Find size of an array in Perl

Question

I seem to have come across several different ways to find the size of an array  What is the difference between these three methods   my  arr    2   print scalar  arr    First way to print array size  print   arr    Second way to print array size  my  arrSize    arr  print  arrSize    Third way to print array size

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Use int  array  as it threats the argument as scalar

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To use the second way  add 1   print   arr   1    Second way to print array size

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This gets the size by forcing the array into a scalar context  in which it is evaluated as its size   print scalar  arr    This is another way of forcing the array into a scalar context  since it s being assigned to a scalar variable   my  arrSize    arr    This gets the index of the last element in the array  so it s actually the size minus 1  assuming indexes start at 0  which is adjustable in Perl although doing so is usually a bad idea    print   arr    This last one isn t really good to use for getting the array size  It would be useful if you just want to get the last element of the array   my  lastElement    arr   arr     Also  as you can see here on Stack nbsp Overflow  this construct isn t handled correctly by most syntax highlighters

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Example   my  a    undef  undef   my  size    a   warn  Size        a      Size  1  It s not the size warn  Size       size    Size  2

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There are various ways to print size of an array  Here are the meanings of all  Let   s say our array is my  arr    3 4   Method 1  scalar This is the right way to get the size of arrays  print scalar  arr     Prints size  here 2  Method 2  Index number   arr gives the last index of an array  So if array is of size 10 then its last index would be 9  print   arr        Prints 1  as last index is 1 print   arr   1    Adds 1 to the last index to get the array size  We are adding 1 here  considering the array as 0-indexed  But  if it s not zero-based then  this logic will fail  perl -le  local      4  my  arr    3  4   print   arr   1       prints 6  The above example prints 6  because we have set its initial index to 4  Now the index would be 5 and 6  with elements 3 and 4 respectively  Method 3  When an array is used in a scalar context  then it returns the size of the array my  size    arr  print  size      Prints size  here 2  Actually  method 3 and method 1 are same

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The    Perl variable types    section of the perlintro documentation contains  The special variable   array tells you the index of the last element of an array  print  mixed   mixed           last element  prints 1 23  You might be tempted to use   array   1 to tell you how many items there are in an array  Don   t bother  As it happens  using  array where Perl expects to find a scalar value     in scalar context     will give you the number of elements in the array  if   animals  lt  5            The perldata documentation also covers this in the    Scalar values    section   If you evaluate an array in scalar context  it returns the length of the array   Note that this is not true of lists  which return the last value  like the C comma operator  nor of built-in functions  which return whatever they feel like returning   The following is always true  scalar  whatever       whatever   1   Some programmers choose to use an explicit conversion so as to leave nothing to doubt   element count   scalar  whatever     Earlier in the same section documents how to obtain the index of the last element of an array   The length of an array is a scalar value  You may find the length of array  days by evaluating   days  as in csh  However  this isn   t the length of the array  it   s the subscript of the last element  which is a different value since there is ordinarily a 0th element

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As numerous answers pointed out  the first and third way are the correct methods to get the array size  and the second way is not  Here I expand on these answers with some usage examples   array name evaluates to the length of the array   the size of the array   the number of elements in the array  when used in a scalar context  Below are some examples of a scalar context  such as  array name by itself inside if or unless  of in arithmetic comparisons such as    or     All of these examples will work if you change  array name to scalar  array name   This would make the code more explicit  but also longer and slightly less readable  Therefore  more idiomatic usage omitting scalar   is preferred here  my  a    undef  q    0  1      All of these test whether  array  has four elements  print q array has four elements  if  a    4  print q array has four elements  unless  a    4   a    4 and print q array has four elements      a    4  and print q array has four elements      All of the above print    array has four elements    All of these test whether array is not empty  print q array is not empty  if  a  print q array is not empty  unless   a   a and print q array is not empty       a  and print q array is not empty      All of the above print    array is not empty

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To find the size of an array use the scalar keyword   print scalar  array    To find out the last index of an array there is     Perl default variable   It gives the last index of an array  As an array starts from 0  we get the size of array by adding one to      print    array 1     Example   my  a   qw 1 3 5   print scalar  a    n   print   a 1    n     Output   3  3

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From perldoc perldata  which should be safe to quote      The following is always true   scalar  whatever       whatever   1     Just so long as you don t   whatever   and mysteriously increase the size or your array      The array indices start with 0    and     You can truncate an array down to nothing by assigning the null list    to it  The following are equivalent        whatever             whatever   -1     Which brings me to what I was looking for which is how to detect the array is empty  I found it if   empty    -1

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All three give the same result if we modify the second one a bit   my  arr    2  4  8  10    print  First result  n   print scalar  arr    print   n nSecond result  n   print   arr   1    Shift numeration with  1 as it shows last index that starts with 0   print   n nThird result  n   my  arrSize    arr  print  arrSize

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First  the second is not equivalent to the other two    array returns the last index of the array  which is one less than the size of the array   The other two are virtually the same  You are simply using two different means to create scalar context  It comes down to a question of readability   I personally prefer the following   say 0  array             Represent  array as a number   I find it clearer than  say scalar  array        Represent  array as a scalar   and  my  size    array  say  size    The latter looks quite clear alone like this  but I find that the extra line takes away from clarity when part of other code  It s useful for teaching what  array does in scalar context  and maybe if you want to use  size more than once

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The first and third ways are the same  they evaluate an array in scalar context  I would consider this to be the standard way to get an array s size   The second way actually returns the last index of the array  which is not  usually  the same as the array size

[perl] Find size of an array in Perl

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