Functionally creating variables using string names in R -

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The same argument I am trying to generate a function to create a bunch of columns on a data frame containing the same naming conventions Unfortunately, I bump into some strange behavior while creating the variable, and I hope someone can tell what's going on. 

  df < - data.frame (var1 = c (1,2,3), var2 = c (3,4,5), var3 = c ("foo", "bar", "baz")) doesnotwork & lt; - function (df, varname) {df [paste (varname, "_square", sep = ""]]

dfBad in var1 Instead of a variable called a variable var1 and a variable named var1_squared as I had hoped.

The function below All values ​​of the original variables around this value, according to the name of the new variable, then only the new variable will have the same operation , But it is unpleasant in a way, and I am not sure what will happen if I need to use logic in many variables. 

  Works <- Function (df, varname) {df [paste (varname, "_square", sep = [2] (Previous) [2] (Previous) [2] (2) (2) Df)} dfGood <- Works ( Df, "var1") dfGood var1 var2 var3 var1_square 1 1 3 foo 1 2 2 4 times 4 3 3 5 baz 9

Here no guidance is highly appreciated, especially if the variable Swings between names of strings and column objects for references A good way of doing this.

You are missing a comma. 

  df < - dat A.frame (var1 = c (1,2,3), var2 = c (3,4,5), var3 = c ("foo", "bar", "falcon")) - Function (df, varnama) {df [, paste (varna, "_square", sep = ""]]

Edit: Okay then the above answer works, but this is actually the question of Does not answer why the other works.

For example: 

  Multiplication Works & lt; - Function (df, varname) {df [paste (varname, "_square", sep = "")] & lt; - Df [varname] * 2 return (df)} As

As all other non-exponential operators do, if we look at the data. Frame Operator source code, See the bit: 

  Ops.data.frame ... if (. "%" ("+", "-", "*", "/", "%%" , "% /%")) {Name (value) & lt; - cn data.frame (value, line.name = rn, check.name = FALSE, check.rows = FALSE)} other matrix (list, value, recursive = FALSE, use.names = FALSE), nrow = nr, dimnames = List (rn, cn)) ...

Actually it is saying that if the operator is one of them, then a data with the given names. Return the frame, otherwise return a matrix with the given names. For some reason, "^" operator is not the only one listed, we can very easily confirm it: 

  df < - data.frame (var1 = c (1,2,3), var2 = c (3,4,5), var3 = c ("foo", "bar", "falcon")) square (df ["var1" "] * 2)> [1]" data.frame "category (df [" var1 "] ^ 2) & gt; [1] With "matrix" exponentonone, and  only  exponentiation, the dimnames of the matrix reverse the new column names of your data. Frame when you assign it R is strange, comic, that means that you can also get your code to work by  as.data.frame ()  wrap around  
 If you use  actually  using your initial function strange: 
 
  ❥ name (dfBad) [1] "var1" "var2" "Var3" "var1_square" ❥ dfBad var1 var2 var3 var1 var1 1 1 3 Foo 1 2 2 4 times 4 3 3 5 buz 9 ❥ str (dfBad) 'data.frame': 3 object 4 variables: $ var1: num 1 2 3 $ var2: num 3 4 5 $ var3: factor w / 3 level "bar", "falcon", "foo": 3 1 2 $ var1_square: num [1: 3, 1] 1 4 9 .. - attr (*, "Dimnames") = 2's list .. .. $: zero .. .. $: chr "var1"  
 
 R < knows  The column is the correct name, but you show the name of the matrix you have left in it.

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