How to initialize a vector with fixed length in R

Question

How to initialize a vector with fixed length in R ??

For example, I want a vector of characters with length of 10??

Solution 1

It's good that you ask because pre-allocating long vectors before for-loops that will be assigning results to long objects are made more efficient by not needing to successively lengthen vectors.

?vector

X <- vector(mode="character", length=10)

This will give you empty strings which get printed as two adjacent double quotes, but be aware that there are no double-quote characters in the values themselves. That's just a side-effect of how print.default displays the values. They can be indexed by location. The number of characters will not be restricted, so if you were expecting to get 10 character element you will be disappointed.

>  X[5] <- "character element in 5th position"

>  X
 [1] ""                                  ""                                 
 [3] ""                                  ""                                 
 [5] "character element in 5th position" ""                                 
 [7] ""                                  ""                                 
 [9] ""                                  "" 
>  nchar(X)
 [1]  0  0  0  0 33  0  0  0  0  0

> length(X)
[1] 10

Technically, lists are "vectors" in R, so this is a recommended (even necessary) practice for constructing lists with for-loops:

obj <- list( length(inp_vec) )
for ( i in seq_along(inp_vec) ){
       obj[[i]] <- some_func( inp_vec[i] )
       }

@Tom reminds us that the default for 'mode' is logical and that R has a fairly rich set of automatic coercion methods, although I find his suggestion vector(,10) to be less clear than would be logical(10) which is its equivalent.

Solution 2

If you want to initialize a vector with numeric values other than zero, use rep

n <- 10
v <- rep(0.05, n)
v

which will give you:

[1] 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05

Solution 3

The initialization method easiest to remember is

vec = vector(,10); #the same as "vec = vector(length = 10);"

The values of vec are: "[1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE" (logical mode) by default.

But after setting a character value, like

vec[2] = 'abc'

vec becomes: "FALSE" "abc" "FALSE" "FALSE" "FALSE" "FALSE" "FALSE" "FALSE" "FALSE" "FALSE"", which is of the character mode.

Solution 4

Just for the sake of completeness you can just take the wanted data type and add brackets with the number of elements like so:

x <- character(10)

View more solutions

Comments

[Thomas]

Or just character(10).

[IRTFM]

Right. Just type character to see how these are equivalent.

[ToBeGeek]

How to initialise a matrix with 100 rows, 4 columns ?

[Roland]

@ToBeGeek Have you considered reading the documentation? help("matrix")