R help - Jul 2016 - Improving performance by rewriting for loops into apply functions

Hello,

I have a slight performance issue that I'd like to solve by rewriting a
short bit of code that uses for loops so that it would use apply in order
to get some performance gains. My problem is that I can't modify the
variables that are passed to apply function during apply functions
execution and use it's latest results. My first thought was to use apply
functions but if this isn't possible I'm open to other suggestions.

For example:
path1 = matrix(c(1,2,3,4,5), ncol=1);
path2 = matrix(c(1,2,3,4,5,6), ncol=1);
apply(path1, 2, function(x, path2){
  tmp = x*path2[x+1];
  path2[x+1] = tmp;
  return(tmp);
}, path2)

In the code above, path2 should have its elements updated in the course of
the apply function and its value should be use in the next iteration while
executing apply function but that doesn't happen. It seems that when a
variable is passed into apply function (path2) it is immutable.

BR Ale?

	[[alternative HTML version deleted]]

If you complain to the doctor that it hurts when you ram your head into the
wall, (s)he is going to tell you to not so that. What do you expect us to say?

You seem full of misinformation. The apply family functions do not necessarily
speed anything up... they are just more compact than for loops.

And yes, their arguments are immutable... as are pretty much all arguments to
functions other than environments. So your screwdriver is not going to work on
that nail.

Also, working with N x 1 matrices is silly... this is not Matlab.

In many cases you can restructure the problem so that you don't need
incremental calculation... but I am not really sure what you wanted in this
case. If this were a for loop the code you have looks to me like it would
produce

path1 <- c(1,2,3,4,5)
path2 <- c(1,2,3,4,5,6)
path1 * path2[ -1 ]

Maybe it is time for you to read "The R Inferno"? 

Also please read the Posting Guide, which warns you to post plain text (so your
email does not get corrupted when it is stripped by the mailing list).
-- 
Sent from my phone. Please excuse my brevity.

On July 22, 2016 4:16:18 PM PDT, "Ale? Grm" <grm.ales at
gmail.com> wrote:
>Hello,
>
>I have a slight performance issue that I'd like to solve by rewriting a
>short bit of code that uses for loops so that it would use apply in
>order
>to get some performance gains. My problem is that I can't modify the
>variables that are passed to apply function during apply functions
>execution and use it's latest results. My first thought was to use
>apply
>functions but if this isn't possible I'm open to other suggestions.
>
>For example:
>path1 = matrix(c(1,2,3,4,5), ncol=1);
>path2 = matrix(c(1,2,3,4,5,6), ncol=1);
>apply(path1, 2, function(x, path2){
>  tmp = x*path2[x+1];
>  path2[x+1] = tmp;
>  return(tmp);
>}, path2)
>
>In the code above, path2 should have its elements updated in the course
>of
>the apply function and its value should be use in the next iteration
>while
>executing apply function but that doesn't happen. It seems that when a
>variable is passed into apply function (path2) it is immutable.
>
>BR Ale?
>
>	[[alternative HTML version deleted]]
>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.

Hi,

The apply here does exactly what you would expect. (Your problem seems to be
with the function that you have written. For some reason, you want to change the
values of path2 but are passing it as a variable o a function. The global value
of path2 will not change.) For that, you have to also return path2.

Try:

path1 <- matrix(c(1,2,3,4,5), ncol=1);
path2 <- matrix(c(1,2,3,4,5,6), ncol=1);
apply(X = path1, MAR = 2, function(x, path2 = path2){
    tmp <- x*path2[x+1]
    path2[x+1] <- tmp
    list(tmp, path2)
}, path2 = path2)


Btw, it is good for your sanity to make the distinction between assignments and
arguments, and also name your arguments for your function when you call them. I
have done that. It makes understanding your code at a glance easier.

As a personal preference, I have no idea what this is supposed to do, but you
may be better off avoiding apply here. I am always nervous about passing
arguments such as you do with the x in the apply function. But it is correct,
and hopefully does what you want. so I will say nothing more.

HTH!

Best wishes,
Ranjan


On Fri, 22 Jul 2016 18:16:18 -0500 Ale? Grm <grm.ales at gmail.com> wrote:
> Hello,
> 
> I have a slight performance issue that I'd like to solve by rewriting a
> short bit of code that uses for loops so that it would use apply in order
> to get some performance gains. My problem is that I can't modify the
> variables that are passed to apply function during apply functions
> execution and use it's latest results. My first thought was to use
apply
> functions but if this isn't possible I'm open to other suggestions.
> 
> For example:
> path1 = matrix(c(1,2,3,4,5), ncol=1);
> path2 = matrix(c(1,2,3,4,5,6), ncol=1);
> apply(path1, 2, function(x, path2){
>   tmp = x*path2[x+1];
>   path2[x+1] = tmp;
>   return(tmp);
> }, path2)
> 
> In the code above, path2 should have its elements updated in the course of
> the apply function and its value should be use in the next iteration while
> executing apply function but that doesn't happen. It seems that when a
> variable is passed into apply function (path2) it is immutable.
> 
> BR Ale?
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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