forming a word from an array of its bytes

[pidloop]

Oh great ones! I have an array containing four integral values, each of which we may assume is less than 256. I wish to combine them into one 32 bit native integer in little-endian format.

The following code does what I want by printing 67305985:

my @array = (1, 2, 3, 4);
my $val = 0;
for (my $i = 0; $i < 4; $i++) {
    $val |= $array[$i]<<(8*$i);
}
printf "%d\n", $val;

which you may convince yourself is correct by running

perl -e 'printf "%d\n", (4<<24)+(3<<16)+(2<<8)+1')

My question is: how can I do this without a loop? I have played around with pack, unpack and join but have not yet found a way.

Many thanks for your time and wisdom.

[jcb]

Obvious method: unroll the loop: (untested)

my @array = (1, 2, 3, 4);
my $val = ($array[0]) | ($array[1]<< 8) | ($array[2]<<16) | ($array[3]
+<<24);
printf "%d\n", $val;

If clarity is less of a concern, try unpack/pack:

my @array = (1, 2, 3, 4);
my $val = unpack 'L', pack 'C4', @array;
printf "%d\n", $val;

[pidloop]

Doh! Never thought of combining unpack and pack that way. Works great, many thanks.

[jcb]

The simple way to understand that combination is that pack converts "stuff" to an octet string and unpack converts an octet string to "stuff" according to template strings. Unpacking with a different template than was used for packing reinterprets the octet string, essentially "type-punning" the data. (Type-punning is a C trick where the element of a C union object that is read is not the element that was most recently written.)

This explanation is how I remember when to use pack and when to use unpack.