in java i can cast number to byte for example
System.err.println((byte)13020);
the result will be
-36
how can i do the same in php ?
don't tell me you can't do that in PHP. it might not be straightforward but it most definitely is doable.
Peter Perháč
2010-07-29 10:53:14
@Peter: How do you cast to a data type that doesn't exist?
Daniel Egeberg
2010-07-29 10:56:11
He doesn't care about the actual data type used. He wants the number handled as a byte-cast would in java
gnud
2010-07-29 10:59:46
he doesnt ask you to CAST it, just convert it.
Peter Perháč
2010-07-29 12:39:23
+2
A:
Maybe do a modulo 256 expression (if unsigned) or modulo 256 and minus 128.
$val = ($val % 256) - 128
This work if you only want a value. If you want a real-one-byte, maybe pack() function will help here.
Edit: Right, 0 would be 128, so maybe this solution do will works:
$val = (($val+128) % 256) - 128
killer_PL
2010-07-29 10:52:48
I saw this has become the accepted answer. However, there's still some nasty corner cases which are not handled propery. As an example, putting in `$val = -512;` gives the surprising result of `-256` instead of 0 (since -512 decimal == 0xfffffe00 in 32 bit) This is especially funny since -256 cannot be represented in 8 bits. So I'll make it explicit: **don't use the modulo operator to do bit operations**, ever. It's asking for trouble.
mvds
2010-07-30 13:58:03
+4
A:
echo ($a&0x7f) + ($a&0x80?-128:0);
edit this mimics what should actually happen for a signed 8-bit value. When the MSB (bit 7) is zero, we just have the value of those 7 bits. With the MSB set to 1, we start at -128 (i.e. 1000000b == -128d).
You could also exploit the fact that PHP uses integer values internally:
$intsize = 64; // at least, here it is...
echo ($a<<($intsize-8))/(1<<($intsize-8));
so you shift the MSB of the byte to the MSB position of the int as php sees it, i.e. you add 56 zero bits on the right. The division "strips off" those bits, while maintaining the sign of the value.
mvds
2010-07-29 10:55:18