r - odd behavior of print within mapply -


i seeing unexpected behavior (to me anyway) when print() included side effect in function wrapped in mapply().

for example, works expected (and yes know it's not how add vectors):

mapply(function(i,j) i+j, i=1:3, j=4:6) # returns [1] 5 7 9

and this:

mapply(function(i,j) paste(i, "plus", j, "equals", i+j), i=1:3, j=4:6)  # returns [1] "1 plus 4 equals 5" "2 plus 5 equals 7" "3 plus 6 equals 9"

but doesn't:

mapply(function(i,j) print(paste(i, "plus", j, "equals", i+j)), i=1:3, j=4:6) # returns: # [1] "1 plus 4 equals 5" # [1] "2 plus 5 equals 7" # [1] "3 plus 6 equals 9" # [1] "1 plus 4 equals 5" "2 plus 5 equals 7" "3 plus 6 equals 9"

what's going on here? haven't used mapply() in while, maybe no-brainer... i'm using r version 3.4.0.

print both prints argument , returns value.

p <- print("abc") # [1] "abc" p # [2] "abc"

so each element gets printed, vector of stuff gets returned (and printed). try e.g. invisible(mapply(...)) or m <- mapply(...) comparison.

fwiw cat() returns null ...


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