Basically the title. I'm trying to display floats in a string, but the result ends up being "3.0e-2" (In the example of the float "0.03"). How do I get in decimal form in Haskell? Thanks to anyone who answers.
For pretty formatting values of built-in types, use printf:
> printf "%f" (0.03 :: Float)
0.03
See the linked docs for a full explanation and all possible formatting options.
Related
Consider this variable
X = "1_2_3_4",
How to I get X into a binary string format?
So like this
<<"1_2_3_4">>
I'm getting all manner of errors trying to figure it out.
Thanks in advance,
Snelly.
Use the built in conversion function.
list_to_binary(X).
Thanks Dave! I had a quite a complex function concatinating variables
and strings/integers/atoms etc.
In case you are interested:
convert(L) ->
convert(L, <<>>).
convert([], Bin) ->
Bin;
convert([H|T], Bin) ->
convert(T, <<Bin/binary, H>>).
(what I was thinking of as Strings) as lists! Confusing
I think the reason that it's confusing is because sometimes the shell prints out a list as a string and sometimes it doesn't. In my opinion, things would be much clearer if the shell always output a list as a list unless you specifically requested a string.
How can i use printf over a list?
I have a list of numbers and i want to print them all by respecting a format (ex: %.3f). I tried to use map over printf, but it does not work. So, i have no idea. Can somebody help me with this? Any ideas are acceptable. Is there a way to create a string from a list respecting a custom format?
printf can produce strings instead of just printing them to stdout. This
is because it is overloaded on its result type (it's also part of machinery
that makes it variadic).
import Text.Printf
main :: IO ()
main = putStrLn . unwords $ printf "%.3f" <$> ([1..10] :: [Double])
That should do the trick.
BTW, printf is not type safe and can blow at run time. I recommend you use
something like
formatting.
I am a starting out programmer and have my first few programming classes. We started off with functional programming, in this case using Haskell. I've managed to complete a few assignments already, but seem to have gotten stuck in one point and was hoping to get some help with it.
In order to not bore you with the entire code, my program right now is extracting a list of commands from a text file. I need to turn this list into a set of coordinates. What I mean is something along the lines of:
function :: [String] -> (Int, Int, Char)
where the function will receive, for example, the list ["0 0 N"] and output the coordinates and direction (0, 0, N).
I tried doing:
function [x y o] = (show x, show y, read o)
which would work if it were just Integers. I can't seem to get the Char part to work. I appologize if it's such a noobie question, but bear with me, please, I'm really new to all of this.
Thank you and best regards!
For your specific test case this should work:
function [(x:' ':y:' ':o:_)] = (read [x], read [y], o)
If your string contains spaces you need to match on them as well if you want to do it like that.
But that's probably not what you actually want. It would break for inputs like ["12 23 S"] or ["3 5 W", "2 8 E"].
If your input is actually a list of Strings like your signature says you should probably write two functions: One that deals with a single String and one that applies your other function to all Strings in the list. Look at the functions map and words and think about how you can use them to solve your problem.
I am trying to write a Forth interpreter in Haskell. There are many sub problems and categories to accomplish this, however, I am trying to accomplish the most basic of steps, and I have been at it for some time in different approaches. The simple input case I am trying to get to is "25 12 +" -> [37]. I am not worried about the lists in Forth are backwards from Haskell, but I do want to try and accommodate the extensibility of the input string down the road, so I am using Maybe, as if there is an error, I will just do Nothing.
I first tried to break the input string into a list of "words" using Prelude's words function. From there I used Prelude's reads function to turn it into a list of tuples (Int,String). So this works great, up until I get to a command "word", such as the char + in the sample problem.
So how do I parse/interpret the string's command to something I can use?
Do I create a new data structure that has all the Forth commands or special characters? (assuming this, how do I convert it from the string format to that data type?)
Need anything else, just ask. I appreciate the help thinking this through.
read is essentially a very simple string parser. Rather than adapting it, you might want to consider learning to use a parser combinator library such as Parsec.
There are a bunch of different tutorials about parser combinators so you'll probably need to do a bit of reading before they 'click.' However, the first example in this tutorial is quite closely related to your problem.
import Text.Parsec
import Text.Parsec.String
play :: String -> Either ParseError Integer
play s = parse pmain "parameter" s
pmain :: Parser Integer
pmain = do
x <- pnum `chainl1` pplus
eof
return x
pnum = read `fmap` many1 digit
pplus = char '+' >> return (+)
It's a simple parser that evaluates arbitrarily long lists:
*Main> play "1+2+3+4+5"
Right 15
It also produces useful parse errors:
*Main> play "1+2+3+4+5~"
Left "parameter" (line 1, column 10):
unexpected '~'
expecting digit, "+" or end of input
If you can understand this simple parser, you should be able to work out how to adapt it to your particular problem (referring to the list of generic combinators in the documentation for Text.Parsec.Combinator). It will take a little longer at first than using read, but using a proper parsing library will make it much easier to achieve the ultimate goal of parsing Forth's whole grammar.
This may seem like a stupid question, but what are the symbols used for string replacement in string.format? can someone point me to a simple example of how to use it?
string.format in Lua follows the same patterns as Printf in c:
https://cplusplus.com/reference/cstdio/printf/
There are some exceptions, for those see here:
http://pgl.yoyo.org/luai/i/string.format
Chapter 20 of PiL describes string.format near the end:
The function string.format is a
powerful tool when formatting strings,
typically for output. It returns a
formatted version of its variable
number of arguments following the
description given by its first
argument, the so-called format string.
The format string has rules similar to
those of the printf function of
standard C: It is composed of regular
text and directives, which control
where and how each argument must be
placed in the formatted string.
The Lua Reference says:
The format string follows the same
rules as the printf family of standard
C functions. The only differences are
that the options/modifiers *, l, L, n,
p, and h are not supported and that
there is an extra option, q.
The function is implemented by str_format() in strlib.c which itself interprets the format string, but defers to the C library's implementation of sprintf() to actually format each field after determining what type of value is expected (string or number, essentially) to correspond to each field.
There should be "Lua Quick Reference" html file in your hard disk, if you used an installation package.
(for example: ../Lua/5.1/docs/luarefv51.html)
There you'll find, among other things,
string.format (s [, args ])
Formatting directives
Formatting field types
Formatting flags
Formatting examples
To add to the other answers: Lua does have a boolean data type, where C does not. C uses numbers for that, where 0 is false and everything else is true.
However, to format a boolean in a String in Lua,
local text = string.format("bool is %d", truth)
gets (at least in Hammerspoon):
bad argument #2 to 'format' (number expected, got boolean)
You can instead use %s for booleans (as for strings):
local text = string.format("bool is %s", truth)