assignment operator - Are `=` and `

Is it just a style preference?

As far as I can tell, they are the same.

I see many people prefer the "longer" <- version and I can't tell why (perhaps keeping away from = and == confusions?)

Answer

No, they are not exactly the same: the = operator cannot be used everywhere that <- can.

The operators <- and = assign into the environment in which they are evaluated. 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.

There are also differences in scope. See this answer for more details.

Which is better depends on who you ask.

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Reading from "Introducing Monte Carlo Methods with R", by Robert and Casella:

"The assignment operator is =, not to be confused with ==, which is the Boolean operator for equality. An older assignment operator is <- and, for compatibility reasons, it still remains functional, but it should be ignored to ensure cleaner programming. (As pointed out by Spector, P. (2009). 'Data Manipulation with R' - Section 8.7., an exception is when using system.time, since = is then used to identify keywords)

Source

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On the other hand, Google's R style guide recommends using <-:

Assignment

Use <-, not =, for assignment.

GOOD:
x <- 5

BAD:
x = 5