Dear R user

I have data created using code below

b<-matrix(2:21,nrow=4)
b[,1:3]=NA
b[4,2]=5
b[3,1]=6

Now the data is

> b
         [,1]  [,2]   [,3]  [,4]  [,5]
[1,]   NA   NA   NA   14   18
[2,]   NA   NA   NA   15   19
[3,]      6   NA   NA   16   20
[4,]   NA    5     NA    17   21


I want to keep data in column 4 greater than 15 and the value in column 1 &
2 either greater than 4 or is 'NA'. So I would like to have
my outcome as below...

[3,]   6   NA NA 16 20
[4,] NA 5 NA 17 21

I thought something like the code below gonna to work but it only returns
the last row,e.g "NA 5 NA 17 21". ...

bb<-b[which( (b[,2]>4 | b[,2]==NA) & (b[,1]>4 | b[,1]==NA) & b[,4]>15) ,])


Please could anyone help?

Many thanks in advance

HJ

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