► |  | How to Design Programs Teachpacks

1 | Ht DP Teachpacks
2 | Ht DP/ 2e Teachpacks

▼ | 2 | Ht DP/ 2e Teachpacks

2.1 | Batch Input/ Output: "batch-io.rkt"
2.2 | Image Guide
2.3 | Images: "image.rkt"
2.4 | Worlds and the Universe: "universe.rkt"
2.5 | i Tunes: "itunes.rkt"
2.6 | Abstraction: "abstraction.rkt"
2.7 | Planet Cute Images
2.8 | Porting World Programs to Universe

► | 2.1 | Batch Input/ Output: "batch-io.rkt"

2.1.1 | IO Functions
2.1.2 | Web Functions
2.1.3 | Testing

On this page:

2.1.1`` IO Functions
read-file
read-1strings
read-lines
read-words
read-words/ line
read-words-and-numbers/ line
read-csv-file
read-csv-file/ rows
read-xexpr
read-plain-xexpr
write-file
2.1.2`` Web Functions
read-xexpr/ web
read-plain-xexpr/ web
url-exists?
xexpr?
xexpr-as-string
url-html-neighbors
2.1.3`` Testing
simulate-file

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#### 2.1`` Batch Input/Output: "batch-io.rkt"

Matthias Felleisen

(require 2htdp/batch-io) | package: htdp-lib

The batch-io teachpack introduces several functions and a form for reading content from files and one function for writing to a file.

##### 2.1.1`` IO Functions

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:

- > procedure (read-file f) → string?
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or 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, for 1, am done "

  assuming the file named "data.txt" has this shape:

  > hello world
  > good bye
  > i, for 1, 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.

- > procedure (read-1strings f) → (listof 1string?)
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or 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"
  > ","
  > " "
  > "f"
  > "o"
  > "r"
  > " "
  > "1"
  > ","
  > " "
  > "a"
  > "m"
  > " "
  > "d"
  > "o"
  > "n"
  > "e"
  > " ")

  Note how this function reproduces all parts of the file faithfully, including spaces and newlines.

- > procedure (read-lines f) → (listof string?)
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or 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, for 1, 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.

  If the last line is not terminated by a newline, the functions acts as if there were one.

- > procedure (read-words f) → (listof string?)
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or 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," "for" "1," "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".

- > procedure (read-words/line f) → (listof (listof string?))
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or the content of file f and produces it as a list of lists, one per line; each line is represented as a list of strings.

  Example:

  > > (read-words/line "data.txt")
  > '(("hello" "world") ("good" "bye") () ("i," "for" "1," "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.

  If the last line is not terminated by a newline, the functions acts as if there were one.

- > procedure (read-words-and-numbers/line f) → (listof (or number? string?))
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or the content of file f and produces it as a list of lists, one per line; each line is represented as a list of strings and numbers.

  Example:

  > > (read-words-and-numbers/line "data.txt")
  > '(("hello" "world") ("good" "bye") () ("i," "for" "1," "am" "done"))

  The results is like the one that read-words/line produces, except strings that can be parsed as numbers are represented as numbers.

  If the last line is not terminated by a newline, the functions acts as if there were one.

- > procedure (read-csv-file f) → (listof (listof any/c))
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or 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.

- > procedure (read-csv-file/rows f s) → (listof X?)
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))
  > s : (-> (listof any/c) X?)

  reads the standard input device (until closed) or the content of file f and produces it as 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.

- > procedure (read-xexpr f) → xexpr?
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or the content of file f and produces it as an X-expression, including whitespace such as tabs and newlines.

  Assumption: the file f or the selected input device contains an XML element. It assumes the file contains HTML-like text and reads it as XML.

  Example:

  > > (read-xexpr "data.xml")
  > '(pre () "\nhello world\ngood bye\n\ni, for 1, am done\n")

  assuming the file named "data.xml" has this shape:

  > <pre>
  > hello world
  > good bye
  > i, for 1, am done
  > </pre>

  Note how the result includes "\\n" for the newlines.

- > procedure (read-plain-xexpr f) → xexpr?
  > f : (or/c 'standard-in 'stdin (and/c string? file-exists?))

  reads the standard input device (until closed) or the content of file f and produces it as an X-expression, without whitespace.

  Assumption: the file f or the selected input device contains an XML element and the content of this element are other XML elements and whitespace. In particular, the XML element does not contain any strings as elements other than whitespace.

  Example:

  > > (read-plain-xexpr "data-plain.xml")
  > '(pre
  > ()
  > (line ((text "hello world")))
  > (line ((text "good bye")))
  > (line ())
  > (line ((text "i, for 1, am done"))))

  assuming the file named "data-plain.xml" has this shape:

  > <pre>
  > <line text="hello world" />
  > <line text="good bye" />
  > <line />
  > <line text="i, for 1, am done" />
  > </pre>

  Compare this result with the one for read-xexpr.

There is only one writer function at the moment:

- > procedure (write-file f cntnt) → string?
  > f : (or/c 'standard-out 'stdout string?)
  > cntnt : string?

  sends cntnt to the standard output device or turns cntnt into the content of file f, located in the same folder (directory) as the program. If the write succeeds, the function produces the name of the file (f); otherwise it signals an error.

  Example:

  > > (if (string=? (write-file "output.txt" "good bye") "output.txt")
  > (write-file "output.txt" "cruel world")
  > (write-file "output.txt" "cruel world"))
  > "output.txt"

  After evaluating this examples, the file named "output.txt" looks like this: cruel world Explain why.

Warning: The file IO functions in this teachpack are platform dependent. That is, as long as your programs and your files live on the same platform, you should not have any problems reading the files that programs wrote and vice versa. If, however, one of your programs writes a file on a Windows operating system and if you then copy this output file to a Mac, reading the copied text file may produce extraneous “return” characters. Note that this describes only one example of possible malfunction; there are other cases when trans-platform actions may cause this teachpack to fail.

##### 2.1.2`` Web Functions

All functions that read a web-based XML consume a URL and possibly additional arguments. They assume that the computer is connected to specified part of the web, though they tolerate non-existent web pages (404 errors)

- > procedure (read-xexpr/web u) → xexpr?
  > u : string?

  reads the content of URL u and produces the first XML element as an xexpr? including whitespace such as tabs and newlines. If possible, the function interprets the HTML at the specified URL as XML. The function returns #f if the web page does not exist (404)

- > procedure (read-plain-xexpr/web u) → xexpr?
  > u : string?

  reads the content of URL u and produces the first XML element as an xexpr? without whitespace. If possible, the function interprets the HTML at the specified URL as XML. The function returns #f if the web page does not exist (404)

- > procedure (url-exists? u) → boolean?
  > u : string?

  ensures that the specified URL u does not produce a 404 error.

- > procedure (xexpr? u) → boolean?
  > u : any?

  checks that the given value is an X-expression in the following sense:

  > ; Xexpr is one of:
  > ; – symbol?
  > ; – string?
  > ; – number?
  > ; – (cons symbol? (cons [List-of Attribute] [List-of Xexpr]))
  > ; – (cons symbol? [List-of Xexpr])
  > ;
  > ; Attribute is:
  > ; (list symbol? string?)
  > ; (list 'a "some text") is called an a-Attribute
  > ; and "some text" is a's value.

  Note that full Racket uses a wider notion of X-expression.

- > procedure (xexpr-as-string x) → string?
  > x : xexpr?

  renders the given X-expression as a string.

- > procedure (url-html-neighbors u) → (listof string?)
  > u : string?

  retrieves the content of URL u and produces the list of all URLs that refer to .html pages via an <a> tag.

##### 2.1.3`` Testing

> syntax (simulate-file process str ...)

simulates a file system for the function process, which reads a file and may produce one. Note: this form is under development and will be documented in a precise manner after it is finalized and useful for a wide audience.
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