Global and local variables in R

Question

I am a newbie for R  and I am quite confused with the usage of local and global variables in R   I read some posts on the internet that say if I use   or  lt - I will assign the variable in the current environment  and with  lt  lt - I can access a global variable when inside a function   However  as I remember in C   local variables arise whenever you declare a variable inside brackets     so I m wondering if this is the same for R  Or is it just for functions in R that we have the concept of local variables   I did a little experiment  which seems to suggest that only brackets are not enough  am I getting anything wrong        x matrix 1 10 2 5    print x 2 2    1  4

User · Answer

A bit more along the same lines   attrs  lt -     attrs a  lt - 1  f  lt - function d        attrs a  lt - d    f 20  print attrs a    will print  1   attrs  lt -     attrs a  lt - 1  f  lt - function d       attrs a  lt  lt - d    f 20  print attrs a    Will print  20

User · Answer

lt - does assignment in the current environment   When you re inside a function R creates a new environment for you   By default it includes everything from the environment in which it was created so you can use those variables as well but anything new you create will not get written to the global environment   In most cases  lt  lt - will assign to variables already in the global environment or create a variable in the global environment even if you re inside a function   However  it isn t quite as straightforward as that   What it does is checks the parent environment for a variable with the name of interest   If it doesn t find it in your parent environment it goes to the parent of the parent environment  at the time the function was created  and looks there   It continues upward to the global environment and if it isn t found in the global environment it will assign the variable in the global environment   This might illustrate what is going on   bar  lt -  global  foo  lt - function        bar  lt -  in foo      baz  lt - function            bar  lt -  in baz - before  lt  lt -          bar  lt  lt -  in baz - after  lt  lt -          print bar            print bar      baz       print bar     gt  bar  1   global   gt  foo    1   in foo   1   in baz - before  lt  lt -   1   in baz - after  lt  lt -   gt  bar  1   global    The first time we print bar we haven t called foo yet so it should still be global - this makes sense   The second time we print it s inside of foo before calling baz so the value  in foo  makes sense   The following is where we see what  lt  lt - is actually doing   The next value printed is  in baz - before  lt  lt -  even though the print statement comes after the  lt  lt -   This is because  lt  lt - doesn t look in the current environment  unless you re in the global environment in which case  lt  lt - acts like  lt -   So inside of baz the value of bar stays as  in baz - before  lt  lt -   Once we call baz the copy of bar inside of foo gets changed to  in baz  but as we can see the global bar is unchanged  This is because the copy of bar that is defined inside of foo is in the parent environment when we created baz so this is the first copy of bar that  lt  lt - sees and thus the copy it assigns to   So  lt  lt - isn t just directly assigning to the global environment    lt  lt - is tricky and I wouldn t recommend using it if you can avoid it   If you really want to assign to the global environment you can use the assign function and tell it explicitly that you want to assign globally   Now I change the  lt  lt - to an assign statement and we can see what effect that has   bar  lt -  global  foo  lt - function        bar  lt -  in foo         baz  lt - function            assign  bar    in baz   envir    GlobalEnv            print bar      baz       print bar    bar   1   global  foo     1   in foo    1   in foo  bar   1   in baz    So both times we print bar inside of foo the value is  in foo  even after calling baz   This is because assign never even considered the copy of bar inside of foo because we told it exactly where to look   However  this time the value of bar in the global environment was changed because we explicitly assigned there   Now you also asked about creating local variables and you can do that fairly easily as well without creating a function     We just need to use the local function   bar  lt -  global    local will create a new environment for us to play in local       bar  lt -  local      print bar       1   local  bar   1   global

User · Answer

Variables declared inside a function are local to that function  For instance   foo  lt - function         bar  lt - 1   foo   bar   gives the following error  Error  object  bar  not found   If you want to make bar a global variable  you should do   foo  lt - function         bar  lt  lt - 1   foo   bar   In this case bar is accessible from outside the function   However  unlike C  C   or many other languages  brackets do not determine the scope of variables  For instance  in the following code snippet   if  x  gt  10        y  lt - 0   else       y  lt - 1     y remains accessible after the if-else statement   As you well say  you can also create nested environments  You can have a look at these two links for understanding how to use them    http   stat ethz ch R-manual R-devel library base html environment html http   stat ethz ch R-manual R-devel library base html get html   Here you have a small example   test env  lt - new env    assign  var   100  envir test env    or simply test env var  lt - 100  get  var     var cannot be found since it is not defined in this environment get  var   envir test env    now it can be found

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