How to use apply, cat and print, without getting N

问题:

I am trying to use cat() as functions inside apply(). I can almost make R do what I want, but I'm getting some very confusing (to me) NULLS at the end of the return. Here is a silly example, to highlight what I'm getting.

val1 <- 1:10
val2 <- 25:34
values <- data.frame(val1, val2)
apply(values, 1, function(x) cat(x[1], x[2], fill=TRUE))

This "works" in that R accepts it and it runs, but I don't understand the results.

> apply(values, 1, function(x) cat(x[1], x[2], fill=TRUE))
1 25
2 26
3 27
4 28
5 29
6 30
7 31
8 32
9 33
10 34
NULL

But, I want to get:

> apply(values, 1, function(x) cat(x[1], x[2], fill=TRUE))
1 25
2 26
3 27
4 28
5 29
6 30
7 31
8 32
9 33
10 34

So, how do I remove that final NULL?

回答1:

The NULL is the R interpreter printing the value of the expression you typed in - the apply. You can either assign it somewhere:

junk = apply(values, 1, function(x) cat(x[1], x[2], fill=TRUE))

in which case it wont get printed, or wrap it in 'invisible':

invisible(apply(values, 1, function(x) cat(x[1], x[2], fill=TRUE)))

Note that its only when you run this interactively that each line is printed, if it's in a function you won't see it.

回答2:

Do you really need the apply() to loop through your content?

> print(values, row.names=FALSE)
 val1 val2
    1   25
    2   26
    3   27
    4   28
    5   29
    6   30
    7   31
    8   32
    9   33
   10   34

回答3:

As Dirk pointed out this is not the way to print thing in R. Usually you would assign the result to a variable and then print it. No side effects, so to say.

Your problem stems from the cat functions, which prints to the terminal as a side effect, but returns NULL.

Try

a <- cat("blabla\n")  
a

If you really want to use apply for printing, there are two solutions. Wrap into invisible call

invisible(apply(values, 1, function(x) invisible(cat(x[1], x[2], fill=TRUE))))

or, just assign the result (NULL) to a temporary value

t <- apply(values, 1, function(x) invisible(cat(x[1], x[2], fill=TRUE)))