Hello All,
I want to divide a number by integer value 3, without using ‘/’ or ‘*’. I am only allowed to use << and >>. Program should out put nearing value. For example.
8/3 = 2.66 and answer with shift can be 3.
7.3 = 2.33 and answer can be 2.
I tried doing this but there is more deviation.
Please let me know..Code:#define DIVIDE_BY_3(uS) ((uS<<4)>>6)
can you use + and -? conditionals?
how did you come up with that piece of code, and why do you expect it to work? unless i dont have enough coffee in me, that divides by four.
Last edited by moi; 08-19-2004 at 01:52 PM.
hello, internet!
Yes we can use + and - but not conditional as they add extra bytes and clock cycles.
Thanks
Using bit shifting has a bigger chance of increasing code size and execution time then using conditionals
A few other things to consider: Are you diving ints, doubles, unsigned, signed, postive, negative?
Last edited by Thantos; 08-19-2004 at 02:00 PM.
Well why don't you help the guy out and go chug some coffee!unless i dont have enough coffee in me, that divides by four.
aww come on, i leave work in 25 minutes! i dont need to be hyper nowOriginally Posted by sean_mackrory
but anyway, that macro, putting aside its ability to drop bits off the top, is the same as >> 2, which is just a straight division by 4
hello, internet!
Just by looking at some bit patterns I am pretty confident that it would be neigh impossible to divide by a non power of 2 using bit shifting without some serious lose (or possible) lose of data
Dividing sixteen bit unsigned value.
I'd just like to echo Thantos. Look at how inaccurate your first examples are - and those numbers give a pretty best-case scenario.
using only + and >>, but really reaaly stupid imo
21/64 is roughly 1/3, so it kinda works, but it truncuates, not roundsCode:unsigned short div3 (unsigned short z) { unsigned long x = z; // make sure no overflow x = x + x + x + x + x + x + x + x + x + x + x + x + x + x + x + x + x + x + x + x + x; // multiply by 21 x = x >> 6; // divide by 64 return (unsigned short) x; // should not overflow }
perhaps there is a better solution
hello, internet!
I think there is no solution until I add few more bytes.
?
by the way, using the processor's integer divison is going to be way faster than any sort of hack
hello, internet!
Just use '/' for the love of everything holy. If you're looking to speed up your program then I'd start looking at algorithm changes/rewrites instead of trying to figure out how to divide by 3 faster.
If you understand what you're doing, you're not learning anything.
Lets find out: 65535 (highest 16 bit unsigned) * 21 = 1376235 / 64 = 21503.671875
65535 / 3 = 21845
Difference: 341.328125
Just a little bit off.
The best bet is to use the subtraction method in a loop. Unless you are working with an ancient chip it can evaluate simple conditions quicker then it can do bit shifting or adding a number 21 times
example:
Code:int div_3(int remainder) { int quotient=0; for (;remainder>=3;remainder-=3,quotient++); if ( remainder >= 2) quotient++; return quotient; }
You all apparently missed this statement:
That seems to clearly state that this is an assignment. Be it by self-study in a book, or a class, or whatever, it still sounds like an assignment, since they've stated that they're "only allowed to use". Thus, the above doesn't apply.I am only allowed to use << and >>.
Quzah.
Hope is the first step on the road to disappointment.
