How would you read input file in Scheme?

Question

I am trying to input data from a .txt file into a scheme structure. Each element is separated by a tab in the data file and each structure set is on a new line. I want to be able to read in the data from one line into a structure and make a list of each structure set in the file. Any suggestions?

Answer 1

Sounds like a CSV file with tabs instead of commas. If you're using PLT Scheme (DrScheme/mzscheme) http://planet.plt-scheme.org/display.ss?package=csv.plt&owner=neil">neil's csv library is probably what you want.

Here is the documentation.

Here is how to load it remotely:

(require (planet neil/csv:1:2/csv))

At least, that's what the instructions say. On my slightly oodate DrScheme, this is what worked:

(require (planet "csv.ss" ("neil" "csv.plt" 1 (= 1))))

Answer 2

Not really sure what structures you had in mind, but say you had a text file like the following:

--> cat blah.txt 
foo bar baz
1 2 3 4 5
aa bb cc dd ee

You could convert it directly into a list of lists in scheme using sed:

--> echo "(define mylist '("`sed -e 's/\(.*\)/(\1)/' blah.txt`"))" > foo.txt

which then produces the following file:

--> cat foo.txt 
(define mylist '((foo bar baz) (1 2 3 4 5) (aa bb cc dd ee)))

And now all you have to do is load the file into scheme:

(load "foo.txt")

And you can access the structure via the `mylist' variable.