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Equivalent code?

Hello Everyone:

In the following example seq_address is an unsigned long and obj->cmd_reg[] is an unsigned char array.

seq_address = (ulong)obj->cmd_reg[3] << 24
            + (ulong)obj->cmd_reg[2] << 16
            + (ulong)obj->cmd_reg[1] <<  8
            + (ulong)obj->cmd_reg[0];

seq_address = (ulong)obj->cmd_reg[3] * 16777216
            + (ulong)obj->cmd_reg[2] * 65536
            + (ulong)obj->cmd_reg[1] * 256
            + (ulong)obj->cmd_reg[0];

I want to combine the four bytes of data from the array and store it as an unsigned long in seq_address. I first tried the code in the first example but it does not work (seq_address is always 0). The code in the second example works. Does anyone see a reason both examples should not do the exact same thing.

-Walt

0 Jon Young over 23 years ago

"+" has higher precedence than "<<". You wanted:

seq_address = ( (ulong)obj->cmd_reg[3] << 24 )
            + ( (ulong)obj->cmd_reg[2] << 16 )
            + ( (ulong)obj->cmd_reg[1] <<  8 )
            +   (ulong)obj->cmd_reg[0];

0 Mik Kleshov over 23 years ago
Extra parentheses seem to have fixed it:

seq_address = ((ulong)obj->cmd_reg[3] << 24) + ((ulong)obj->cmd_reg[2] << 16) + ((ulong)obj->cmd_reg[1] << 8) + (ulong)obj->cmd_reg[0];
I could never memorize all those priorities. I try to use parentheses whenever I'm in doubt :-))
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0 Walt Conley over 23 years ago in reply to Jon Young

Ah, that make sense. Thanks a million.

-Walt
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0 Jon Ward over 23 years ago in reply to Walt Conley

FYI,

Here is a link to the C precedence table:

http://www.isthe.com/chongo/tech/comp/c/c-precedence.html

Jon
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