Paste value after certain delimiter - r

Question

I have data in the following format:
In Column A:
String1__String2__String3
In Column B:
Value
I would like to paste the Value into the String after the first delimiter like this:
String1__Value__String2__String3
The crucial part of the code I am using now (where I paste the value) is the following line:
df2 <-cbind(df[1],apply(df[,2:ncol(df)],2,function(i)ifelse(is.na(i), NA, paste(df[,1],i,sep="_"))))
With this code it append the value after the string, like this:
String1__String2__String3__Value
Is there an easy way to rearrange this so the Values will be pasted at the correct place. Or do I have to redo the complete code ?
Thanks
Update, Example:
Column A:
Jennifer__DoesSomething__inaCity
Column B:
2
Result now:
Jennifer__DoesSomething__inaCity__2
Desired result:
Jennifer__2__DoesSomething__inaCity
The strings Jennifer, DoesSomething, inaCity change and are not the same length. Only the delimiter stays the same. I want to paste after the first delimiter.
Thanks !

Answer 1

Here is an idea. Using sub we only replace the first seen pattern. So using mapply we replace all the numbers in one column with their corresponding strings on the second column.
mapply(function(x, y) sub('__', paste0('__', y, '__'), x), df$v1, df$v2)
# atsfs__dsfgg__sdgsdg eeee__FFFF__GGGG
#"atsfs__3__dsfgg__sdgsdg" "eeee__5__FFFF__GGGG"
DATA
dput(df)
structure(list(v1 = c("atsfs__dsfgg__sdgsdg", "eeee__FFFF__GGGG"
), v2 = c(3, 5)), .Names = c("v1", "v2"), row.names = c(NA, -2L
), class = "data.frame")