thanks all,
Luke