How to insert one element into a vector?

There must be a million ways of doing this!

Actually I'd say there were no ways of doing this, since I dont think 
you can actually insert into a vector - you have to create a new vector 
that produces the illusion of insertion!

  Here's a Q+D function that fails if you try and insert at the start, 
or at the end. Its very D.

insert <- function(v,e,pos){
   return(c(v[1:(pos-1)],e,v[(pos):length(v)]))
}

  > v=c(1,2,3,4,6)

  > insert(v,5,5)
  [1] 1 2 3 4 5 6

Yay.

  > insert(v,5,1)
  [1] 1 5 1 2 3 4 6

Oops.

  Cant be bothered to fix it, the principle is there.

Baz

michael watson (IAH-C) wrote:

There must be a million ways of doing this!

  Actually I'd say there were no ways of doing this, since I dont
think you can actually insert into a vector - you have to create a
new vector that produces the illusion of insertion!

  Here's a Q+D function that fails if you try and insert at the
start, or at the end. Its very D.

insert <- function(v,e,pos){
   return(c(v[1:(pos-1)],e,v[(pos):length(v)]))
}

Pretty much what 'append' does.

Deepayan

Barry Rowlingson <B.Rowlingson at lancaster.ac.uk> writes:

michael watson (IAH-C) wrote:

There must be a million ways of doing this!

  Actually I'd say there were no ways of doing this, since I dont
think you can actually insert into a vector - you have to create a new
vector that produces the illusion of insertion!

  Here's a Q+D function that fails if you try and insert at the start,
or at the end. Its very D.

insert <- function(v,e,pos){
   return(c(v[1:(pos-1)],e,v[(pos):length(v)]))
}

This is actually an exercise in a certain introductory book on R, with
the intention of having the student do the obvious c(v[1:4],5,v[5:5])
style computation and maybe think a little about the edge effects. I
only recently saw the somewhat unfortunately named append() function,
which has been in S/R ever since the blue book...:

append(v,5,4)

[1] 1 2 3 4 5 6

O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907

Pretty much what 'append' does.

A shame then, that help.search("insert") doesn't find 'append'! I cant 
think why anyone looking for a way of _inserting_ a value in the middle 
of a vector would think of looking at "append"!

  Python has separate insert and append methods for vectors.

  >>> x=[1,2,3,4,6]
  >>> x.insert(4,5)
  >>> x
  [1, 2, 3, 4, 5, 6]
  >>> x.append(99)
  >>> x
  [1, 2, 3, 4, 5, 6, 99]


Barry

Deepayan Sarkar wrote:

Pretty much what 'append' does.

A shame then, that help.search("insert") doesn't find 'append'! I cant think 
why anyone looking for a way of _inserting_ a value in the middle of a vector 
would think of looking at "append"!

Yes, this should be fixed.

Python has separate insert and append methods for vectors.

I don't think this is a good idea unless there are efficiency issues. 
You could always define one: the definition is fairly simple.
    insert<-append

 	-thomas

Deepayan Sarkar wrote:

Pretty much what 'append' does.

  A shame then, that help.search("insert") doesn't find 'append'! I cant 
think why anyone looking for a way of _inserting_ a value in the middle 
of a vector would think of looking at "append"!

  Python has separate insert and append methods for vectors.

  >>> x=[1,2,3,4,6]
  >>> x.insert(4,5)
  >>> x

  [1, 2, 3, 4, 5, 6]

  >>> x.append(99)
  >>> x

  [1, 2, 3, 4, 5, 6, 99]

So has R. R's 'insert' is called 'append', and R's 'append' is called
'c'. Counter-intuitively though, and I'm happy that Peter Dalgaard
didn't know that 'append' inserts: it gives some hope to us ordinary
mortals.

cheers, jazza

Jari Oksanen <jarioksa at sun3.oulu.fi>