How to make a condition that if a value (that is a character) starts with a specific word, then I need to print a vector?

I have this dataframe that I applied ifelse function and startsWith function

df = data.frame(x = c('part1',letters[1:3],'part2',letters[5:6]))

y = 5:6

df$y = ifelse(startsWith(df$x,'part'),y,NA)
df

However the output is

      x  y
1 part1  5
2     a NA
3     b NA
4     c NA
5 part2  5
6     e NA
7     f NA

The column y includes 5 and 5 instead of 5 and 6. I don’t know what the error is I am making. Thanks for the help.

>Solution :

ifelse probably isn’t the right tool here.

If you want a one-liner solution in base R you could use replace:

within(df, y <- replace(rep(NA, nrow(df)), startsWith(df$x, 'part'), y))
#>       x  y
#> 1 part1  5
#> 2     a NA
#> 3     b NA
#> 4     c NA
#> 5 part2  6
#> 6     e NA
#> 7     f NA

Although I guess most folks would probably just create an NA column and write y into the matching indices.

df$y <- NA
df$y[startsWith(df$x, 'part')] <- y

df
#>       x  y
#> 1 part1  5
#> 2     a NA
#> 3     b NA
#> 4     c NA
#> 5 part2  6
#> 6     e NA
#> 7     f NA

Both approaches of course assume that the number of matches is the same as the length of y, and you probably need some extra logic to check that this is definitely the case.

Created on 2022-11-10 with reprex v2.0.2

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