Extract Row on Criteria

See if this gets you further along:

?order
?rev
?"["

df <- data.frame(x = 1:20, rsamp = sample(1:20,20), Lets  

=LETTERS[1:20])

ord <- order(df$rsamp)

# creates a sorted vector of row numbers

df[ord,]

   x rsamp Lets
16 16     1    P
14 14     2    N
18 18     3    R
15 15     4    O
1   1     5    A
3   3     6    C
9   9     7    I
19 19     8    S
20 20     9    T
10 10    10    J
7   7    11    G
17 17    12    Q
13 13    13    M
8   8    14    H
11 11    15    K
12 12    16    L
2   2    17    B
6   6    18    F
4   4    19    D
5   5    20    E
#rev ... reverses

df[rev(ord),]

   x rsamp Lets
5   5    20    E
4   4    19    D
6   6    18    F
2   2    17    B
12 12    16    L
11 11    15    K
8   8    14    H
13 13    13    M
17 17    12    Q
7   7    11    G
10 10    10    J
20 20     9    T
19 19     8    S
9   9     7    I
3   3     6    C
1   1     5    A
15 15     4    O
18 18     3    R
14 14     2    N
16 16     1    P
# successive applications of [ , ][ , ] can be very powerful
# this is the first 10 of the reverse sorted by rsamp df

df[rev(ord),][1:5,]

   x rsamp Lets
5   5    20    E
4   4    19    D
6   6    18    F
2   2    17    B
12 12    16    L

And next time, ... please use dput( ) for offering a sample dataframe.

-- 
David Winsemius


On Mar 5, 2009, at 9:57 PM, Bob Roberts wrote:

Hi,
 I have a data frame and I want to extract the entire rows that  
have the 20 largest numbers in column Z. How would I go about doing  
that? Thanks.




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