I was trying some coding challenge in Hackerank, and I was wondering how to read a line of space-separated integers using OCaml.
I know if the input integers are newline separated,
then we could do
let recread_lines () =
try line = read_line ()
in line:: read_lines()
with End_of_file->[]
and then convert the list to list of int using
let ()=
let input = read_lines ()
in List.map int_of_string input.
I also know that we can read the first integer from a line of integers by doing:
let n1 =Scanf.scanf " %d" (fun x -> x)
From C, I know scanf method scans the line looking for structure defined in the parameter and then store the scanned values into the variables. I don't see the use of (fun x->x) at the end of the scanf method in OCaml.
I don't know how to read a line of integers though, so if you can point me in the right direction, I would be able to work on those challenges!
Here's a function to read a line of space-separated integers from stdin:
let read_ints () =
let line = read_line () in
let ints = Str.split (Str.regexp " *") line in
List.map int_of_string ints
This function will raise End_of_file if there are no more lines in the input.
You can adapt the regular expression if there are actually tabs mixed with the spaces (say).
(FWIW I have found over the years that the scanf family of functions is prone to behaving badly in the face of unexpected input. This isn't a problem with OCaml in particular, it's the same for all languages that have scanf. So I haven't used scanf for quite a few decades.)
Related
NOTE: I'm totally Newbie in Standard ML. I merely have basic F# knowledge.
This is a good ol' code in C
#include <stdio.h>
int main()
{
char str[100]; // size whatever you want
scanf("%s", str);
printf("%s\n", str);
return 0;
}
now, I want to make a Standard ML-version-equivalent of this code. so I tried this:
val str = valOf (TextIO.inputLine TextIO.stdIn)
val _ = print str
but my SML/NJ says this:
uncaught exception Option
raised at: smlnj/init/pre-perv.sml:21.28-21.34
I googled it, and I also searched this site, but I cannot find any solution which doesn't cause error.
does anyone knows it?
EDIT: I tried this code:
fun main =
let val str = valOf (TextIO.inputLine TextIO.stdIn)
in
case str
of NONE => print "NONE\n"
| _ => print str
end
but it also makes error:
stdIn:1.6-1.10 Error: can't find function arguments in clause
stdIn:4.9-6.33 Error: case object and rules don't agree [tycon mismatch]
rule domain: 'Z option
object: string
in expression:
(case str
of NONE => print "NONE\n"
| _ => print str)
This answer was pretty much given in the next-most recent question tagged sml: How to read string from user keyboard in SML language? -- you can just replace the user keyboard with stdin, since stdin is how you interact with the keyboard using a terminal.
So you have two problems with this code:
fun main =
let val str = valOf (TextIO.inputLine TextIO.stdIn)
in
case str
of NONE => print "NONE\n"
| _ => print str
end
One problem is that if you write fun main then it has to take arguments, e.g. fun main () = .... The () part does not represent "nothing" but rather exactly one thing, being the unit value.
The other problem is eagerness. The Option.valOf function will crash when there is no value, and it will do this before you reach the case-of, making the case-of rather pointless. So what you can do instead is:
fun main () =
case TextIO.inputLine TextIO.stdIn of
SOME s => print s
| NONE => print "NONE\n"
Using the standard library this can be shortened to:
fun main () =
print (Option.getOpt (TextIO.inputLine TextIO.stdIn, "NONE\n"))
I encourage you to read How to read string from user keyboard in SML language?
I know that in order to load a program in OCaml one has to type #use "source_code_file.ml" in toplevel where source_code_file.ml is the file we want to use.
My program reads input from stdin. In the command line i have a txt file that with redirection is used to act as stdin. Can i do this in toplevel? I would like to this because in toplevel i can easily see what type variables have and if things are initialized with the correct values.
If you're on a Unix-like system you can use Unix.dup2 to do almost any kind of input redirection. Here is a function with_stdin that takes an input file name, a function, and a value. It calls the function with standard input redirected from the named file.
let with_stdin fname f x =
let oldstdin = Unix.dup Unix.stdin in
let newstdin = Unix.openfile fname [Unix.O_RDONLY] 0 in
Unix.dup2 newstdin Unix.stdin;
Unix.close newstdin;
let res = f x in
Unix.dup2 oldstdin Unix.stdin;
Unix.close oldstdin;
res
If your function doesn't consume the entire input the leftover input will confuse the toplevel. Here's an example that does consume its entire input:
# let rec linecount c =
try ignore (read_line ()); linecount (c + 1)
with End_of_file -> c;;
val linecount : int -> int = <fun>
# with_stdin "/etc/passwd" linecount 0;;
- : int = 86
#
This technique is too simple if you wanted to interleave interactions with the toplevel with calls to your function to consume just part of its input. I suspect that would make things too complicated to be worth the effort. It would be much easier (and perhaps better overall) to rewrite your code to work with an explicitly specified input channel.
I have an OCaml function that converts a string to an array. What is the canonical way of writing a "main" function to call this and print the array.
let createArray pattern patArray =
(* some unimportant way of setting all the elements in the array patArray
based on the string pattern *)
let main () =
let pattern = "Pattern" in
let patArray = Array.create (String.length pattern) 0 in
let res = createArray pattern patArray in
Array.iter ~f:(printf "%d ") patArray;; <------------------
main ()
1) In the above, if I leave out the ';;' , it does not work. What is the significance of that?
2) Instead of using a dummy binding "res" , can I somehow just write two statements to be executed sequentially , like so:
createArray pattern patArray
Array.iter ~f:(printf "%d ") patArray
Without the ;;, the parser cannot know that the main () call following that line is supposed to be a stand-alone expression (whitespace is not significant here).
You can use the following idiom instead:
let main () = ...
let () = main ()
The let () = expr idiom will evaluate an expression of type unit at that point. The initial let informs the parser that a new top-level let construct begins. Using ;; is an alternative way to tell the parser about the end of a top-level construct, but is primarily intended for interactive use.
In order to evaluate two expressions sequentially, separate them with a semicolon (use parentheses or begin ... end if you're unsure about precedence rules). For example:
let patArray = Array.create (String.length pattern) 0 in
createArray pattern patArray;
Array.iter ~f:(printf "%d ") patArray
Or, using begin and end to make precedence clearer:
let patArray = Array.create (String.length pattern) 0 in begin
createArray pattern patArray;
Array.iter ~f:(printf "%d ") patArray
end
Without the ;, the parser would not know whether Array.iter on the next line is supposed to an additional argument to the createArray call.
I am new to OCaml, and now I am trying to make a simple REPL.
let rec repl () =
print_prompt () ;
let input = Scanf.scanf "%s" (fun x -> x) in
if input = "" then repl ()
else print_endline input
let print_prompt () = print_string "> "
The problem now i am having is: when program starts, it does not display prompt immediately. It waits for my input and prints prompt along with my input.
What I want is:
> "user_input"
"user_input"
But i am getting :
"user_input"
> "user_input"
How can I fix this?
Using readline instead of Scanf :
val read_line : unit -> string
Flush standard output, then read characters from standard input until a newline character is encountered. Return the string of all characters read, without the newline character at the end.
Well, you didn't show the print_promt implementation, but I can guess, that it uses some buffered io function like print_string or printf. They print into an intermediate buffer and data will not be displayed unless flush is called. You can use flush or flush_all functions to do this manually. Also you can use a special specificator %! in printf formats string:
open Printf
let print_prompt () = printf "> %!"
This is almost certainly a buffering problem. In your print_prompt function, flush the standard output:
flush stdout
I am trying to put an input string into sub-string arrays. The number of data in the input file are less than 10 but unknown. The number of spaces between each data is also unclear.
Example:
Asd B Cwqe21 Ddsw Eww
I am quite novice to Fortran, so I do not know which format I should use. My problem is that I do not know the number of data (here I assumed that there are 5), so how can I make the code work?
I tried the following which did not work:
CHARACTER (LEN=100), DIMENSION(10) :: string
READ (1,*) (string,I=1,10)
It seems that the error I got was because there was no 6th string to read and put into string(6).
I tried using the "Index" to find the space, but since I do not know how many spaces are in the string, it did not help me.
I don't know if this is more or less elegant/efficient than the standard approach in M.S.B's comment, but an interesting alternative.
integer istart,nw
character (len=100) line,wd,words(100)
open(1,file='t.dat')
read(1,'(a)')line
istart=1
nw=0
do while(len(trim(line(istart:))).gt.0)
read(line(istart:),*)wd
istart=istart+index(line(istart:),trim(wd))+len(trim(wd))
nw=nw+1
words(nw)=trim(wd)
enddo
write(*,*)trim(line)
write(*,*)('/',trim(words(k)),k=1,nw),'/'
end
An inefficient approach that is simple to program is to try to read the maximum number of items, and if this fails to successively try to read one fewer items until the read is successful, as shown below:
program xread_strings
integer, parameter :: nw = 10
character (len=1000) :: text
character (len=20) :: words(nw)
integer :: i,ierr,nread
text = "Asd B Cwqe21 Ddsw Eww"
nread = 0
do i=nw,1,-1
read (text,*,iostat=ierr) words(:i)
if (ierr == 0) then
nread = i
exit
end if
end do
if (nread > 0) write (*,*) "read ",nread," words: ",("'"//trim(words(i)) // "' ",i=1,nread)
end program xread_strings
! g95 Output:
! read 5 words: 'Asd' 'B' 'Cwqe21' 'Ddsw' 'Eww'