[R] error when copy and transform within a data frame

[Al R]

# trying to do a copy and a transform within a data frame, but getting the
"arguments imply differing number of rows" error, and I'm not sure why

a=c(1,2,3)
b=c(2,3,4)
c=c("Yes","No","Yes")
d=c("No","Yes","No")

df=data.frame(a,b,c,d)

# the following works fine!
df = transform(df, new=sapply(df[,c(1,2)], FUN = function(x) { x^2 } ))

# but the following doesn't work:

num_value = function(x) {
	if (x == "Yes") { return(1) }
	else  if (x == "No") { return(0) }
 	else return(NA)
}

df = transform(df, new=sapply(df[,c(3,4)], FUN = num_value ))

# generates this error..  
Error in data.frame(list(a = c(1, 2, 3), b = c(2, 3, 4), c = c(2L, 1L,  : 
  arguments imply differing number of rows: 3, 2

# thanks for the help!

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