Thread: undo bitshift

i dont know how u can do it by using bitwise opeartor but i just want to say that left shift is the same as multiplying "literal" by 2 raised to the power "variable". So u can divide "result" by "literal" and take its square root to get the value of "variable".

sqrt is not the right thing to use here, I'm pretty sure. Also, it returns a floating point value, so using %x is definitely wrong (unless you actually want to see what the floating point value looks like as a integer - but that's unlikely).

I guess you could do:

Code:

x = (int)(log(result / literal) / log(2));

But it's probably quicker to do:

Code:

int c = 0;
int x = result / literal;
while (x) {  c++; x >>= 1; }

c is the number of bits literal was shifted.

--
Mats

Ok, yes, or use while(x > 1).

--
Mats

Originally Posted by young turk

sounds right but when I try it, it doesn't work:

Code:

#include <math.h>
#include <stdio.h>

int main(int argc, char **argv) {

int variable = 3;
int literal = 0xf0;
int result = literal << variable;
int rl = result/literal;
printf("literal: %x\n",literal);
printf("result: %x\n",result);
printf("result/literal %x \n",rl);
printf("sqrt(result/literal): %x\n",sqrt(rl));

}

gives me

Code:

$./test
literal: f0
result: 780
result/literal 8 
sqrt(result/literal): 667f3bcd

have I missed something?

sorry i was wrong with the square root part. But going in the similar manner and taking the log(with base 2) of 'rl' will give the value of 'variable'.
I have modified your code by taking the above mentioned into consideration and changed various values of 'variable' and the result was satisfactory.

Code:

#include <math.h>
#include <stdio.h>

int main(void) {

int variable = 5;//changed it to 2,3,4 etc.
int literal = 4;
int result = literal << variable;
float rl = (float)result/literal;
printf("literal: %d\n",literal);
printf("result: %d\n",result);
printf("result/literal %f \n",rl);
printf("variable=sqrt(result/literal): %f\n",log(rl)/log(2));//log(with base 2) of rl. In C log with a base of 10 is given so i changed it accordingly
getch();
return 0;

}