Storing loop output in a dataframe in R

Question

I want to know how to store the values of the complete loop output into a single dataframe in R. For example,

for(i in unique(x$id)){
    .
    .
    .
    y=output of one iteration}

At the end of each iteration, I am getting the output in y. But I want to store output of all iterations into y. How do I do that in R?

You should seriously consider using lapply. It's designed for this. — Roland
– Roland, Commented Aug 6, 2015 at 18:18

penguin2718 · Accepted Answer · 2015-08-06 18:24:38Z

24

You can do this simply by

y  <- NULL;
for (i in unique(x$id))
 { 
  tmp <- [output of one iteration]
  y <- rbind(y, tmp)
 }

answered Aug 6, 2015 at 18:24

penguin2718

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3 Comments

Roland Over a year ago

Don't grow an object in a loop. It's slow. Pre-allocate to the size you need!

Leyla Alkan Over a year ago

How do you mean? Could you please show what needs to be fixed here in order for the code to run faster? @Roland

Peter Mortensen Over a year ago

Inadvertent Shlemiel the painter’s algorithm?

cr1msonB1ade · Accepted Answer · 2015-08-06 18:19:21Z

6

You can begin with y as an empty data.frame as in: y <- data.frame(). Then bind the rows to this data.frame at the end of each iteration as in: y <- rbind.data.frame(y, [output of one interation]). But you can also make this a little more tight by wrapping it in an lapply and do.call as in:

y <- do.call(rbind.data.frame,
             lapply(unique(x$id),
                    function(i){
       ...;
       return([output of one iteration])}))

answered Aug 6, 2015 at 18:19

cr1msonB1ade

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