[R] extraction

[Heberto Ghezzo]

Hello everybody
I am trying to extract some observations from a data frame, ie the 
subjects that belong to a given group, and although t-test etc work 
if I try to obtain the number of subjects in the subgroup i get some 
funny numbers. All the subjects with NA for group are included in 
the subgroups 'group==1' etc.
Is this a bug?
for example in Windoze

R : Copyright 2001, The R Development Core Team
Version 1.2.2  (2001-02-26)


> z1 <- rnorm(100)
> z2<- rep(1:5,20)
> z3 <- z2
> z3[z1>1] <- NA
> z1[z2==1]
 [1]  1.16747148  0.58088013  1.00958356  1.35477371 -
1.19790468  0.07028495 -0.67707300
 [8] -0.10999675  1.78765730  0.52044707  0.47965798 -
2.05768565  1.16349068 -0.75485570
[15] -0.70517830  0.86419410 -0.03514043 -0.84861275 -
0.50284191  0.64971596
> z1[z3==1]
 [1]          NA  0.58088013          NA          NA          NA          NA  
        NA
 [8] -1.19790468          NA  0.07028495 -0.67707300 -0.10999675   
       NA          NA
[15]          NA  0.52044707  0.47965798 -2.05768565          NA -
0.75485570          NA
[22] -0.70517830  0.86419410          NA -0.03514043 -0.84861275   
       NA -0.50284191
[29]          NA          NA          NA  0.64971596
> 
 
the first with z2==1 gives me the 20 subjects OK but the second 
should have given me less than 20 but it returns 32! it adds all the 
NA's for z3.
I would have thought that z3==1 excluded all the NA's
Thanks for any explanation.


R. Heberto Ghezzo  Ph.D.
Meakins-Christie Labs
McGill University
Montreal - Canada
heberto at meakins.lan.mcgill.ca
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