Different behavior in using '=' versus '

I would like to know why there is a difference between using = and <- while assigning a dataframe.

Case a: Using =

set.seed(100);a <- data.frame(a1=rnorm(10),a2=sample(c(1,0),10,replace=TRUE))

Case b: Using <-

set.seed(100);b <- data.frame(b1 <- rnorm(10),b2 <- sample(c(1,0),10,replace=TRUE))

Why is there the following difference? Why does not second method retain the variable/column names?

> a
           a1 a2
1 -0.50219235  0
2  0.13153117  0
3 -0.07891709  1
4  0.88678481  1
5  0.11697127  0

>b

  b1....rnorm.5. b2....sample.c.1..0...5..replace...TRUE.
1    -0.50219235                                        0
2     0.13153117                                        0
3    -0.07891709                                        1
4     0.88678481                                        1
5     0.11697127                                        0

Answer

Within functions '=' is used as naming or referring the function to a variable of a particular name and <- refers to the assignment function. When R runs it will first resolve '<-" functions within your function parameters. It will then name the variable wither the thing to the left of the equal sign or the full expression in this case "b1 <- rnorm(10)". Finally it will resolve the function (in this case data.frame).

You almost always want to use the '=' within a function. There can be cases where you may want to nest an assignment "<-" but normally this will make your code ridiculous to read.