can define a new column because it is factor

I have a dataset like this:

I would like to create a new column

i gave that code

df$newrisk <- 0.5*df$magnitude + 0.6*df$aarea + 3*df$platarea

I got an error message for df$platarea?
BUt the platarea will only increase when it is "yes".
How can I code that???? the code is right if I omit df$platarea, but i would also include df$platarea but don’t know how??

>Solution :

We can create a logical vector

i1 <- df$platarea == "yes"
df$newrisk[i1] <- with(df, 0.5 * magnitude[i1] + 0.6 * area[i] + 3)

If it is only to change the + 3 *, multiply by the logical vector so that FALSE (or 0 will return 0 and TRUE for ‘yes’ will return 3 as -3 *1 = 3)

df$newrisk <- with(df, 0.5 * magnitude + 0.6 * area + 3 *i1)