R: How to use a "if"-like-function on vectors but no "ifelse" because if no it should do nothing

Question

I try to overwrite multiple variables at once (months in a kind of calendar) with an value of another variable based on a condition of a third variable. If the condition is false, nothing should happen.

I thought the If-function would be helpful because one could omit the else but no, it does not work for vectors.

    DF1[,3:4] <- if (DF1$v7==5) {DFDF1$v5}  
the condition has length > 1 and only the first element will be used

On the other hand, fitting for a vector is ifelse, but it tells me

    DF1[,3:4] <- ifelse (DF1$v7==5, DF1$v5, )
"argument "no" is missing, with no default", 

So what should I do to achieve my goal?

Thanks for any advice, beginners as me are grateful!

PS: Variables are just examples.

EDIT: I am looking for an way, where I can address the variables that should be replaced by column-number as in DF1[, 3:4] because otherwise I would have to type up to 12 different variable names (in my original DF).

Answer 1

Do one column after the other. Regarding the else condition, just use the original value

for (i in 3:4){
 DF[,i] <- ifelse(DF$v7 == 5, DF1$v5, DF[,i])
}