R has something called lazy evaluation what does that mean

Question

I've just started to learn R and have read about something called lazy evaluation. I dont understand what that means as it does not exist in any other language. Can somebody explain?

Answer 1

Let me explain this with an example. Consider the following code, here there are two paramaters passed, a and b. But only a is being used in the function.

F <- function(a,b)
{
                a^2
}

So when we call the function with just one parameter it assumes that one parameter is the value for the variable that is being used. So if we call the function like this, 

F(2)

it assumes 2 to be a's value. Calling the function F with just one parameter doesn’t give any error. 

Comments

While your example is correct about b not being evaluated, your reason is not totally right: F uses 2 for the value of a not because it is used in the function, but because a is the first parameter of F. If F were defined as 

F <- function(a,b)
{
                b^2
}

then calling F(2) would give an error, because a would be given the value of 2, and b has no value.

Lazy evaluation is implemented in a lot of functional languages, like Scala, Haskell, R, and can be used in eager languages like Python (think range function) or even C++.