R – How to create and access a Perl hash with scalar keys whose values are arrays

arraysperlreference

In Perl 5.10, how do I create and access a hash with scalar keys whose values are arrays?

#Doing this does not work properly.
%someHash= ("0xfff" => ('Blue', 'Red', 'Yellow'));
@arr = @fileContents{"0xfff"};
print @arr;

When I print the array, the only thing that prints is "ARRAY('randmemAddr')". When I do a foreach loop on @arr, only the first element is printed. I have then concluded that I have not stored the array properly in the hash.

Best Answer

My original answer posted working code, but didn't really explain the problem with yours. This is expanded a bit to correct that. Your example had two problems. First, you had a problem when making the reference. You need to use [ ] instead of the standard parentheses in order to create a reference (to an anonymous array). Second, when you tried to get at the reference, you left off one set of brackets. You want to put the reference itself inside @{ } in order to get at the whole array. (Also, and this may be a typo: you have no $ before filecontents.)

The code here is essentially from perldoc perldsc. I highly recommend it. Also very useful if you're new to references in Perl is perldoc perlreftut. Both tutorials discuss how to make and get at references in a variety of situations. Finally, you can find a good cheat sheet for references in a post on PerlMonks.

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


my %HoA = (
    flinstones      => [ qw/fred barney/ ],
    jetsons         => [ qw/george jane elroy/ ],
);

for my $family (keys %HoA) {
    print "Members of the \u$family:\n";
    print "\t @{ $HoA{$family} }\n";
}

ES5 version use `Array.concat`:

var array1 = ["Vijendra", "Singh"];
var array2 = ["Singh", "Shakya"];

console.log(array1.concat(array2));

ES6 version use destructuring

const array1 = ["Vijendra","Singh"];
const array2 = ["Singh", "Shakya"];
const array3 = [...array1, ...array2];

Since there is no 'built in' way to remove duplicates (ECMA-262 actually has Array.forEach which would be great for this), we have to do it manually:

Array.prototype.unique = function() {
    var a = this.concat();
    for(var i=0; i<a.length; ++i) {
        for(var j=i+1; j<a.length; ++j) {
            if(a[i] === a[j])
                a.splice(j--, 1);
        }
    }

    return a;
};

Then, to use it:

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];
// Merges both arrays and gets unique items
var array3 = array1.concat(array2).unique();

This will also preserve the order of the arrays (i.e, no sorting needed).

Since many people are annoyed about prototype augmentation of Array.prototype and for in loops, here is a less invasive way to use it:

function arrayUnique(array) {
    var a = array.concat();
    for(var i=0; i<a.length; ++i) {
        for(var j=i+1; j<a.length; ++j) {
            if(a[i] === a[j])
                a.splice(j--, 1);
        }
    }

    return a;
}

var array1 = ["Vijendra","Singh"];
var array2 = ["Singh", "Shakya"];
    // Merges both arrays and gets unique items
var array3 = arrayUnique(array1.concat(array2));

For those who are fortunate enough to work with browsers where ES5 is available, you can use Object.defineProperty like this:

Object.defineProperty(Array.prototype, 'unique', {
    enumerable: false,
    configurable: false,
    writable: false,
    value: function() {
        var a = this.concat();
        for(var i=0; i<a.length; ++i) {
            for(var j=i+1; j<a.length; ++j) {
                if(a[i] === a[j])
                    a.splice(j--, 1);
            }
        }

        return a;
    }
});

Best Answer

Related Solutions

R – Why do you need $ when accessing array and hash elements in Perl

Javascript – How to merge two arrays in JavaScript and de-duplicate items

ES5 version use Array.concat:

ES6 version use destructuring

Related Topic

ES5 version use `Array.concat`: