How to avoid date formatted values getting converted to numeric when assigned to a matrix or data frame?

r matrix as.date
Joes de Natris picture Joes de Natris · Jan 17, 2017 · Viewed 13.6k times · Source

I have run into an issue I do not understand, and I have not been able to find an answer to this issue on this website (I keep running into answers about how to convert dates to numeric or vice versa, but that is exactly what I do not want to know).

The issue is that R converts values that are formatted as a date (for instance "20-09-1992") to numeric values when you assign them to a matrix or data frame. For example, we have "20-09-1992" with a date format, we have checked this using class(). 

as.Date("20-09-1992", format = "%d-%m-%Y")
class(as.Date("20-09-1992", format = "%d-%m-%Y"))

We now assign this value to a matrix, imaginatively called Matrix:

Matrix <- matrix(NA,1,1)
Matrix[1,1] <- as.Date("20-09-1992", format = "%d-%m-%Y")
Matrix[1,1]
class(Matrix[1,1])

Suddenly the previously date formatted "20-09-1992" has become a numeric with the value 8298. I don't want a numeric with the value 8298, I want a date that looks like "20-09-1992" in date format.

So I was wondering whether this is simply how R works, and we are not allowed to assign dates to matrices and data frames (somehow I have managed to have dates in other matrices/data frames, but it beats me why those other times were different)? Is there a special method to assigning dates to data frames and matrices that I have missed and have failed to deduce from previous (somehow successful) attempts at assigning dates to data frames/matrices?

Answer

akaDrHouse picture akaDrHouse · Jan 17, 2017

I don't think you can store dates in a matrix. Use a data frame or data table. If you must store dates in a matrix, you can use a matrix of lists.

Matrix <- matrix(NA,1,1)
Matrix[1,1] <- as.list(as.Date("20-09-1992", format = "%d-%m-%Y"),1)
Matrix
[[1]]
[1] "1992-09-20"

Edited: I also just re-read you had this issue with data frame. I'm not sure why.

mydate<-as.Date("20-09-1992", format = "%d-%m-%Y")
mydf<-data.frame(mydate)
mydf

      mydate
1 1992-09-20

Edited: This has been a learning experience for me with R and dates. Apparently the date you supplied was converted to number of days since origin. Origin is defined as Jan 1st,1970. To convert this back to a date format at some point

Matrix
     [,1]
[1,] 8298

as.Date(Matrix, origin ="1970-01-01")
[1] "1992-09-20"

Related questions

R memory management / cannot allocate vector of size n Mb

I am running into issues trying to use large objects in R. For example: > memory.limit(4000) > a = matrix(NA, 1500000, 60) > a = matrix(NA, 2500000, 60) > a = matrix(NA, 3500000, 60) Error: cannot allocate vector of size 801.1 Mb > a = matrix(NA, 2500000, 60) …

Read More
Subscript out of bounds - general definition and solution?

When working with R I frequently get the error message "subscript out of bounds". For example: # Load necessary libraries and data library(igraph) library(NetData) data(kracknets, package = "NetData") # Reduce dataset to nonzero edges krack_full_nonzero_edges <- …

Read More
Select rows of a matrix that meet a condition

In R with a matrix: one two three four [1,] 1 6 11 16 [2,] 2 7 12 17 [3,] 3 8 11 18 [4,] 4 9 11 19 [5,] 5 10 15 20 I want to extract the submatrix whose rows have column three = 11. That is: one two three four [1,] 1 6 11 16 [3,] 3 8 11 18 [4,] 4 9 11 19 I want to do this without looping. I am new to R so this …

Read More