Question

I have a project where a function receives four 8-bit characters and needs to convert the resulting 32-bit IEEE-754 float to a regular Perl number. It seems like there should be a faster way than the working code below, but I have not been able to figure out a simpler pack function that works.

It does not work, but it seems like it is close:

$float = unpack('f', pack('C4', @array[0..3]);  # Fails for small numbers 

Works:

@bits0 = split('', unpack('B8', pack('C', shift))); @bits1 = split('', unpack('B8', pack('C', shift))); @bits2 = split('', unpack('B8', pack('C', shift))); @bits3 = split('', unpack('B8', pack('C', shift))); push @bits, @bits3, @bits2, @bits1, @bits0;  $mantbit = shift(@bits); $mantsign = $mantbit ? -1 : 1; $exp = ord(pack('B8', join('',@bits[0..7]))); splice(@bits, 0, 8);  # Convert fractional float to decimal for (my $i = 0; $i < 23; $i++) {     $f = $bits[$i] * 2 ** (-1 * ($i + 1));     $mant += $f; } $float = $mantsign * (1 + $mant) * (2 ** ($exp - 127)); 

Anyone have a better way?

Answer 1

I’d take the opposite approach: forget unpacking, stick to bit twiddling.

First, assemble your 32 bit word. Depending on endianness, this might have to be the other way around:

my $word = ($byte0 << 24) + ($byte1 << 16) + ($byte2 << 8) + $byte3; 

Now extract the parts of the word: the sign bit, exponent and mantissa:

my $sign = ($word & 0x80000000) ? -1 : 1; my $expo = (($word & 0x7F800000) >> 23) - 127; my $mant = ($word & 0x007FFFFF | 0x00800000); 

Assemble your float:

my $num = $sign * (2 ** $expo) * ( $mant / (1 << 23)); 

There’s some examples on Wikipedia.

• Tested this on 0xC2ED4000 => -118.625 and it works.
• Tested this on 0x3E200000 => 0.15625 and found a bug! (fixed)
• Don’t forget to handle infinities and NaNs when $expo == 255