dataframe - R: how to choose the ith number in a data frame by ID -

here data:

id         day            advance id1         1              na id1         1              na id1         1              na id1         2              0.14 id1         2              na id1         3              0.22 id1         3              na id1         3              na id1         2              na id1         4              -0.96 id1         4              na id2         1              na id2         1              na id2         2              1.54 id3         1              na id3         2              -0.48 

i want choose advance of ith (i = 2 ,..., 30) day of each id, here expected result 2nd day:

id1      0.14 id2      1.54 id3     -0.48 

hope answer! thanks!

using base r, try

no.na <- na.omit(x) no.na[no.na$day == 2, c("id", "advance")]      id advance 4  id1    0.14 14 id2    1.54 16 id3   -0.48 

a 1 liner per @akrun be

na.omit(x[x$day == 2, c("id", "advance")])