What's wrong with my R function for computing conditional means? -

i've begun teaching myself r, , decided try create function calculates mean of variable in data frame conditional on variable in same data frame equalling value. here code:

conditional.mean <- function( df, tv, cvar, cval ) {   col1 <- df$tv   col2 <- df$svar   subdf <- data.frame( col1, col2 )    tv.cond <- subdf[ which(col2==cval), 1 ]    x <- mean( tv.cond )    return( x ) } 

df data frame, tv target variable conditional mean computed, cvar variable used in condition, , cval value checked cvar.

when run code above, function outputs na , gives following warning: "in mean.default(tv.cond) : argument not numeric or logical: returning na." don't understand why saying this, since tv.cond should vector of values of target variable cvar = cval.

thanks help!