r - How can I use loops for and if -

r - How can I use loops for and if -

this illustration dataset

> v1 v2 1 a1 5437 2 a1 5437 3 a1 5437 4 a2 1819 5 a2 1758 6 a2 1212 7 a2 1212 8 a3 1456

i want compute unique values column v2, result be:

a1 1 a2 3 a3 1

i have started write code, don't have thought - how should like:

old_id <- a[1,2] old_art <- a[2,1] (i in nrow(a)){ if (old_id == a[1,i+2] && old_art == a[i+2,1]){ new_id[i] <- old_id[1,i+2] new_art[i] <- } <- i+1 }

i know simple solution like:

tapply(a[,2], a[,1], function(t) length(unique(t)))

but task utilize loop function - , if

this sounds homework. for loops run through elements in vector on right hand side of in. means for loop increment automatically , don't need i <- i+1.

hence, for loop should this

for (i in 1:nrow(a)) { < code > } # <- + 1 # no need this!

notice i in 1:nrow(a) , no i in nrow(a). haven't checked code, forsyntax. remember, for loops functions; so

for (i in 1:3) { print(i) } #[1] 1 #[1] 2 #[1] 3

is same as

`for`(i, 1:3, print(i)) #[1] 1 #[1] 2 #[1] 3

see ?"for".

r loops if-statement for-loop tapply