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Selecting a random number with some probability

Asked by Aftab Ahmed Khan on 25 Feb 2015
Latest activity Commented on by Luciano Anastassiou on 22 May 2019
Hello Everyone, I am using this one line of code to generate a single value either to be 1 or 2 with equal probability but my question is that how can i select the value to be 1 with 60% probability and the value to be 2 with 40% probability ? Thank you.
select=randi(2,1,1);

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5 Answers

Answer by Torsten
on 25 Feb 2015
 Accepted Answer

x=rand;
if x<0.6
select=1;
else
select=2;
end
Best wishes
Torsten.

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Thank you all for the response. Take care.

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Answer by pankaj singh on 10 Mar 2018

The simplest technique is to use inbuilt Matlab function 'randscr'.
Suppose you want to generate M by N matrix of W, X, Y, and Z with probabilities i,j,k, and l. Then use
out = randsrc(M,N,[W,X,Y,Z;i,j,k,l]); % i+j+k+l = 1;
In your case, as you want a single value to be generated, your M x N = 1 x 1 matrix; the values are 1 with 60% probability (i.e. 0.6) and 2 with 40% (i.e. 0.4) probability, therefore use this;
out = randsrc(1,1,[1,2;0.6,0.4]);
Note that the above is just an example. You can create any matrix size with any number of values. The sum of probabilities must be equal to 1.

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Answer by Daniel
on 25 Feb 2015

You can use rand, which gives uniform distribution and look if the number is below or above 0.6.
if(0.6 <= rand()){
select = 1;
} else {
select = 2;
}
That should give 60/40 chances. There are more elegant ways to do that though.

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Could you please give me a bit explanation why this way makes sense or some documentation that I can read to gain some intuition? Thanks
Hi Trung Khoa Le,
The intuition is simply that "rand" generates a random number between 0 and 1. Then when you apply the "if" constraint, you are telling the system to only give out the result "select = 1" when that random number is below 0.6. Otherwise it will give out "select = 2".
If you repeated this 1000s of times, it would give out "select = 1" 60% of the time, because 60% of those random numbers between 0 and 1 will be below 0.6, and the other 40% of the time it will give out "select = 2".

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Answer by Jos (10584)
on 25 Feb 2015
Edited by Jos (10584)
on 25 Feb 2015

For two values it is simple
VAL = [10 20] % 2 values
P = .8 % probabbility of selecting first value
Ndraw = 20 % number of draws
R = rand(Ndraw,1) < P
SEL = VAL(2 - R) % use as index into VAL
For more complicated cases you might be interested inTake a look at my RANDP function, which picks random values with relative probabilities.

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Answer by Ka Mirul on 14 Nov 2017

I have create a video about generating random number in MATLAB
https://www.youtube.com/watch?v=MyH3-ONYL_k
Hope that will help you

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