[R] Problem with na.omit when using length()

[Daniel Malter]

This looks buggish to me (though at least non-intuitive), but I am almost
sure there is an explanation for why the b==0 condition includes the NAs.
You find a way to circumvent it in the last two lines of the example below.

a=c(1,1,1,0,0,0)
b=c(1,NA,0,1,NA,0)
sno=rnorm(6)

na.omit(length(sno[a==1 & b==0]))
sno[a==1 & b==0]
length(sno[a==1 & b==0])

which(a==1&b==0)
sno[which(a==1&b==0)] 

Daniel

-------------------------
cuncta stricte discussurus
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Von: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] Im
Auftrag von Viju Moses
Gesendet: Tuesday, October 06, 2009 12:52 AM
An: r-help at r-project.org
Betreff: [R] Problem with na.omit when using length()

I'm seeing what looks to me like odd behaviour when I use na.omit on a
simple "length" function, as follows.

 > sno
  [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22
23 24 25 26 27 28 29 30 31 32 33 34
 > a
  [1] 0 1 0 0 1 1 0 0 0 0 0 0 0 0 1 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0  >
b
  [1]  1  1  0  1  1  1  0  0 NA  0  0  0 NA  0  1 NA  0  1  0  0  0  0 NA
0  0  0  0 NA  0 NA  0  1  0  0

#NA refers to no data available.

 > df=data.frame(sno,a,b)
# I'm pasting the sorted data frame below:
 > sortdf=df[order(a,b),]
 > sortdf
    sno a  b
3    3 0  0
7    7 0  0
8    8 0  0
10  10 0  0
11  11 0  0
12  12 0  0
14  14 0  0
17  17 0  0
20  20 0  0
21  21 0  0
22  22 0  0
24  24 0  0
25  25 0  0
26  26 0  0
27  27 0  0
29  29 0  0
31  31 0  0
33  33 0  0
34  34 0  0
1    1 0  1
4    4 0  1
9    9 0 NA
13  13 0 NA
23  23 0 NA
28  28 0 NA
30  30 0 NA
19  19 1  0
2    2 1  1
5    5 1  1
6    6 1  1
15  15 1  1
18  18 1  1
32  32 1  1
16  16 1 NA

#Now I wish to count howmany records have a=1 AND b=0. From the lower
section of that sorted dataframe we see the answer is 1 (record # 19). 
But instead I'm seeing 2. Probably counting record # 16 also.

 > na.omit(length(sno[a==1 & b==0]))
[1] 2

I'd be grateful to anyone who can point out what I'm doing wrong.

Regards.

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