# Is it possible to set an uncertain parameter (ureal) to a fixed value?

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Francesco Roscia on 15 Jul 2019
Answered: Vimal Rathod on 23 Jul 2019
Hi,
It is the first time that I use the Robust Control Toolbox. I have a MIMO system (uss) which depends on some uncertain parameters (ureal) and I d like to plot its principal gains for some specific values ot these parameters. Do you know how to set the value of the uncertain parameter to a fixed value in the admissible range?
Thanks!

Vimal Rathod on 23 Jul 2019
The uncertain parameter has no property to set itself to a fixed value but if you need to plot on a fixed value, you can set the range parameter of ureal to very close value to the fixed value so that ureal variable nearly behaves as a fixed value.
Here is an example.
a = ureal('a' , 1);
% limiting the range to a fixed value but not equal to fixed value
a.Range = [0.99999999999999 , 1.0000000000001];
%Limiting the percentage of uncertainty to a mininum
a.Percentage = [-0.00000000000001 , 0.000000000000001];
% generating 1000 samples of the uncertain variable and plotting it on histogram
hsample = usample(a , 1000);
hist(reshape(hsample,[1000,1]),20);
a.Range = [0.99999999999999 , 1.0000000000001];
The histogram of 1000 elements is the following:
Here you can see that the uncertain value behaves as a fixed value when the parameters of the ureal variable are set properly.