About the use of lapply

Question

Someone sent me the following code, which repeats the same command (mvrnorm) 10 times, in a list format.

dat <- lapply(1:10,mvrnorm, n = 10, Sigma=matrix(.3, 3, 3), mu = rep(0, 3))

The code works.

However, when I try the following command, it did not work and I do not understand why it does not work. I expect it to count the number of columns in 'x' ten times repeatedly:

dat <- lapply(1:10, ncol, x=matrix(.3, 4, 4))

Error in FUN(X[[i]], ...) : unused argument (X[[i]])

Basically, I am trying to understand in what situation the following format works:

lapply(1:10, function, ...)

If anyone can explain to me why it works when the function is mvrnorm (first example) but not ncol (second example)?

Answer 1

How to reproduce that error with as few lines as possible in R:

test = function(){    #method takes no parameters
    print("ok")
}
lapply(1:10, test)     #passing in 1 through 10 into test, 
                       #error happens here

Throws the error:

Error in FUN(X[[i]], ...) : unused argument (X[[i]])
Calls: lapply -> FUN
Execution halted

The method 'test' takes no arguments, and you passed 1 argument to it with lapply.

How to fix it:

Make sure the function passed to sapply takes the same number of arguments that you are feeding it:

test = function(foobar){     #method takes one parameter
    print("ok")
}
lapply(1:10, test)           #passing in 1 through 10 into test,
                             #this runs correctly, printing ok 10 times.