What are the differences between and - assignment operators in R

Question

What are the differences between the assignment operators   and  lt - in R    I know that operators are slightly different  as this example shows  x  lt - y  lt - 5 x   y   5 x   y  lt - 5 x  lt - y   5   Error in  x  lt - y    5   could not find function   lt - lt -    But is this the only difference

User · Accepted Answer

What are the differences between the assignment operators = and <- in R?

As your example shows, = and <- have slightly different operator precedence (which determines the order of evaluation when they are mixed in the same expression). In fact, ?Syntax in R gives the following operator precedence table, from highest to lowest:

…
‘-> ->>’           rightwards assignment
‘<- <<-’           assignment (right to left)
‘=’                assignment (right to left)
…

But is this the only difference?

Since you were asking about the assignment operators: yes, that is the only difference. However, you would be forgiven for believing otherwise. Even the R documentation of ?assignOps claims that there are more differences:

The operator <- can be used anywhere, whereas the operator = is only allowed at the top level (e.g., in the complete expression typed at the command prompt) or as one of the subexpressions in a braced list of expressions.

Let’s not put too fine a point on it: the R documentation is wrong. This is easy to show: we just need to find a counter-example of the = operator that isn’t (a) at the top level, nor (b) a subexpression in a braced list of expressions (i.e. {…; …}). — Without further ado:

x
# Error: object 'x' not found
sum((x = 1), 2)
# [1] 3
x
# [1] 1

Clearly we’ve performed an assignment, using =, outside of contexts (a) and (b). So, why has the documentation of a core R language feature been wrong for decades?

It’s because in R’s syntax the symbol = has two distinct meanings that get routinely conflated (even by experts, including in the documentation cited above):

The first meaning is as an assignment operator. This is all we’ve talked about so far.
The second meaning isn’t an operator but rather a syntax token that signals named argument passing in a function call. Unlike the = operator it performs no action at runtime, it merely changes the way an expression is parsed.

So how does R decide whether a given usage of = refers to the operator or to named argument passing? Let’s see.

In any piece of code of the general form …

‹function_name›(‹argname› = ‹value›, …)
‹function_name›(‹args›, ‹argname› = ‹value›, …)

… the = is the token that defines named argument passing: it is not the assignment operator. Furthermore, = is entirely forbidden in some syntactic contexts:

if (‹var› = ‹value›) …
while (‹var› = ‹value›) …
for (‹var› = ‹value› in ‹value2›) …
for (‹var1› in ‹var2› = ‹value›) …

Any of these will raise an error “unexpected '=' in ‹bla›”.

In any other context, = refers to the assignment operator call. In particular, merely putting parentheses around the subexpression makes any of the above (a) valid, and (b) an assignment. For instance, the following performs assignment:

median((x = 1 : 10))

But also:

if (! (nf = length(from))) return()

_{Now you might object that such code is atrocious (and you may be right). But I took this code from the base::file.copy function (replacing <- with =) — it’s a pervasive pattern in much of the core R codebase.}

The original explanation by John Chambers, which the the R documentation is probably based on, actually explains this correctly:

[= assignment is] allowed in only two places in the grammar: at the top level (as a complete program or user-typed expression); and when isolated from surrounding logical structure, by braces or an extra pair of parentheses.

In sum, by default the operators <- and = do the same thing. But either of them can be overridden separately to change its behaviour. By contrast, <- and -> (left-to-right assignment), though syntactically distinct, always call the same function. Overriding one also overrides the other. Knowing this is rarely practical but it can be used for some fun shenanigans.

User · Answer

x   y   5 is equivalent to x    y   5   because the assignment operators  group  right to left  which works   Meaning  assign 5 to y  leaving the number 5  and then assign that 5 to x    This is not the same as  x   y    5  which doesn t work  Meaning  assign the value of y to x  leaving the value of y  and then assign 5 to  umm     what exactly   When you mix the different kinds of assignment operators   lt - binds tighter than    So x   y  lt - 5 is interpreted as x    y  lt - 5   which is the case that makes sense   Unfortunately  x  lt - y   5 is interpreted as  x  lt - y    5  which is the case that doesn t work   See  Syntax and  assignOps for the precedence  binding  and grouping rules

User · Answer

Google s R style guide simplifies the issue by prohibiting the     for assignment  Not a bad choice   https   google github io styleguide Rguide xml  The R manual goes into nice detail on all 5 assignment operators   http   stat ethz ch R-manual R-patched library base html assignOps html

User · Answer

According to John Chambers  the operator   is only allowed at  the top level   which means it is not allowed in control structures like if  making the following programming error illegal    gt  if x   0  1 else x Error  syntax error   As he writes   Disallowing the new assignment form     in control expressions avoids programming errors  such as the example above  that are more likely with the equal operator than with other S assignments    You can manage to do this if it s  isolated from surrounding logical structure  by braces or an extra pair of parentheses   so if   x   0   1 else x would work   See http   developer r-project org equalAssign html

User · Answer

This may also add to understanding of the difference between those two operators   df  lt - data frame        a   rnorm 10         b  lt - rnorm 10      For the first element R has assigned values and proper name  while the name of the second element looks a bit strange   str df     data frame   10 obs  of  2 variables       a               num  0 6393 1 125 -1 2514 0 0729 -1 3292          b    rnorm 10   num  0 2485 0 0391 -1 6532 -0 3366 1 1951       R version 3 3 2  2016-10-31   macOS Sierra 10 12 1

User · Answer

The difference in assignment operators is clearer when you use them to set an argument value in a function call   For example   median x   1 10  x       Error  object  x  not found   In this case  x is declared within the scope of the function  so it does not exist in the user workspace   median x  lt - 1 10  x         1   1  2  3  4  5  6  7  8  9 10   In this case  x is declared in the user workspace  so you can use it after the function call has been completed     There is a general preference among the R community for using  lt - for assignment  other than in function signatures   for compatibility with  very  old versions of S-Plus   Note that the spaces help to clarify situations like  x lt -3   Does this mean assignment  x  lt - 3   Or less than  x  lt  -3     Most R IDEs have keyboard shortcuts to make  lt - easier to type   Ctrl     in Architect  Alt   - in RStudio  Option   - under macOS   Shift   -  underscore  in emacs ESS     If you prefer writing   to  lt - but want to use the more common assignment symbol for publicly released code  on CRAN  for example   then you can use one of the tidy   functions in the formatR package to automatically replace   with  lt -   library formatR  tidy source text    x 1 5   arrow   TRUE     x  lt - 1 5     The answer to the question  Why does x  lt - y   5 throw an error but not x  lt - y  lt - 5   is  It s down to the magic contained in the parser    R s syntax contains many ambiguous cases that have to be resolved one way or another   The parser chooses to resolve the bits of the expression in different orders depending on whether   or  lt - was used   To understand what is happening  you need to know that assignment silently returns the value that was assigned   You can see that more clearly by explicitly printing  for example print x  lt - 2   3    Secondly  it s clearer if we use prefix notation for assignment   So   x  lt - 5   lt -  x  5    same thing  y   5     y  5     also the same thing   The parser interprets x  lt - y  lt - 5 as     lt -  x    lt -  y  5     We might expect that x  lt - y   5 would then be    lt -  x      y  5     but actually it gets interpreted as        lt -  x  y   5    This is because   is lower precedence than  lt -  as shown on the  Syntax help page

User · Answer

The operators  lt - and   assign into the environment in which they are evaluated  The operator  lt - can be used anywhere  whereas the operator   is only allowed at the top level  e g   in the complete expression typed at the command prompt  or as one of the subexpressions in a braced list of expressions

[r] What are the differences between "=" and "<-" assignment operators in R?

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