problem with using randsample

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mehdi J 2018-11-7

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https://ww2.mathworks.cn/matlabcentral/answers/428554-problem-with-using-randsample

评论： Steven Lord 2020-10-6

Hi, I have a set that its members change in a loop and I want to select just one member of it randomly and evaluate the result. the written code is as below:

CandidateNode = randsample(UnvisitedNode,1);

it works good but sometimes its result is not acceptable. for example when UnvisitedNode=[3] running this code have different result. sometimes CandidateNode=[1], sometimes CandidateNode=[2] and sometimes CandidateNode=[3] how could I fix it? tanx in advanced

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KALYAN ACHARJYA 2018-11-7

Sorry, the question is not understood. Kindly clarify?

mehdi J 2018-11-7

在 MATLAB Online 中打开

as you know: "y = randsample(population,k) returns a vector of k values sampled uniformly at random, without replacement, from the values in the vector population." but when the population is a vector with one member (for example population=[5]), for k=1 you will receive meaningless result.

>> randsample([5],1)

ans =

1

>> randsample([5],1)

ans =

4

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Answer 1

Jeff Miller 2018-11-7

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https://ww2.mathworks.cn/matlabcentral/answers/428554-problem-with-using-randsample#answer_345733

When the first parameter of randsample is a single number k, the assumption is that you want a random sample from the integers 1:k. It looks like you will have to check numel(UnvisitedNode) and return its value when numel=1.

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Steven Lord 2020-10-6

在 MATLAB Online 中打开

Alternately you could use randi instead of randsample for this particular use case.

% Build some sample data
numberOfNodes = randi(10); % Random number of nodes
UnvisitedNode = (1:numberOfNodes).^2
% Choose one of the UnvisitedNodes
selectedNode = UnvisitedNode(randi(numel(UnvisitedNode)))

This is a variant of the suggestion given in the description of the population input argument on the documentation page for the randsample function:

"If population is a numeric vector containing only nonnegative integer values, and population can have the length 1, then use y = population(randsample(length(population),k)) instead of y = randsample(population,k)."

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Answer 2

Aaron Schnydrig 2020-10-6

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https://ww2.mathworks.cn/matlabcentral/answers/428554-problem-with-using-randsample#answer_506366

The question is quite old, but for the ones finding it over Google (like I did):

The simplest answer would be the following:

CandidateNode = randsample(repmat(UnvisitedNode, 2, 1),1)

The repmat() function uses every value of your vector twice. Therefore, it will not change the probability of a certain element. However, it will make sure that your vector always has more than one element and is therefore used as a population.