 Jeroen -5 years ago 312
Javascript Question

# Sampling a random subset from an array

What is a clean way of taking a random sample, without replacement from an array in javascript? So suppose there is an array

``````x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15]
``````

and I want to randomly sample 5 unique values; i.e. generate a random subset of length 5. To generate one random sample one could do something like:

``````x[Math.floor(Math.random()*x.length)];
``````

But if this is done multiple times, there is a risk of a grabbing the same entry multiple times. Tim Down

I suggest shuffling a copy of the array using the Fisher-Yates shuffle and taking a slice:

``````function getRandomSubarray(arr, size) {
var shuffled = arr.slice(0), i = arr.length, temp, index;
while (i--) {
index = Math.floor((i + 1) * Math.random());
temp = shuffled[index];
shuffled[index] = shuffled[i];
shuffled[i] = temp;
}
return shuffled.slice(0, size);
}

var x = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15];
var fiveRandomMembers = getRandomSubarray(x, 5);
``````

Note that this will not be the most efficient method for getting a small random subset of a large array because it shuffles the whole array unnecessarily. For better performance you could do a partial shuffle instead:

``````function getRandomSubarray(arr, size) {
var shuffled = arr.slice(0), i = arr.length, min = i - size, temp, index;
while (i-- > min) {
index = Math.floor((i + 1) * Math.random());
temp = shuffled[index];
shuffled[index] = shuffled[i];
shuffled[i] = temp;
}
return shuffled.slice(min);
}
``````
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