2.1 Batch Input/Output: "batch-io.ss"
| (require 2htdp/batch-io) |
The batch-io teachpack introduces several functions and a form for reading content from files and one function for writing to a file.
All functions that read a file consume the name of a file and possibly
additional arguments. They assume that the specified file exists in the
same folder as the program; if not they signal an error:
(read-file f) → string? f : (and/c string? file-exists?) reads the content of file f and produces it as a string, including newlines .Example:
> (read-file "data.txt") "hello world \n good bye \n\ni am done "
assuming the file named "data.txt" has this shape:hello world
good bye
i am done
Note how the leading space in the second line translates into the space between the newline indicator and the word "good" in the result.(read-1strings f) → (listof 1string?) f : (and/c string? file-exists?) reads the content of file f and produces it as a list of one-char strings, one per character .Example:
> (read-1strings "data.txt") '("h" "e" "l" "l" "o" " " "w" "o" "r" "l" "d" " " "\n" " " "g" "o" "o" "d" " " "b" "y" "e" " " "\n" "\n" "i" " " "a" "m" " " "d" "o" "n" "e" " ")
Note how this function reproduces all parts of the file faithfully, including spaces and newlines.(read-lines f) → (listof string?) f : (and/c string? file-exists?) reads the content of file f and produces it as a list of strings, one per line .Example:
> (read-lines "data.txt") '("hello world " " good bye " "" "i am done ")
when "data.txt" is the name of the same file as in the preceding item. And again, the leading space of the second line shows up in the second string in the list.(read-words f) → (listof string?) f : (and/c string? file-exists?) reads the content of file f and produces it as a list of strings, one per white-space separated token in the file .Example:
> (read-words "data.txt") '("hello" "world" "good" "bye" "i" "am" "done")
This time, however, the extra leading space of the second line of "data.txt" has disappeared in the result. The space is considered a part of the separator that surrounds the word "good".(read-words/line f) → (listof string?) f : (and/c string? file-exists?) reads the content of file f and produces it as a list of lists, one per line; each line is represented as a list of white-space separated tokens .Example:
> (read-words/line "data.txt") '(("hello" "world") ("good" "bye") () ("i" "am" "done"))
The results is similar to the one that read-words produces, except that the organization of the file into lines is preserved. In particular, the empty third line is represented as an empty list of words.(read-csv-file f) → (listof (listof any/c)) f : (and/c string? file-exists?) reads the content of file f and produces it as a list of lists of comma-separated values .Example:
> (read-csv-file "data.csv") '(("hello" "world") ("good" "bye") ("i" "am" "done"))
where the file named "data.csv" has this shape:hello, world
good, bye
i, am, done
It is important to understand that the rows don’t have to have the same length. Here the third line of the file turns into a row of three elements.- reads the content of file f and produces it as list of rows, each constructed via s
Examples:
> (read-csv-file/rows "data.csv" (lambda (x) x)) '(("hello" "world") ("good" "bye") ("i" "am" "done"))
> (read-csv-file/rows "data.csv" length) '(2 2 3)
The first example shows how read-csv-file is just a short form for read-csv-file/rows; the second one simply counts the number of separated tokens and the result is just a list of numbers. In many cases, the function argument is used to construct a structure from a row.
There is only one writer function at the moment:
(write-file f cntnt) → boolean? f : string? cntnt : string? turns cntnt into the content of file f, located in the same folder (directory) as the program. If the file exists when the function is called, the function produces true; otherwise it produces false.Example:
> (if (write-file "output.txt" "good bye") (write-file "output.txt" "cruel world") (write-file "output.txt" "cruel world")) #t
After evaluating this examples, the file named "output.txt" looks like this:cruel world
Explain why.