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POOptimal method of comparing a vector of numbers to values in another vector
 

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  1. POOptimal method of comparing a vector of numbers to values in another vector
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    <p>Suppose I have two vectors of values:</p> <pre><code>a &lt;- c(1,3,4,5,6,7,3) b &lt;- c(3,5,1,3,2) </code></pre> <p>And I want to apply some function, <code>FUN</code>, to each of the inputs of <code>a</code> against the whole of <code>b</code>, what's the most efficient way to do it.</p> <p>More specifically, in this case for each of the elements in <code>a</code> I want to know for each value of 'a', how many of the elements in <code>b</code> are greater than or equal to that value. The naïve approach is to do the following:</p> <pre><code>sum(a &lt; b) </code></pre> <p>Of course, this doesn't work as it attempts to iterate over each of the vectors in parallel and gives me the warning: </p> <blockquote> <p>longer object length is not a multiple of shorter object length </p> </blockquote> <p>The output, btw, of that command is <code>3</code>.</p> <p>However, in my situation, what I'd like to see is an output that is:</p> <pre><code>0 2 4 4 5 5 2 </code></pre> <p>Of course, I realize I can do it using a for loop as such:</p> <pre><code>out &lt;- c() for (i in a) { for (i in a) { out[length(out) + 1] = sum(b&lt;i)} } </code></pre> <p>Likewise, I could use <code>sapply</code> as such:</p> <pre><code>sapply(a, function(x)sum(b&lt;x)) </code></pre> <p>However, I'm trying to be a good R programmer and stay away from for loops and <code>sapply</code> seems to be very slow. Are there other alternatives?</p> <p>For what it's worth, I'm doing this a couple of million times where <code>length(b)</code> is always less than <code>length(a)</code> and <code>length(a)</code> ranges from 1 to 30.</p>
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      1. USTony Breyal
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    1. CODo both the vectors `a,b` vary in each of your million iterations, or is one of them fixed?
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      1. USPrasad Chalasani
    2. COBoth vectors are generated multiple different times while going through all of the data. So, yes, they do vary, each one has about 10000 different values.
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