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[原创]C语言一题
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发表于: 2010-10-5 01:05
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为了方便大家辨别，我人肉OCR一下。
2.71：
You just started working for a company that is implementing a set of procedures
to operatate on a data structure where 4 signed bytes are packed into a 32-bit
unsigned. Bytes within the word are numbered from 0 (least significant) to 3
(most significant). You have been assigned the task of implementing a function
for a machine using two's-complement arithmetic and arithemetic right shifts with
following prototype:
/* Declaration of data type where 4 bytes are packed
into an unsigned */
typedef unsigned packet_t;
/* Extract byte from word. Return as signed integer */
int xbyte(packet_t word, int bytenum);
That is, the function will extract the designed byte and sign extend it to be a
32-bit int.
Your predecessor (who was fired for incompetence) wrote following cod
/* Failed attempt at xbyte */
int xbyte(packet_t word, int bytenum)
{
return (word >> (bytenum << 3)) & 0xFF;
}
A. What is wroing with this code?
B. Give a correct implementation of the function that uses only left and righ
shifts, along with one subtraction.
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A. 不用说了，显然错的。
B. 大概算了一下，目前无法突破减法的次数限制，用了两次减法。等待高人 
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