"cannot open file" means the wrong folder

◈ 8 cards

orders <- read.csv("orders.csv") reads the 12-row table with header = TRUE and sep = "," by default. "cannot open file 'orders.csv': No such file or directory" is a working-directory problem — getwd(), setwd(), or an RStudio Project. write.csv(agg, "agg.csv", row.names = FALSE) — forget the argument and a junk first column appears.

The line that starts every script

Maple & Birch's January orders are in orders.csv — one header row and twelve data rows, comma-separated. One call reads it into a data frame:

> orders <- read.csv("orders.csv")
> orders
   order_id prov channel amount units order_date
1      1001   ON  Online    420     3 2025-01-06
2      1002   QC   Store    180     1 2025-01-08
3      1003   ON   Store    760     5 2025-01-15
4      1004   BC  Online    250     2 2025-01-21
5      1005   AB  Online   1320     8 2025-02-03
6      1006   ON  Online    510     4 2025-02-10
7      1007   QC  Online     90     1 2025-02-14
8      1008   BC   Store    640     4 2025-02-27
9      1009   ON   Store    305     2 2025-03-04
10     1010   AB   Store    980     6 2025-03-11
11     1011   QC   Store    215     2 2025-03-19
12     1012   BC  Online    470     3 2025-03-25

This is the frame every lesson from here to Module 14 uses. read.csv() has three defaults worth knowing, all visible in args(read.csv): header = TRUE (the first row becomes the column names), sep = "," (the separator), and — since R 4.0.0 — text stays chr. For a semicolon-separated European export, read.csv2() or sep = ";".

Worked example — the error everyone meets first

Run the same line from the wrong folder:

> orders <- read.csv("orders.csv")
Error in file(file, "rt") : cannot open the connection
In addition: Warning message:
In file(file, "rt") :
  cannot open file 'orders.csv': No such file or directory

The file is fine. The CSV is fine. R is fine. R looked for orders.csv in its working directory — the folder it treats as "here" — and the file is somewhere else. Three fixes, in order of preference:

An RStudio Project. File → New Project in the folder that holds the data; every script in the project then opens with that folder as the working directory.
getwd() prints the current working directory; setwd("C:/Users/you/afm112") changes it. On Windows, write the path with forward slashes or doubled backslashes — a single \ is an escape character inside a string.
file.choose() opens a file picker and returns the full path: read.csv(file.choose()).

The message to memorise is the warning's last line: "cannot open file … No such file or directory" = the path, not the file.

Writing a result back

After Module 12's province summary, agg holds four rows. write.csv() saves it — but its default row.names = TRUE writes the row names as a first column with an empty header:

"","prov","total"
"1","AB",2300
"2","BC",1360

Open that in Sheets and column A is junk. The habit:

> write.csv(agg, "agg.csv", row.names = FALSE)
"prov","total"
"AB",2300
"BC",1360

Type the read and the write

The questions cover the error's cause, the separator default, and the missing row.names argument.

CRISP-DM: read.csv() is data understanding → collect initial data; write.csv() is deployment → produce final report.