What is the difference between push and unshift in Perl?

0 votes
asked Feb 19, 2010 by abhijit-k-rao

Can someone please explain why push behaves the way as shown below?

Basically I am trying to print values of an array populated by push as well unshift.

When I try to print array contents populated by push using array indexes, It always prints the element at the top of the array, whereas array populated by unshift prints contents of array based on array index. I don't understand why.

with unshift

#!/usr/bin/perl
@names = ("Abhijit","Royal Enfield","Google");
@numbers=();
$number=1;
$i=0;
foreach $name (@names) {
    #print $_ . "\n";
    $number=$number+1;
    #push(@numbers,($number));
    unshift(@numbers,($number));
    print("Array size is :" . @numbers . "\n");
    $i=$i+1;
    print("Individual Elements are:" . @numbers[i] . "\n");
    pop(@numbers);
}

rhv:/var/cl_ip_down>./run.sh
Array size is :1
Individual Elements are:2
Array size is :2
Individual Elements are:3
Array size is :3
Individual Elements are:4

without unshift

#!/usr/bin/perl
@names = ("Abhijit","Royal Enfield","Google");
@numbers=();
$number=1;
$i=0;
foreach $name (@names) {
    #print $_ . "\n";
    $number=$number+1;
    push(@numbers,($number));
    #unshift(@numbers,($number));
    print("Array size is :" . @numbers . "\n");
    $i=$i+1;
    print("Individual Elements are:" . @numbers[i] . "\n");
}

rhv:/var/cl_ip_down>./run.sh
Array size is :1
Individual Elements are:2
Array size is :2
Individual Elements are:2
Array size is :3
Individual Elements are:2

/without pop/

#!/usr/bin/perl
@names = ("Abhijit","Royal Enfield","Google");
@numbers=();
$number=1;
$i=0;
foreach $name (@names) {
    #print $_ . "\n";
    $number=$number+1;
    #push(@numbers,($number));
    unshift(@numbers,($number));
    print("Array size is :" . @numbers . "\n");
    $i=$i+1;
    print("Individual Elements are:" . @numbers[i] . "\n");
    #pop(@numbers);
}

rhv:/var/cl_ip_down>./run.sh
Array size is :1
Individual Elements are:2
Array size is :2
Individual Elements are:3
Array size is :3
Individual Elements are:4

with pop

#!/usr/bin/perl
@names = ("Abhijit","Royal Enfield","Google");
@numbers=();
$number=1;
$i=0;
foreach $name (@names) {
    #print $_ . "\n";
    $number=$number+1;
    #push(@numbers,($number));
    unshift(@numbers,($number));
    print("Array size is :" . @numbers . "\n");
    $i=$i+1;
    print("Individual Elements are:" . @numbers[i] . "\n");
    pop(@numbers);
}

rhv:/var/cl_ip_down>./run.sh
Array size is :1
Individual Elements are:2
Array size is :1
Individual Elements are:3
Array size is :1
Individual Elements are:4

3 Answers

0 votes
answered Jan 19, 2010 by b-roth

unshift is used to add a value or values onto the beginning of an array:

Does the opposite of a shift. Or the opposite of a push, depending on how you look at it.

The new values then become the first elements in the array.

push adds elements to the end of an array:

Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY.

0 votes
answered Feb 19, 2010 by zaid

You really should be using use strict; and use warnings; in your code. Having them activated will allow you to identify errors in your code.

Change all instances of the following:

foreach $name (@names) -> for my $i (@names) as you don't do anything with the elements in the @names array.

@numbers[i] -> $numbers[$i] as this is where you've made a not uncommon mistake of using an array slice rather than referring to an array element.

This is not C. Every 'variable' has to have a sigil ($, @, %, &, etc.) in front of it. That i should really be $i.


As for the difference between push and shift, the documentation explains:

perldoc -f push:

push ARRAY,LIST

Treats ARRAY as a stack, and pushes the values of LIST onto the end of ARRAY. The length of ARRAY increases by the length of LIST. ... Returns the number of elements in the array following the completed "push".

perldoc -f unshift:

unshift ARRAY,LIST

Does the opposite of a shift. Or the opposite of a push, depending on how you look at it. Prepends list to the front of the array, and returns the new number of elements in the array.


To put it ASCII-matically...

        +---------+           +-----------+        +---------+ 
<-----  | ITEM(S) |  ----->   | (@) ARRAY | <----- | ITEM(S) | ----->
 shift  +---------+  unshift  +-----------+  push  +---------+   pop
                              ^           ^
                              FRONT       END
0 votes
answered Sep 15, 2017 by pavel

Note that

the preallocated array is balanced toward the 0 end of the array (meaning there is more free space at the far end of the list than there is before the list's 0 element). This is done purposely to make pushes more efficient than unshifts. http://www.perlmonks.org/?node_id=17890

Although lists do quite fine as "Perl is smartly coded because the use of lists as queues was anticipated (Ibid.)".

For comparison, in various JavaScript engines shift/unshift on arrays seems to be significantly slower.

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