How to order files in R according to certain “date parts” of their names

Question

My question is similar to this question: How to change the order of display of list.files(): for example based on part of the whole name of files

but my case is differnt. I used the solution provided:

              a=a[order(gsub('.*_(\\d{8})[.].*','\\1',a))]

but did not work for my case because the naming in my case is different from that provided in that question.

I have several files in a directory. The naming of the files is complex,for instance :

 file.img
 file.img

I want to list the files so that I can work with them, it seemed that R listed them in a certain order. R ordered the files alphabetically even if the data is not ordered correctly.for example these two names of files:

How can I tell R to change the default display of list.files , and order the files based on _yearmonthday only which represents yearmonthday in all files:

to list files in R we use this :

I used:

      mixsort 

but did not ordered them as I want

Answer 1

I think you just need to change the pattern of the gsub function. For example:

xx <- 'SM_OP_20120330T000000_20120330T235959_245_001_7_ssture.img'
gsub('.*_(\\d{8}).*','\\1',xx)
"20120330"

So , the whole solution is :

 a <- list.files("D:\\semon", "*.img", full.names = TRUE)
 a  <- a[order(as.numeric(gsub('.*_(\\d{8}).*','\\1',a)))]

EDIT add an example:

   a <- list('SM_OP_20120330T000000_20120330T235959_245_001_7_ssture.img', ##2012-mars-30
          'SM_RE_20101130T000000_20110427T235959_245_001_7_ssture.img', ##2010-nov-30
          'SM_RE_20100901T000000_20090127T235959_245_001_7_ssture.img', ##2010-sep-01
          'SM_RE_20100904T000000_20090427T235959_245_001_7_ssture.img') ##2010-sep-04


a[order( as.numeric(gsub('.*_(\\d{8}).*','\\1',a)))]

[[1]]
[1] "SM_RE_20100901T000000_20090127T235959_245_001_7_ssture.img"

[[2]]
[1] "SM_RE_20100904T000000_20090427T235959_245_001_7_ssture.img"

[[3]]
[1] "SM_RE_20101130T000000_20110427T235959_245_001_7_ssture.img"

[[4]]
[1] "SM_OP_20120330T000000_20120330T235959_245_001_7_ssture.img"