Generate an array of random numbers

GAMS
1

Dear Friends,
I have defined set W and parameter Coef(W) as follows:

Set W /1*10/;
Parameter Coef(W);

I would like to randomly assign binary values to Coef(W); where about %A of elements in W get 1, and %(100-A) of them get 0. For instance, if A is equal to 70, therefore, Coef(W) could be one of the following arrays:

[1 1 0 1 1 0 1 1 0 1]
or
[0 1 1 1 0 0 1 1 1 1]
or
...

Can anyone please help me?

Kind regards,
Amirhossein

2

Hi, this is a good challenged! I think this can work.

*reset the seed for the random number generator.
execseed = 1 + gmillisec(jnow);

Set W /1*10/
select(W);
alias(W,Wp);
select(W)=NO;

Parameter Coef(W)
          p
          A /70/
;
loop(W$(ord(w) le floor(A*card(W)/100)),
p = uniformInt(1,card(W));
while( sum(Wp$(ord(Wp) eq p),1) eq sum(Wp$(ord(Wp) eq p and select(Wp)),1),
p = uniformInt(1,card(W));
);
select(Wp)$(ord(Wp) eq p) = yes;
Coef(Wp)$(ord(Wp) eq p) = 1;
);

Coef(Wp)$(not select(Wp)) = 0;


display Coef;

Bye!

3

This does what you want. Setting the seed for the random number generator is not required, but it is useful to get different results for different runs.

Set W /1*10/;
Parameter Coef(W);
scalar a / 0.7 /;
* set the seed in some random way, based on the time
execseed = gmillisec(jnow);
coef(w) = [uniform(0,1) < a];
display coef;
4

Hi dirkse, i like your compact implementation.
I tested your code but sometimes I got less than seven items with a value of 1 and sometimes more.
i’m right?

5

You are right. Unfortunately, the compact method of Mr. dirkse does not always work properly. The suggested method by Mr. Manassaldi works perfectly. Thanks.

6

It seems we need to clarify what the original poster intended. If the intent was to draw a number of samples from a {0,1} distribution, then you would expect to get different numbers of 1s when using different random number seeds. But perhaps the intent was to just permute a given number of 1s. In that case, the problem could be stated more clearly, without using percentages. For example, how would you get 70% of 11 items? If what you want is to randomly set k members of a parameter to 1, you can do this with the shuffle option:

https://www.gams.com/latest/docs/UG_OptionStatement.html?search=shuffle

set i /1*10/;
parameter c(i);
scalar k 'number of 1s in c' / 4 /;
c(i)$[ord(i) <= k] = 1;
* set the seed in some random way, based on the time
execseed = gmillisec(jnow);
option shuffle = c;
display c;
7

I did not know the shuffle option. Thanks for sharing dirkse!