How does this work?

[punchinello]

Here is the code:

if(i&~077)

,
where i is an int value. The condition is supposed to be testing whether i is within a proper range. But how? Thank you very much!

[mattjbond]

~077 is the unary of the octal value 77;

octal 77 will correspond to the low order 6 bits being set of a word. In 32 bit hex word

0x0000003f.

the unary of this flips all the set bits to 0 and all 0 bits to 1 giving

0xffffffc0

this is then anded to mask off all the bits this corresponds to, in this case all the bits above bit 6.

so essentially, i = i &~077; will clear the low order six bits. if your value falls in this range then
i&~077 will return 0 else it will return a value above zero.

So in this case, if i < 64 then

i&~077 evaluates to 0 and any value of i >= 64 evaluates to i.

[punchinello]

Thank you very much! That was such a help!!