median of column with awk

Question

How can I use AWK to compute the median of a column of numerical data?

I can think of a simple algorithm but I can't seem to program it:

What I have so far is:

sort | awk 'END{print NR}' 

And this gives me the number of elements in the column. I'd like to use this to print a certain row (NR/2). If NR/2 is not an integer, then I round up to the nearest integer and that is the median, otherwise I take the average of (NR/2)+1 and (NR/2)-1.

Answer 1

With awk you have to store the values in an array and compute the median at the end, assuming we look at the first column:

sort -n file | awk ' { a[i++]=$1; } END { print a[int(i/2)]; }'

Sure, for real median computation do the rounding as described in the question:

sort -n file | awk ' { a[i++]=$1; }
    END { x=int((i+1)/2); if (x < (i+1)/2) print (a[x-1]+a[x])/2; else print a[x-1]; }'

Answer 2

This awk program assumes one column of numerically sorted data:

#/usr/bin/env awk
{
    count[NR] = $1;
}
END {
    if (NR % 2) {
        print count[(NR + 1) / 2];
    } else {
        print (count[(NR / 2)] + count[(NR / 2) + 1]) / 2.0;
    }
}

Sample usage:

sort -n data_file | awk -f median.awk