I have a matrix of the following form:

    time
id    0  2  4  6  9 12 14
  3   9  8 NA NA NA NA NA
  7   3 NA  3 NA  3 NA  4
  13 11  6  7 NA  5 NA  6
 .....

I hope for each row to select the last observation which is not 'NA'.

For example, for the first row, id=3, the value I want to select is 8
for the second row, id=7, the value I want to select is 4
for the third row, id=13, the value I want to select is 6

....

if it would be easier for you to demonstrate for me, here I include the code
to generate the above three-row matrix:

a1=c(9,8,rep(NA,5))
a2=c(3,NA,3,NA,3,NA,4)
a3=c(11,6,7,NA,5,NA,6)
matrix=rbind(a1,a2,a3)

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