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Powers of two

Posted on 2008-10-14
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Last Modified: 2012-05-05
hi,

  number = log(input)/log(2);

could anyone pls help me...to get the power of two of the input, i am doing this but i got incorrect answer for 8 ...

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Question by:zizi21
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11 Comments
 
LVL 6

Expert Comment

by:RishadanPort
ID: 22717021
Can't you use the math class pow function?

pow(number, power);
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Author Comment

by:zizi21
ID: 22717044
let say someone input 8,

i need to give the answer 3...
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LVL 6

Expert Comment

by:RishadanPort
ID: 22717063
ah so you do logs..

Unfortunately the problem is because the log function returns a double number, and double numbers can't be stored 100 percent accurately.

You will probably get some invalid result because of this.

Try using the "ceil, and floor" functions to help you get the correct answer.
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LVL 6

Expert Comment

by:RishadanPort
ID: 22717114
You actually could just just try using sprintf

like this:

double result = <log calculation>;

here is an example of only printing only 1 decimal
printf("%8.1d" result);  <-- creates a entire width of 8, and only 1 decimal space.

You can read up on this stuff.

here is a link I found:
http://cermics.enpc.fr/~ts/C/FUNCTIONS/format.html
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LVL 6

Expert Comment

by:RishadanPort
ID: 22717116
correction:

printf("%8.1d" result);  
--> should be printf("%8.1d", result);
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Author Comment

by:zizi21
ID: 22717132
thanks...
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Author Comment

by:zizi21
ID: 22717136
it works with ceil..can i assume it is correct or do i need to check for floor ??
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LVL 6

Accepted Solution

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RishadanPort earned 500 total points
ID: 22717171
you can't assume.

There are a few problems. with the approach with using the ceil / floor.

1. What happens if you do a log / log and the actual answer should have a decimal...
Then when you use the ceil / floor to the nearest integer, you will obviously get the wrong answer.

I suggest that instead of using ceil/floor, you simply do the division, and only print out a certain precision of decimal places. You may want to add other tests, but I am unsure exactly how to approach with 1 single method that is the absolute best.

Usually, the programmer just rounds to a specific decimal place
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Author Comment

by:zizi21
ID: 22717208
thanks

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LVL 53

Expert Comment

by:Infinity08
ID: 22718415
>> to get the power of two of the input,

If the input will be an integer value, this is relatively easy, since you have binary operators.
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LVL 6

Expert Comment

by:RishadanPort
ID: 22724260
I agree; I think I probably should have mentioned using bitwise operators, but I wasn't 100 percent clear on if he would be doing this approach only for powers of 2....

Have a look up on "Binary bitwise operators"

Operators: ">>" and "<<" are quite helpful
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