Re: forming a word from an array of its bytes

by jcb (Parson)

on Apr 07, 2021 at 22:35 UTC ( [id://11130976]=note: print w/replies, xml )

Need Help??

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;
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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;
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Re^2: forming a word from an array of its bytes by pidloop (Novice) on Apr 08, 2021 at 04:29 UTC
Doh! Never thought of combining unpack and pack that way. Works great, many thanks.	[reply]
Re^3: forming a word from an array of its bytes by jcb (Parson) on Apr 08, 2021 at 23:29 UTC
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`.	[reply] [d/l] [select]

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