should have added: dim(x)[2L] -> length(x)
Am 31.03.20 um 16:21 schrieb Prof. Dr. Matthias Kohl:
Dear Ivan,
if I enter ncol in the console, I get
function (x)
dim(x)[2L]
<bytecode: 0x5559e9429030>
<environment: namespace:base>
indicating that function dim is called. Function dim has a method for
data.frame; see methods("dim").
The dim-method for data.frame is
dim.data.frame
function (x)
c(.row_names_info(x, 2L), length(x))
<bytecode: 0x5559eb80da40>
<environment: namespace:base>
Hence, it calls length on the provided data.frame. In addition, some
"magic" with .row_names_info is performed, where
base:::.row_names_info
function (x, type = 1L)
.Internal(shortRowNames(x, type))
<bytecode: 0x5559ece50160>
<environment: namespace:base>
Best
Matthias
Am 31.03.20 um 16:10 schrieb Ivan Calandra:
Thanks Ivan for the answer.
So it confirms my first thought that these two functions are equivalent
when applied to a "simple" data.frame.
The reason I was asking is because I have gotten used to use length() in
my scripts. It works perfectly and I understand it easily. But to be
honest, ncol() is more intuitive to most users (especially the novice)
so I was thinking about switching to using this function instead (all my
data.frames are created from read.csv() or similar functions so there
should not be any issue). But before doing that, I want to be sure that
it is not going to create unexpected results.
Thank you,
Ivan
--
Dr. Ivan Calandra
TraCEr, laboratory for Traceology and Controlled Experiments
MONREPOS Archaeological Research Centre and
Museum for Human Behavioural Evolution
Schloss Monrepos
56567 Neuwied, Germany
+49 (0) 2631 9772-243
https://www.researchgate.net/profile/Ivan_Calandra
On 31/03/2020 16:00, Ivan Krylov wrote:
On Tue, 31 Mar 2020 14:47:54 +0200
Ivan Calandra <calan...@rgzm.de> wrote:
On a simple data.frame (i.e. each element is a vector), ncol() and
length() will give the same result.
Are they just equivalent on such objects, or are they differences in
some cases?
I am not aware of any exceptions to ncol(dataframe)==length(dataframe)
(in fact, ncol(x) is dim(x)[2L] and ?dim says that dim(dataframe)
returns c(length(attr(dataframe, 'row.names')), length(dataframe))), but
watch out for AsIs columns which can have columns of their own:
x <- data.frame(I(volcano))
dim(x)
# [1] 87 1
length(x)
# [1] 1
dim(x[,1])
# [1] 87 61
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--
Prof. Dr. Matthias Kohl
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