How to create an empty R vector to add new items

Question

I want to use R in Python, as provided by the module Rpy2. I notice that R has very convenient [] operations by which you can extract the specific columns or lines. How could I achieve such a function by Python scripts?

My idea is to create an R vector and add those wanted elements into this vector so that the final vector is the same as that in R. I created a seq(), but it seems that it has an initial digit 1, so the final result would always start with the digit 1, which is not what I want. So, is there a better way to do this?

Answer 1

I pre-allocate a vector with

> (a <- rep(NA, 10))
 [1] NA NA NA NA NA NA NA NA NA NA

You can then use [] to insert values into it.

Answer 2

You can create an empty vector like so

vec <- numeric(0)

And then add elements using c()

vec <- c(vec, 1:5)

However as romunov says, it's much better to pre-allocate a vector and then populate it (as this avoids reallocating a new copy of your vector every time you add elements)

Answer 3

I've also seen

x <- {}

Now you can concatenate or bind a vector of any dimension to x

rbind(x, 1:10)
cbind(x, 1:10)
c(x, 10)

Answer 4

To create an empty vector use:

vec <- c();

Please note, I am not making any assumptions about the type of vector you require, e.g. numeric.

Once the vector has been created you can add elements to it as follows:

For example, to add the numeric value 1:

vec <- c(vec, 1);

or, to add a string value "a"

vec <- c(vec, "a");

Answer 5

As pointed out by Brani, vector() is a solution, e.g.

newVector <- vector(mode = "numeric", length = 50)

will return a vector named "newVector" with 50 "0"'s as initial values. It is also fairly common to just add the new scalar to an existing vector to arrive at an expanded vector, e.g.

aVector <- c(aVector, newScalar)

Answer 6

In rpy2, the way to get the very same operator as "[" with R is to use ".rx". See the documentation about extracting with rpy2

For creating vectors, if you know your way around with Python there should not be any issue. See the documentation about creating vectors