[R] Vectorizing a loop

[Jeff Newmiller]

On Tue, 7 Feb 2012, Alexander Shenkin wrote:

> Hello Folks,
>
> I'm trying to vectorize a loop that processes rows of a dataframe.  It
> involves lots of conditionals, such as "If column 10 == 3, and if column
> 3 is True, and both column 5 and 6 are False, then set column 4 to True".
>
> So, for example, any ideas about vectorizing the following?
>
> df = data.frame( list(a=c(1,2,3,4), b=c("a","b","c","d"), c=c(T,F,T,F),
> d=NA, e=c(F,F,T,T)) )
>
> for (i in 1:nrow(df)) {
>
>    if (df[i,3] %in% c(FALSE,NA) & (df[i,1] > 2 | df[i,5]) ) {
>        df[i,4] = 1
>    }
>
>    if (df[i,5] %in% c(TRUE, NA) & df[i,2] == "b") {
>        df[i,4] = 2
>        df[i,5] = T
>    }
>
> }

Your code attempts to do some things with NA that won't behave the way
you expect them to.  Specifically, you cannot use %in% to test for NA,
and you cannot give the "if" function an NA.  It only appears to work 
because you don't actually give it a complete set of test values 
consistent with your tests in the loop. My guess at your intent is:

df <- data.frame( list( a=c(1,2,3,4,5)
                       , b=c("a","b","c","d","e")
                       , c=c(TRUE,FALSE,TRUE,FALSE,NA)
                       , d=NA
                       , e=c(FALSE,FALSE,TRUE,TRUE,NA)
                       ) )
tmpdf <- df

for (i in 1:nrow(df)) {

     if ( ( is.na(df[i,3]) || !df[i,3] ) && ( df[i,1] > 2 || ( is.na( 
df[i,5] ) || df[i,5] ) ) ) {
         df[i,4] <- 1
     }

     if ( ( is.na( df[i,5] ) || df[i,5] ) && df[i,2] == "b" ) {
         df[i,4] <- 2
         df[i,5] <- TRUE
     }

}

df2 <- df
df <- tmpdf

# intermediate logical vectors for clarity
tmp <- ( is.na(df[[3]]) | !df[[3]] ) & ( df[[1]] > 2 | df[[5]] )
tmp2 <- ( is.na(df[[5]]) | df[[5]] ) & df[[2]] == "b"
df[ tmp, "d" ] <- 1
df[ tmp2, "d" ] <- 2
df[ tmp2, "e" ] <- TRUE

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