[R] strange behaviour with loops and lists

[Liviu Andronic]

Dear all,
I encountered this strange behaviour with loops and lists. Consider this:
xl <- list()
for(i in 5:7){##loop over numeric vector
xl[[i]] <- rnorm(i)
}
> xl
[[1]]
NULL

[[2]]
[1] -0.4448192 -1.3395014

[[3]]
[1] 1.3214195 -1.2968560 -0.6327795


The above lists contained a NULL element for some reason. While the code below:
xl <- list()
for(i in as.character(2:3)){##loop over character vector
xl[[i]] <- rnorm(i)
}
> xl
$`2`
[1] -1.139506 2.894280

$`3`
[1] 0.0599175 1.0793515 0.4296049


This resulting list contains no extraneous elements. Is this normal? Why?

Thanks,
Liviu

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[arun]

Hi,
xl<- vector("list",7)

for(i in 5:7){##loop over numeric vector
     xl[[i]] <- rnorm(i)
 }
 xl

#[[1]]
#NULL
#
#[[2]]
#NULL
#
#[[3]]
#NULL
#
#[[4]]
#NULL
#
#[[5]]
#[1]  0.3266762  0.4316069  1.2290551 -0.6725783  1.6159861
#
#[[6]]
#[1] -2.8560618 -0.5694743 -0.7325862  1.6786160  0.3883842 -0.3991854
#
#[[7]]
#[1] -1.22443607  0.66940158  0.97476608 -0.85964961  0.06331827 -0.40658831
#[7]  0.24824287

xlNew<- vector("list",4)

for(i in as.character(2:3)){##loop over character vector

    xlNew[[as.numeric(i)]] <- rnorm(i)
}

xlNew
#[[1]]
#NULL
#
#[[2]]
#[1] -2.155804  1.780388
#
#[[3]]
#[1]  0.4192816 -1.0142512 -0.2125988
#
#[[4]]
#NULL
A.K.

[peter dalgaard]

On May 19, 2013, at 18:08 , Liviu Andronic wrote:

> Dear all,
> I encountered this strange behaviour with loops and lists. Consider this:
> xl <- list()
> for(i in 5:7){##loop over numeric vector
> xl[[i]] <- rnorm(i)
> }
>> xl
> [[1]]
> NULL
>
> [[2]]
> [1] -0.4448192 -1.3395014
>
> [[3]]
> [1] 1.3214195 -1.2968560 -0.6327795
>
>
> The above lists contained a NULL element for some reason. While the code below:
> xl <- list()
> for(i in as.character(2:3)){##loop over character vector
> xl[[i]] <- rnorm(i)
> }
>> xl
> $`2`
> [1] -1.139506 2.894280
>
> $`3`
> [1] 0.0599175 1.0793515 0.4296049
>
>
> This resulting list contains no extraneous elements. Is this normal? Why?

(The first example really had 2:3, not 5:7, right?)

The essential bit is that to assign to the 2nd element of a list, it needs to have at least two elements:

> x <- list()
> x[[2]] <- 123
> x
[[1]]
NULL

[[2]]
[1] 123

assigning to an element with a specific name just requires there is an element of that name:

> x[["2"]] <- 321
> x
[[1]]
NULL

[[2]]
[1] 123

$`2`
[1] 321

In both cases, x will be extended if needed, so that the required element exists. Notice that there is no relation between the name and the number of a list element; e.g., x[["2"]] is the 3rd element in the above example.


--
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd....@cbs.dk Priv: PDa...@gmail.com

[Liviu Andronic]

On Sun, May 19, 2013 at 7:02 PM, peter dalgaard <pda...@gmail.com> wrote:
> (The first example really had 2:3, not 5:7, right?)
>

Indeed. I simplified the example mid-email.

> The essential bit is that to assign to the 2nd element of a list, it needs to have at least two elements:
>

Thanks for the explanations. Regards,
Liviu

>> x <- list()
>> x[[2]] <- 123
>> x
> [[1]]
> NULL
>
> [[2]]
> [1] 123
>
> assigning to an element with a specific name just requires there is an element of that name:
>
>> x[["2"]] <- 321
>> x
> [[1]]
> NULL
>
> [[2]]
> [1] 123
>
> $`2`
> [1] 321
>
> In both cases, x will be extended if needed, so that the required element exists. Notice that there is no relation between the name and the number of a list element; e.g., x[["2"]] is the 3rd element in the above example.
>
>
> --
> Peter Dalgaard, Professor,
> Center for Statistics, Copenhagen Business School
> Solbjerg Plads 3, 2000 Frederiksberg, Denmark
> Phone: (+45)38153501
> Email: pd....@cbs.dk Priv: PDa...@gmail.com
>
>
>
>
>
>
>
>

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