Re: [Help-glpk] power of: calculate 2^x from variable x

[Michael Hennebry]

On Tue, 7 Jun 2011, malekro wrote:

> i am trying to calculate 2^x from a variable x. i already spent several
> hours and with the help of archived posts i thought i have the solution.
> unfortunately i cannot figure out why the code below returns an empty result
> or malforms the x variable, i think the idea is correct.
>
> set R := {0..3};
> set C := {0..2};
>
> # the given numbers that need to be powered by 2
> var x{R}, integer;
>
> # temporary table, each row will later be used for multiplication with 2^c
> var t{R,C}, binary;
>
> # the desired solution 2^x
> var s{R}, integer;
>
> # set the x-th cell in temporary row to 1, rest 0
> s.t. c1{r in R}: sum{c in C} t[r,c] = 1; # exactly one 1 in row
> s.t. c3{r in R, c in C}: c * t[r,c] = x[r]; # set the x[r]-th cell to 1

|R|*|C| constraints with two terms each.
Probably not what you want.
Drop them and the rest should be sufficient.

> s.t. c2{r in R}: sum{c in C} c * t[r,c] = x[r]; # verification so t[r,0] !=
> 1 if x[r] > 0
>
> # multiply the c-th cell of the temporary row with 2^c
> s.t. c4{r in R}: s[r] = sum{c in C} (2 ** c) * t[r,c];

--
Michael   address@hidden
"Pessimist: The glass is half empty.
Optimist:   The glass is half full.
Engineer:   The glass is twice as big as it needs to be."