After trying this simple console input with 5, the result is shown as 53
printfn "Enter no. of blocks: "
let nBlock = System.Console.Read()
printfn "entered value is %O" nBlock
Tried it on the interactive, still getting wrong results. Any solutions please?
You should try something like:
printfn "Enter no. of blocks: "
let nBlock = System.Console.ReadLine() |> System.Int32.Parse
printfn "entered value is %d" nBlock
Explanation:
you code only reads one character - as Lee mentioned
with this you will read a line (ending after you press return) and parse that string into a int.
Remark: maybe you will want to check for a number, you can do this with TryParse:
printfn "Enter no. of blocks: "
let nBlock =
match System.Console.ReadLine() |> System.Int32.TryParse with
| true, n -> n
| false, _ -> -1
printfn "entered value is %d" nBlock
Of course you will have to check for the error case (-1) or change it into a option or something.
System.Console.Read() returns an int, therefore nBlock contains the int representation of the input character '5', which is 53. You can convert it back to a char using Convert.ToChar. Since each character is returned individually, you will need wrapping it into a loop to process the entire input, as described in this article.
A better approach is probably to use Console.ReadLine() to read then entire line and parse that to an int.
Related
I am new to programming so this should be an easy one.
I want to write a code that asks for 3 numbers and then finds the minimum. Something like that:
let main(): Unit =
putline ("Please enter 3 numbers:")
putline ("First number: ")
let a = getline ()
putline ("Second number: ")
let b = getline ()
putline("Third number: ")
let c = getline ()
if (a<b && a<c) then putline ("Minimum:" + a)
elif (b<c && b<a) then putline ("Minimum:" + b)
else putline ("Minimum:" + c)
I am sorry if this is terrible but I am still new to this. Also I am not allowed to use the dictionary. Any advice?
You can use the F# function min, which gives you the minimum of two values.
min 1 2 // 1
To get the minimum of three values you can use it twice:
min (min a b) c
A cleaner way to write this with F# piping is:
a |> min b |> min c
Alternatively, put the items in a list and use List.min:
[ a; b; c ] |> List.min
If, for some reason, you decide to expand beyond three numbers, you could consider using Seq.reduce
let xs = [0;-5;3;4]
xs
|> Seq.reduce min
|> printfn "%d"
// prints -5 to stdout
You can use min as the reducer because it accepts 2 arguments, which is exactly what Seq.reduce expects
Firstly your putline function. I'm assuming that this is supposed to take a value and print it to the console with a newline, so the built in F# command to do this is printfn and you would use it something like this:
let a = 1
printfn "Minimum: %d" a
The %d gets replaced with the value of a as, in this case, a is an integer. You would use %f for a float, %s for a string... the details will all be in the documentation.
So we could write your putline function like this:
let putline s = printfn "%s" s
This function has the following signature, val putline : s:string -> unit, it accepts a string and return nothing. This brings us onto your next problem, you try and say putline ("Minimum:" + a). This won't work as adding a number and a string isn't allowed, so what you could do is convert a to a string and you have several ways to do this:
putline (sprintf "Minimum: %d" a)
putline ("Minimum:" + a.ToString())
sprintf is related to printfn but gives you back a string rather than printing to the console, a.ToString() converts a to a string allowing it to be concatenated with the preceding string. However just using printfn instead of putline will work here!
You also have a logic problem, you don't consider the cases where a == b == c, what's the minimum of 1,1,3? Your code would say 3. Try using <= rather than <
For reading data from the console, there is already an answer on the site for this Read from Console in F# that you can look at.
I've made a function to read a .txt file and turn it into a string.
From here I need help with collecting how many times a word is shown.
But I'm not sure where to go from here and any kind of help with any of the bulletpoints would be greatly appreciated.
Let's go through this step by step then, creating a function for each bit:
Convert words starting with an upper-case to a lower-case word so that all words are lower case.
Split the string into a sequence of words:
let getWords (s: string) =
s.Split(' ')
Turns "hello world" into ["hello"; "world"]
Sort the amount of times a word is shown. A word in this sense is a sequence of characters without whitespaces or punctuation (!#= etc)
Part #1: Format a word in lower without punctuation:
let isNotPunctuation c =
not (Char.IsPunctuation(c))
let formatWord (s: string) =
let chars =
s.ToLowerInvariant()
|> Seq.filter isNotPunctuation
|> Seq.toArray
new String(chars)
Turns "Hello!" into "hello".
Part #2: Group the list of words by the formatted version of it.
let groupWords (words: string seq) =
words
|> Seq.groupBy formatWord
This returns a tuple, with the first part as the key (formatWord) the second part is a list of the words.
Turns ["hello"; "world"; "hello"] into
[("hello", ["hello"; "hello"]);
("world", ["world"])]
Sort from most frequent word shown and to less frequent word.
let sortWords group =
group
|> Seq.sortByDescending (fun g -> Seq.length (snd g))
Sort the list descending (biggest first) by the length (count) of items in the second part - see the above representation.
Now we just need to clean up the output:
let output group =
group
|> Seq.map fst
This picks the first part of the tuple from the group:
Turns ("hello", ["hello"; "hello"]) into "hello".
Now we have all the functions, we can stick them together into one chain:
let s = "some long string with some repeated words again and some other words"
let finished =
s
|> getWords
|> groupWords
|> sortWords
|> output
printfn "%A" finished
//seq ["some"; "words"; "long"; "string"; ...]
Here's another way using Regex
open System.Text.RegularExpressions
let str = "Some (very) long string with some repeated words again, and some other words, and some punctuation too."
str
|> (Regex #"\W+").Split
|> Seq.choose(fun s -> if s = "" then None else Some (s.ToLower()))
|> Seq.countBy id
|> Seq.sortByDescending snd
I am facing a problem that leave me disoriented, since it should be a pretty basic procedure. I am learning how to build simple console applications with F#, but I cannot figure out how to pass an integer as standard input to the program. Please consider:
open System
let square x = x * x
[<EntryPoint>]
let main (argv :string[]) =
printfn "The function will take an input and square it"
printfn "%d squared is %d" 12 (square 12)
Console.ReadLine() |> ignore
0
How can I pass the value to printfn and square the way I can do in C with:
int main(){
int i;
scanf("%d", &i);
printf("\nThe value of variable i is %d.\n", i);
return 0;
}
I tried to adapt let x = Console.ReadLine() to get integers, but no appreciable results. The documentation I consulted, mainly consider string inputs. I am afraid I am missing something important for properly comprehend the basic of F#. Any suggestion, in this sense, would be highly appreciated.
ReadLine () will return a string. As mentioned by #JohnPalmer, it needs to be parsed to an int. The following program should give you the basic idea:
let rec input () =
printf "Input: "
match Core.int.TryParse (stdin.ReadLine ()) with
| true, i -> i
| _ ->
printfn "Invalid input, try again"
input ()
let square x = x * x
[<EntryPoint>]
let main _ =
printfn "The function will take an input and square it"
let i = input ()
printfn "%d squared is %d" i (square i)
0
I am new to learning F# and writing a simple console application, where the user enters a value for distance, and I want some validation to make sure that the input is a number. It also needs to make sure that it is a number, and if it isn't, tell the user and they start again. This is what I have so far:
let distance = 0
while distance = 0 do
System.Console.WriteLine("How far do you want to travel?")
let answer = System.Console.ReadLine()
try
let distance = System.Int32.Parse(answer)
if distance < 0 then
let distance = 0
printfn "Can't use negative numbers"
elif distance = 0 then
printfn "Can't travel a distance of 0"
else
printfn "You are about to travel %A" distance
finally
let distance = 0
printfn "Invalid distance format"
And this is what comes up:
In this example, what I want to happen is for the "Invalid distance format" to not appear, and it moves on to the next part of the app.
How would I make it so that "Invalid distance format" only appears if distance can't be converted to an int via System.Int32.Parse(answer)
Even if try-finally is the complete wrong way to go about doing this, how else would it be done?
Thanks in advance
What you want is try...with
try
let distance = System.Int32.Parse(answer)
. . . . .
with
| _ as ex -> printfn "Invalid Distance Format"
http://fsharpforfunandprofit.com/posts/exceptions/
You could also do something like
...
let attemptedConvert = Int32.TryParse(answer)
let success,convertValue = attemptedConvert
if success then
//other stuff here
else
printfn "Invalid Number Format"
This way there is no exception thrown, and you still have validation if the entry was converted successfully.
As noted below, exceptions can be a more expensive operation than other alternatives, and should be evaluated to see if that is causing unnecessary overhead. Like any process though, this should be evaluated on a case by case basis.
TryParse Method
As you are learning, I would approach this problem by doing something like this.
It makes use of the option type which is useful for handling the 'null' case (i.e. no input). It also uses F#s pattern matching, which is a very powerful alternative to using if, else, elif etc.
//see here - http://fsharpforfunandprofit.com/posts/the-option-type/
let tryParseOption intStr =
try
let i = System.Int32.Parse intStr
Some i
with _ -> None
type Ask =
static member Askdistance (?text)=
let text = defaultArg text ""
printfn "%s" text
System.Console.WriteLine("How far do you want to travel?")
let distance = tryParseOption (System.Console.ReadLine())
match distance with
|None -> Ask.Askdistance("Invalid format distance provided")
|Some(a) when a <0 -> Ask.Askdistance("Can't use negative numbers")
|Some(a) when a =0 -> Ask.Askdistance("Can't travel a distance of 0")
|_ -> printfn "You are about to travel %A" distance.Value
ignore()
//this will keep asking you to input a distance until you put in a correct value (i.e. a positive integer value. Note it will reject a floating point input).
Ask.Askdistance()
While the other answers are correct, here is another way of doing it:
open System
let rec travel() =
printfn "How far do you want to travel?"
let d = Console.ReadLine()
match Int32.TryParse d with
| false, _ ->
printfn "Invalid distance format '%s'" d
travel()
| _, 0 ->
printfn "Can't travel a distance of 0"
travel()
| _, d when d < 0 ->
printfn "Can't use negative number %i" d
travel()
| _, d ->
printfn "You are about to travel %i" d
travel()
In my opinion, this has the following advantages:
does not use exceptions
does not use different concepts for the same thing (printfn vs Console.WriteLine)
does not introduce unnecessary constructs (types, helper functions for already existing functionality)
does not mix responsibilities (print error of this invocation in the next one)
I agree, that learning a new language you should explore the solution space. But then choose a concise, elegant way.
Some other aspects of my answer may be opinionated though. You might want to
use explicit trues instead of _.
choose a different name for the matched, parsed distance. I use shadowing because I think of the parsed value as just being a different representation.
reorder the cases, e.g. having the 'happy path' first resembles a try...catch:
open System
let rec travel() =
printfn "How far do you want to travel?"
let d = Console.ReadLine()
match Int32.TryParse d with
| true, d when d > 0 ->
printfn "You are about to travel %i" d
| true, 0 ->
printfn "Can't travel a distance of 0"
travel()
| true, _ ->
printfn "Can't use negative number %i" d
travel()
| _ ->
printfn "Invalid distance format '%s'" d
travel()
travel()
Another approach would be to separate reading from the console and traveling:
using a reading function which always returns an int:
open System
let rec readInt() =
let d = Console.ReadLine()
match Int32.TryParse d with
| true, d ->
d
| _ ->
printfn "Invalid distance format '%s'" d
readInt()
let rec travel() =
printfn "How far do you want to travel?"
let d = readInt()
if d > 0 then
printfn "You are about to travel %i" d
elif d = 0 then
printfn "Can't travel a distance of 0"
travel()
else
printfn "Can't use negative number %i" d
travel()
travel()
using a reading function that might fail:
open System
let rec readInt() =
let d = Console.ReadLine()
match Int32.TryParse d with
| true, d -> Some d
| _ -> None
let rec travel() =
printfn "How far do you want to travel?"
match readInt() with
| Some d when d > 0 ->
printfn "You are about to travel %i" d
| Some d when d = 0 ->
printfn "Can't travel a distance of 0"
travel()
| Some d ->
printfn "Can't use negative number %i" d
travel()
| None ->
printfn "Invalid distance format '%s'" d
travel()
travel()
Hi everbody I am doing a project with F# but I get this error when ı use let num= line for the following code . I'm new at F# so I can not solve the problem. My code should do this things. User enter a number and calculate the fibonacci but if user enter not a number throw exception
open System
let rec fib n =
match n with
|0->0
|1->1
|2->1
|n->fib(n-1)+fib(n-2);;
let printFibonacci list =
for i=0 to (List.length list)-1 do
printf "%d " (list.Item(i));;
let control = true
while control do
try
printfn "Enter a Number:"
let num:int = Convert.ToInt32(stdin.ReadLine())
with
| :? System.FormatException->printfn "Number Format Exception";
let listFibonacci = [for i in 0 .. num-1->fib(i)]
printFibonacci(listFibonacci)
printfn "\n%A"(listFibonacci)
control<-false
Console.ReadKey(true)
exit 0;;
I'm not an F# expert but I can see 3 problems with the code you posted.
1) As Lasse V Karlsen commented - f# uses the 'offside' rule so your 'fib' expression needs the body indented in. If you are running this in the Visual Studio Shell it should warn you of this by putting a blue squiggly line under the appropriate code.
2) Both 'control' and 'num' are mutable values so need to be declared explicitly as such.
f# is a functional language so by default any expressions are immutable i.e they are not allowed to change state after they have been declared.
In f#, saying 'let n = expr' does not mean 'assign the value of expr to n' like you would in say c# or c++. Instead it means 'n fundamentally is expr' and will be forever much like a mathematical equation.
So if you want to update the value of a variable you use the special '<-' notation which is the equivalent of 'assign the value on rhs to the lhs' and you need to declare that variable as mutable i.e 'this value can be changed later'
So I think both num and control need to be declared at the top of the loop as
let mutable control = false
let mutable num = 0 // or whatever you want the initial value of num to be
As a side note you don't have to explicitly declare num as an int ( you can if you want ) but f# will infer the type for you
If I understand your code correctly, you want to keep asking for input number n until a valid number is given and print fibonacci numbers up to n. In this case, you'd better move the calculation and printing inside the try block. Here's an updated version with formatting.
open System
let rec fib n =
match n with
|0->0
|1->1
|2->1
|n->fib(n-1)+fib(n-2);;
let printFibonacci list =
for i=0 to (List.length list)-1 do
printf "%d " (list.Item(i))
let mutable control = true //you forgot to add the 'mutable' keyword
while control do
try
printfn "Enter a Number:"
let num:int = Convert.ToInt32(stdin.ReadLine())
let listFibonacci = [for i in 0 .. num-1 -> fib(i)]
printFibonacci(listFibonacci)
printfn "\n%A"(listFibonacci)
control <- false
with
| :? System.FormatException -> printfn "Number Format Exception"
//add the ignore statement to drop the resulting ConsoleKeyInfo struct
//or the compiler will complain about an unused value floating around.
Console.ReadKey(true) |> ignore
// exit 0 (* Exit isn't necessary *)
Instead of using an imperative style number entry routine and relying on exceptions for control flow, here's a recursive getNumberFromConsole function you could use as well:
open System
let rec fib n =
match n with
| 0 -> 0
| 1 | 2 -> 1
| n -> fib(n-1) + fib(n-2);;
let printFibonacci list =
for i=0 to (List.length list)-1 do
printf "%d " (list.Item(i))
//alternative number input, using recursion
let rec getNumberFromConsole() =
match Int32.TryParse(stdin.ReadLine()) with
| (true, value) -> value
| (false, _) -> printfn "Please enter a valid number"
getNumberFromConsole()
printfn "Enter a Number:"
let num = getNumberFromConsole()
let listFibonacci = [for i in 0 .. num-1 -> fib(i)]
printFibonacci(listFibonacci)
printfn "\n%A"(listFibonacci)
Console.ReadKey(true) |> ignore
P.S. Thanks for showing me stdin. I never knew it existed. Now I can write some interactive scripts.