What is the logic of this function in R?












16















I'm studying about arguments in R functions, but I have some problem to understand the logic of it.



h <- function(a = 1, b = d){

    d <- (a + 1)^2

    c(a, b)

}



h()

# [1] 1 4


I expected the error message would be return because there is no value of b.
d is created under h function but there is no code like b = d which assign a value to b in function h .



However, the result is [1] 1 4.



How were b and d linked?






r function variable-assignment







share|improve this question





























    16















    I'm studying about arguments in R functions, but I have some problem to understand the logic of it.



    h <- function(a = 1, b = d){
    
    d <- (a + 1)^2
    
    c(a, b)
    
}
    

    
h()
    
# [1] 1 4


    I expected the error message would be return because there is no value of b.
    d is created under h function but there is no code like b = d which assign a value to b in function h .



    However, the result is [1] 1 4.



    How were b and d linked?






    r function variable-assignment







    share|improve this question



























      16












      16








      16


      3






      I'm studying about arguments in R functions, but I have some problem to understand the logic of it.



      h <- function(a = 1, b = d){
      
    d <- (a + 1)^2
      
    c(a, b)
      
}
      

      
h()
      
# [1] 1 4


      I expected the error message would be return because there is no value of b.
      d is created under h function but there is no code like b = d which assign a value to b in function h .



      However, the result is [1] 1 4.



      How were b and d linked?






      r function variable-assignment







      share|improve this question
















      I'm studying about arguments in R functions, but I have some problem to understand the logic of it.



      h <- function(a = 1, b = d){
      
    d <- (a + 1)^2
      
    c(a, b)
      
}
      

      
h()
      
# [1] 1 4


      I expected the error message would be return because there is no value of b.
      d is created under h function but there is no code like b = d which assign a value to b in function h .



      However, the result is [1] 1 4.



      How were b and d linked?






      r function variable-assignment




      r function variable-assignment






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Feb 7 at 14:11









      chrki

      4,73242546




      4,73242546










      asked Feb 7 at 8:04









      Song LeeSong Lee

      913




      913
























          1 Answer
          1






          active

          oldest

          votes


















          18














          Default function arguments values are lazily evaluated in R (i.e. evaluated only when they're needed):



          See the output of this code for an example :



          printme <- function(name,x){cat('evaluating',name,'n');x}
          

          
h <- function(a = printme('a',1), b = printme('b',d)){
          
  cat('computing d...n')
          
  d <- (a + 1)^2
          
  cat('d computedn')
          
  cat('concatenating a and b...n')
          
  c(a, b)
          
  cat('a and b concatenatedn')
          
}
          

          
h()


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating b 
          
a and b concatenated


          As you can see, d is calculated before evaluating the default value of b



          EDIT :



          Furthermore, as correctly pointed out by @BrodieG in the comments, default arguments are evaluated in the function environment; in fact, in the example above, b can be initialized to the value of variable d that is defined inside the function environment.



          Instead, when you specify a argument (without using the default), the expression that assigns the parameter is still lazily evaluated, but this time in the calling environment e.g. :



          # same functions as above, but this time we specify the parameters in the call     
          
h(a=printme('a',123),b=printme('d',d))


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating d 
          
Error in printme("d", d) : object 'd' not found


          Note the error when argument b is evaluated because d cannot be found in the calling environment.






          share|improve this answer





















          • 2





            Might be worth highlighting that default arguments are evaluated in the environment of the function they belong to, not in the calling environment like non-default arguments.

            – BrodieG
            Feb 7 at 13:18











          • @BrodieG: good idea, edited my answer ;)

            – digEmAll
            Feb 7 at 18:28











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          1 Answer
          1






          active

          oldest

          votes








          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          18














          Default function arguments values are lazily evaluated in R (i.e. evaluated only when they're needed):



          See the output of this code for an example :



          printme <- function(name,x){cat('evaluating',name,'n');x}
          

          
h <- function(a = printme('a',1), b = printme('b',d)){
          
  cat('computing d...n')
          
  d <- (a + 1)^2
          
  cat('d computedn')
          
  cat('concatenating a and b...n')
          
  c(a, b)
          
  cat('a and b concatenatedn')
          
}
          

          
h()


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating b 
          
a and b concatenated


          As you can see, d is calculated before evaluating the default value of b



          EDIT :



          Furthermore, as correctly pointed out by @BrodieG in the comments, default arguments are evaluated in the function environment; in fact, in the example above, b can be initialized to the value of variable d that is defined inside the function environment.



          Instead, when you specify a argument (without using the default), the expression that assigns the parameter is still lazily evaluated, but this time in the calling environment e.g. :



          # same functions as above, but this time we specify the parameters in the call     
          
h(a=printme('a',123),b=printme('d',d))


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating d 
          
Error in printme("d", d) : object 'd' not found


          Note the error when argument b is evaluated because d cannot be found in the calling environment.






          share|improve this answer





















          • 2





            Might be worth highlighting that default arguments are evaluated in the environment of the function they belong to, not in the calling environment like non-default arguments.

            – BrodieG
            Feb 7 at 13:18











          • @BrodieG: good idea, edited my answer ;)

            – digEmAll
            Feb 7 at 18:28
















          18














          Default function arguments values are lazily evaluated in R (i.e. evaluated only when they're needed):



          See the output of this code for an example :



          printme <- function(name,x){cat('evaluating',name,'n');x}
          

          
h <- function(a = printme('a',1), b = printme('b',d)){
          
  cat('computing d...n')
          
  d <- (a + 1)^2
          
  cat('d computedn')
          
  cat('concatenating a and b...n')
          
  c(a, b)
          
  cat('a and b concatenatedn')
          
}
          

          
h()


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating b 
          
a and b concatenated


          As you can see, d is calculated before evaluating the default value of b



          EDIT :



          Furthermore, as correctly pointed out by @BrodieG in the comments, default arguments are evaluated in the function environment; in fact, in the example above, b can be initialized to the value of variable d that is defined inside the function environment.



          Instead, when you specify a argument (without using the default), the expression that assigns the parameter is still lazily evaluated, but this time in the calling environment e.g. :



          # same functions as above, but this time we specify the parameters in the call     
          
h(a=printme('a',123),b=printme('d',d))


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating d 
          
Error in printme("d", d) : object 'd' not found


          Note the error when argument b is evaluated because d cannot be found in the calling environment.






          share|improve this answer





















          • 2





            Might be worth highlighting that default arguments are evaluated in the environment of the function they belong to, not in the calling environment like non-default arguments.

            – BrodieG
            Feb 7 at 13:18











          • @BrodieG: good idea, edited my answer ;)

            – digEmAll
            Feb 7 at 18:28














          18












          18








          18







          Default function arguments values are lazily evaluated in R (i.e. evaluated only when they're needed):



          See the output of this code for an example :



          printme <- function(name,x){cat('evaluating',name,'n');x}
          

          
h <- function(a = printme('a',1), b = printme('b',d)){
          
  cat('computing d...n')
          
  d <- (a + 1)^2
          
  cat('d computedn')
          
  cat('concatenating a and b...n')
          
  c(a, b)
          
  cat('a and b concatenatedn')
          
}
          

          
h()


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating b 
          
a and b concatenated


          As you can see, d is calculated before evaluating the default value of b



          EDIT :



          Furthermore, as correctly pointed out by @BrodieG in the comments, default arguments are evaluated in the function environment; in fact, in the example above, b can be initialized to the value of variable d that is defined inside the function environment.



          Instead, when you specify a argument (without using the default), the expression that assigns the parameter is still lazily evaluated, but this time in the calling environment e.g. :



          # same functions as above, but this time we specify the parameters in the call     
          
h(a=printme('a',123),b=printme('d',d))


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating d 
          
Error in printme("d", d) : object 'd' not found


          Note the error when argument b is evaluated because d cannot be found in the calling environment.






          share|improve this answer















          Default function arguments values are lazily evaluated in R (i.e. evaluated only when they're needed):



          See the output of this code for an example :



          printme <- function(name,x){cat('evaluating',name,'n');x}
          

          
h <- function(a = printme('a',1), b = printme('b',d)){
          
  cat('computing d...n')
          
  d <- (a + 1)^2
          
  cat('d computedn')
          
  cat('concatenating a and b...n')
          
  c(a, b)
          
  cat('a and b concatenatedn')
          
}
          

          
h()


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating b 
          
a and b concatenated


          As you can see, d is calculated before evaluating the default value of b



          EDIT :



          Furthermore, as correctly pointed out by @BrodieG in the comments, default arguments are evaluated in the function environment; in fact, in the example above, b can be initialized to the value of variable d that is defined inside the function environment.



          Instead, when you specify a argument (without using the default), the expression that assigns the parameter is still lazily evaluated, but this time in the calling environment e.g. :



          # same functions as above, but this time we specify the parameters in the call     
          
h(a=printme('a',123),b=printme('d',d))


          Console output :



          computing d...
          
evaluating a 
          
d computed
          
concatenating a and b...
          
evaluating d 
          
Error in printme("d", d) : object 'd' not found


          Note the error when argument b is evaluated because d cannot be found in the calling environment.







          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Feb 7 at 18:27

























          answered Feb 7 at 8:17









          digEmAlldigEmAll

          47k989124




          47k989124








          • 2





            Might be worth highlighting that default arguments are evaluated in the environment of the function they belong to, not in the calling environment like non-default arguments.

            – BrodieG
            Feb 7 at 13:18











          • @BrodieG: good idea, edited my answer ;)

            – digEmAll
            Feb 7 at 18:28














          • 2





            Might be worth highlighting that default arguments are evaluated in the environment of the function they belong to, not in the calling environment like non-default arguments.

            – BrodieG
            Feb 7 at 13:18











          • @BrodieG: good idea, edited my answer ;)

            – digEmAll
            Feb 7 at 18:28








          2




          2





          Might be worth highlighting that default arguments are evaluated in the environment of the function they belong to, not in the calling environment like non-default arguments.

          – BrodieG
          Feb 7 at 13:18





          Might be worth highlighting that default arguments are evaluated in the environment of the function they belong to, not in the calling environment like non-default arguments.

          – BrodieG
          Feb 7 at 13:18













          @BrodieG: good idea, edited my answer ;)

          – digEmAll
          Feb 7 at 18:28





          @BrodieG: good idea, edited my answer ;)

          – digEmAll
          Feb 7 at 18:28




















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