Choose random element of vector

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giorgos kivides
giorgos kivides on 24 Jan 2020
Commented: giorgos kivides on 24 Jan 2020
I have a vector and i want to choose 2 random elements x1 and x2.
The x2 element I do not want to be the same element as x1 and i want to select after element x1
For example:
vector = [1,2,3,4,5,6,7,8,9]
element x1 may be the number 2. element x2 cannot be number 1 or number 2.

Answers (5)

Sindar
Sindar on 24 Jan 2020
Edited: Sindar on 24 Jan 2020
% select a random integer between 1 and the last index of the vector
ind_1 = randi([1 length(vector)]);
% select a random integer between ind_1 and the last index of the vector
ind_2 = randi([ind_1+1 length(vector)]);
% select the elements associated with these indices
x1=vector(ind_1);
x2=vector(ind_2);
  1 Comment
Walter Roberson
Walter Roberson on 24 Jan 2020
If length(vector) is randomly choosen for ind_1 then ind_2 has no room to be chosen from.

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Walter Roberson
Walter Roberson on 24 Jan 2020
P1 = randi(length(Vector) - 1);
P2 = randi([P1+1,length(Vector)]) ;
x1 = Vector(P1) ;
x2 = Vector(P2) ;
Note that this is not a fair distribution. You could get a fairer distribution if you dropped the requirement that x2 be chosen strictly after choosing x1:
P = sort(randperm(length(Vector, 2)));
x1 = Vector(P(1));
x2 = Vector(P(2));
This is fairer but violates the condition that x1 be chosen before x2.

James Tursa
James Tursa on 24 Jan 2020
Edited: James Tursa on 24 Jan 2020
k = sort(randperm(numel(vector),2));
x1 = vector(k(1));
x2 = vector(k(2));
This assumes that you always want to be able to pick two numbers. I.e., x1 cannot be vector(end) and x2 cannot be vector(1).

giorgos kivides
giorgos kivides on 24 Jan 2020
Thanks a lot.I need some help. when choosing the two random elements, I would like to take the elements between them and change their order.
for example
vector = [1 2 3 4 5 6 7 8 9]
Random Elements: X1 = number 2. X2 = number 6.
the vector will have the format:
Newvector = [1 2 5 4 3 6 7 8 9]
  2 Comments
James Tursa
James Tursa on 24 Jan 2020
To reverse order:
vector(k(1)+1:k(2)-1) = vector(k(2)-1:-1:k(1)+1);
To have random order:
m = randperm(k(2)-k(1)-1) + k(1) ;
vector(k(1)+1:k(2)-1) = vector(m);

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giorgos kivides
giorgos kivides on 24 Jan 2020
i want something yet.. i have an array and i want to convert to vector. i want in the vector the first elements to be the first row, etc.
for example:
array= [ 1 2 3 4 5; 11 12 13 14 15; 21 22 23 24 25]
the vector will have the format:
vector = [1 2 3 4 5 11 12 13 14 15 21 22 23 24 25]
  2 Comments
Walter Roberson
Walter Roberson on 24 Jan 2020
reshape(array.', 1, [])
giorgos kivides
giorgos kivides on 24 Jan 2020
thanks. And i have the vector and i want to add the number 1 at start of vector. for example:
vector = [2 3 4 5 6]
newvector = [1 2 3 4 5 6].
how to add the number?

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