help mata mm_freq()
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Title

    mm_freq() -- Compute frequency counts

Syntax

        real colvector mm_freq(x [, w, levels])

        real colvector _mm_freq(x [, w, levels])

        real colvector mm_freq2(x [, w])

        real colvector _mm_freq2(x [, w])

    where

            x:  transmorphic matrix containing data, rows are observations

            w:  real colvector containing weights

       levels:  transmorphic matrix containing levels of data, in ascending
                order


Description

    mm_freq() returns the frequency counts of the "levels" (unique rows) of
    x. That is, mm_freq() computes the frequency distribution of x. The
    frequency counts are returned in ascending sort order of the levels of x.

    w specifies weights associated with the observations in x. Omit w, or
    specify w as 1 to obtain unweighted results.

    levels specifies the levels to be counted. levels must be sorted in
    ascending order. If levels is omitted, mm_freq() determines the frequency
    counts of all levels of x. Furthermore, levels==. indicates to determine
    the counts of all levels and, additionally, return the levels of x in
    levels.

    mm_freq2() returns for each row of x the number of duplicates in x (or
    the sum of weights associated with the duplicates, respectively).

    _mm_freq() and _mm_freq2() are like mm_freq() and mm_freq2(), but assume
    that x is sorted.


Remarks

    Example:

        : x = (1\2\3\.\1\3\1\1\2\3\.)
        
        : mm_freq(x,1)
               1
            +-----+
          1 |  4  |
          2 |  2  |
          3 |  3  |
          4 |  2  |
            +-----+

        : x, mm_freq2(x,1)
                1   2
             +---------+
           1 |  1   4  |
           2 |  2   2  |
           3 |  3   3  |
           4 |  .   2  |
           5 |  1   4  |
           6 |  3   3  |
           7 |  1   4  |
           8 |  1   4  |
           9 |  2   2  |
          10 |  3   3  |
          11 |  .   2  |
             +---------+

        : l = (0\1\2)
        
        : f = mm_freq(x,1,l)
        
        : l, f
               1   2
            +---------+
          1 |  0   0  |
          2 |  1   4  |
          3 |  2   2  |
            +---------+

    Note that

        : f = mm_freq(x,1,l=.)
        
        : l, f
               1   2
            +---------+
          1 |  1   4  |
          2 |  2   2  |
          3 |  3   3  |
          4 |  .   2  |
            +---------+

    is equivalent to

        : l = uniqrows(x)
        
        : f = mm_freq(x,1,l)
        
        : l,f
               1   2
            +---------+
          1 |  1   4  |
          2 |  2   2  |
          3 |  3   3  |
          4 |  .   2  |
            +---------+

    Furthermore, mm_freq() may also be used with strings:

        : s = ("a"\"b"\"c"\"b")
        
        : mm_freq(s,1)
               1
            +-----+
          1 |  1  |
          2 |  2  |
          3 |  1  |
            +-----+


Conformability

    mm_freq(x, w, levels), _mm_freq(x, w, levels):
         x:  n x c 
         w:  n x 1 or 1 x 1
    levels:  l x c or levels==.
    result:  l x 1.

    mm_freq2(x, w), _mm_freq2(x, w):
         x:  n x c
         w:  n x 1 or 1 x 1
    result:  n x 1.
    

Diagnostics

    None.


Source code

    mm_freq.mata


Author

    Ben Jann, ETH Zurich, jann@soz.gess.ethz.ch


Also see

    Online:  help for [M-5] uniqrows(), [M-4] utility, mm_levelsof(), 
             moremata