How to choose a single element randomly from a vector

A=[2 3 4 5];

How do I choose any one variable from the vector A randomly each time.

Answer 1

[R]

Hi @Yogesh

To choose a random element from the vector A each time in MATLAB, you can use the randi function to generate a random index and then use that index to access an element from A.

See below:

A = [2 3 4 5];
randomElement = A(randi(length(A)));
disp(randomElement);

     5

This code will select and display a random element from the vector A each time it is run.

To know more about random number generation, refer to: Random Number Generation - MATLAB & Simulink (mathworks.com)

[Yogesh]

Hey thanks man...

Answer 2

[John D'Errico]

I've upvoted the answer by @R. Please accept it. My answer here is only to express some ideas that are really best put in a comment to that answer, but comments often get lost.

When you have a problem you can't solve, one too big of a step to make for your current skills, break it into smaller pieces. Look for ways to approach a solution, in smaller chunks. Always remember to eat a programming elephant one byte at a time.

For example, consider the vector V.

V = [2 3 5 7 11 13 17 19];

Here, a list pf prime numbers. Suppose I want to generate a random prime from that list? First, how do you extract one element? What, for example is the 5th element from that vector?

V(5)

ans = 11

So we know how to index into a vector. Does this help us in any way? Well, possibly. Is there some way we can generate a random index? There are 8 elements in the vector V.

nV = numel(V)

nV = 8

Suppose we could generate a random integer, from the set 1 through 8? Could we then use that number as an index into V?

What tools are there in MATLAB to generate random numbers? The big three are rand, randn, and randi. Of those three, rand and randn generate numbers from continuous distributions, thus uniform and Gaussian. They are not directly useful here. (though with some effort, we could use rand.) But randi should seem useful.

help randi

 RANDI Pseudorandom integers from a uniform discrete distribution.
    R = RANDI(IMAX,N) returns an N-by-N matrix containing pseudorandom
    integer values drawn from the discrete uniform distribution on 1:IMAX.
    RANDI(IMAX,M,N) or RANDI(IMAX,[M,N]) returns an M-by-N matrix.
    RANDI(IMAX,M,N,P,...) or RANDI(IMAX,[M,N,P,...]) returns an
    M-by-N-by-P-by-... array.  RANDI(IMAX) returns a scalar.
    RANDI(IMAX,SIZE(A)) returns an array the same size as A.
 
    R = RANDI([IMIN,IMAX],...) returns an array containing integer
    values drawn from the discrete uniform distribution on IMIN:IMAX.
 
    Note: The size inputs M, N, P, ... should be nonnegative integers.
    Negative integers are treated as 0.
 
    R = RANDI(..., CLASSNAME) returns an array of integer values of class
    CLASSNAME.
 
    R = RANDI(..., 'like', Y) returns an array of integer values with 
    the same data type and complexity (real or complex) as the numeric
    variable Y.
 
    The arrays returned by RANDI may contain repeated integer values.  This
    is sometimes referred to as sampling with replacement.  To get unique
    integer values, sometimes referred to as sampling without replacement,
    use RANDPERM.
 
    The sequence of numbers produced by RANDI is determined by the settings of
    the uniform random number generator that underlies RAND, RANDN, and RANDI.
    RANDI uses one uniform random value to create each integer random value.
    Control that shared random number generator using RNG.
 
    Examples:
 
       Example 1: Generate integer values from the uniform distribution on
       the set 1:10.
          r = randi(10,100,1);
 
       Example 2: Generate an integer array of integer values drawn uniformly
       from 1:10.
          r = randi(10,100,1,'uint32');
 
       Example 3: Generate integer values drawn uniformly from -10:10.
          r = randi([-10 10],100,1);
 
       Example 4: Reset the random number generator used by RAND, RANDI, and
       RANDN to its default startup settings, so that RANDI produces the same
       random numbers as if you restarted MATLAB.
          rng('default');
          randi(10,1,5)
 
       Example 5: Save the settings for the random number generator used by
       RAND, RANDI, and RANDN, generate 5 values from RANDI, restore the
       settings, and repeat those values.
          s = rng
          i1 = randi(10,1,5)
          rng(s);
          i2 = randi(10,1,5) % i2 contains exactly the same values as i1
 
       Example 6: Reinitialize the random number generator used by RAND,
       RANDI, and RANDN with a seed based on the current time.  RANDI will
       return different values each time you do this.  NOTE: It is usually
       not necessary to do this more than once per MATLAB session.
          rng('shuffle');
          randi(10,1,5)
 
    See also RAND, RANDN, RANDPERM, RNG, RANDSTREAM

    Documentation for randi
       doc randi

    Other uses of randi

       codistributed/randi        distributed/randi
       codistributor1d/randi      gpuArray/randi
       codistributor2dbc/randi    RandStream/randi

Do you see that randi can generate random integers from the set 1:nV?

randi(nV)

ans = 3

randi(nV)

ans = 6

Does that help? I hope so. Use indexing to give you what you need.

ind = randi(nV)

ind = 4

V(ind)

ans = 7

The point is, to look for tools that might help you to get at least closer to a solution. Then think about how you might use them, sometimes in combination with oher tools you already know how to use, to then solve your problem.