Question

I have an R data frame:

> tab1
  pat  t conc
1  P1  0  788
2  P1  5  720
3  P1 10  655
4  P2  0  644
5  P2  5  589
6  P2 10  544

I am trying to create a new column for conc as a percentage of conc at t=0 for each patient. As well as many other things, I have tried:

tab1$conct0 <- tab1$conc / tab1$conc[tab1$t == 0  & tab1$pat == tab1$pat]

But I am clearly miles off with the correct code that means “conc WHERE t==0 AND pat == pat for this particular row”

I am sure I could use a for loop or something but hoped there was something easier?

Thanks

Answer 1

I would find the starting concentration for each patient with:

startConc <- tab1[tab1$t == 0,]

which gives (from your example data)

  pat t conc
1  P1 0  788
4  P2 0  644

After that you can use apply

newconc <- apply(tab1, 1, function(x){as.numeric(x[3])/startConc[startConc$pat==x[1],3]})

which gives you

[1] 1.0000000 0.9137056 0.8312183 1.0000000 0.9145963 0.8447205