Fungsi terbalik

31

Bukankah lebih rapi jika fungsi pemrograman bisa terbalik, sama seperti fungsi matematika yang mereka implementasikan?

Tulis fungsi (atau program) yang mengambil satu input xdalam bentuk apa pun, yang menghasilkan ln(x).
Ketika byte dari program disusun ulang / dibalik sehingga byte pertama sekarang adalah byte terakhir, ia harus mengambil satu input xdalam bentuk dan output apa saja e^xsebagai gantinya.

  • Jawaban Anda harus memiliki setidaknya 3 angka penting yang benar.
  • Perkiraan baik-baik saja, selama mereka memiliki setidaknya 3 angka signifikan yang benar.
  • Kode Anda harus dalam bahasa pemrograman yang sama baik maju dan mundur.

Katakanlah program ini mengimplementasikan ln(x):

abc你好

Maka program ini harus mengimplementasikan e^x:

\xBD\xA5\xE5\xA0\xBD\xE4cba

Bintang emas jika Anda menggunakan bahasa tanpa dukungan mengambang.

Ini adalah bentuk kode-golf yang aneh, sehingga program terpendek menang.

Filip Haglund
sumber
4
"Bukankah lebih rapi jika fungsi pemrograman bisa terbalik, seperti fungsi matematika yang mereka implementasikan?" Beberapa bahasa (misalnya J dan Mathematica) sebenarnya dapat melakukan ini untuk beberapa fungsi.
Martin Ender
Selain itu, K2 dapat memperkirakan invers untuk fungsi murni monadik sewenang-wenang melalui kelebihan "invers fungsi" dari dyadic dan triadic ?, yang menggunakan metode garis potong.
JohnE
1
"setidaknya 3 angka signifikan yang benar" - pada kisaran berapa?
TLW
4
Saya menyadari ini sudah sangat terlambat sekarang, tetapi saya pikir ini akan menjadi tantangan yang sangat bagus seandainya komentar tidak diizinkan.
Alex A.
Saya benar-benar memikirkan itu ketika saya datang dengan tantangan ini @AlexA. tetapi lupa tentang hal itu saat menulis posting: P Juga itu akan membuat bahasa "normal" seperti java, c ++ dll pada dasarnya tidak mungkin.
Filip Haglund

Jawaban:

75

Haskell, 11 byte

f=log
pxe=f

dan dalam urutan terbalik:

f=exp
gol=f

Ini berfungsi tanpa trik "komentar". Alih-alih, setiap versi menentukan fungsi ( pxe/ gol) tambahan yang tidak digunakan .

nimi
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49
+1 for gol=f.
Leif Willerts
2
This is also a valid solution in Julia.
Rainer P.
44

APL, 3 bytes

*⊣⍟

This is a function train. Monadic * returns e^x, monadic returns ln(x). is a dyadic function that returns its left argument. Thus, *⊣⍟ is equivalent to just *, and the reverse ⍟⊣* is equivalent to just .

marinus
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22

Jelly, 5 4 bytes

Yay, my first Jelly answer. :) Input is via command-line argument.

Jelly has its own code page so each character is one byte.

eÆÆl

Try it online!

Reversed:

lÆÆe

Try it online!

Explanation

The Æ on its own is an unrecognised token, so it acts the same as a linefeed. That means in either case the main link is only Æl or Æe which is the 2-character built-in for exp() or ln() and is by default performed on the first command-line argument.

Martin Ender
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9

Javascript, 18 bytes

Math.log//pxe.htaM
Neil
sumber
Don't you need a return() or console.log() around it?
OldBunny2800
2
@OldBunny2800 It evaluates to a function, which should be permissible.
Neil
5
Math.ln||pxe.htaM will probably also work.
SuperJedi224
@SuperJedi224 Thanks, that helped me spot the error in my answer!
Neil
@Neil I hadn't even noticed that
SuperJedi224
7

Seriously, 5 bytes

,_.e,

Input, ln, output, then exp on an empty stack (does nothing), and input (does nothing since input is exhausted). Try it online!

Reversed:

,e._,

Try it online!

Mego
sumber
5

Julia, 7 bytes

log#pxe

This is an anonymous function. Assign it to a variable to call it. Evaluates to builtins log or exp plus a comment.

Rainer P.
sumber
1
Same answer works for R
Dason
5

Mathematica, 19 bytes

1&#@pxE+0&0+Log@#&1

Reversed:

1&#@goL+0&0+Exp@#&1

This was interesting to golf! Mathematica has no line comments / implicit string endings, so I couldn't take the simple route. Instead, I used the fact that 0 + x == x, 0 x == 0, and that 1 x == x, no matter what x is! Testing:

In[1]:= (1&#@pxE+0&0+Log@#&1)[x]

Out[1]= Log[x]

In[2]:= (1&#@goL+0&0+Exp@#&1)[x]

         x
Out[2]= E
LegionMammal978
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4

Python2, 73 bytes

io: stdin/stdout

from math import*;print log(input())#))(tupni(pxe tnirp;*tropmi htam morf

inverse:

from math import*;print exp(input())#))(tupni(gol tnirp;*tropmi htam morf
Filip Haglund
sumber
You can shave 10 characters off by using __import__("math"). instead of
TLW
3

CJam, 11 bytes

rdmle#eemdr

Test it here.

Reversed:

rdmee#elmdr

Test it here.

Basically the same comment-trick as the OP's Python answer. e# starts a comment. rd reads the input and ml or me computes the logarithm or exponential.

Martin Ender
sumber
3

Brachylog, 3 bytes

*₁≡

Try it online!

Initially, I had hoped to use ~*, but although *~ computes e^x and successfully ignores the trailing tilde, ~* fails for all integer inputs and hits a float overflow on most non-integer inputs.

Forwards:

       The output
  ≡    is
*₁     the natural logarithm of
       the input.

Backwards:

       The output is
  *    Euler's number to the power of
       the input
≡      passed through the identity predicate
 ₁     with an extraneous subscript.

This uses the identity predicate because, although trailing tildes are tolerated, leading subscripts are not. (If they were, the Brachylog answer would be *₁ alone, which is just the normal builtin for natural log.)

Unrelated String
sumber
2

Vitsy, 5 bytes

This is a program that exits on an error.

EL^rE
E   E  Push java.lang.Math.E
 L     Push log_(top) (input) (ln(input))
  ^    Push (top)^(input)  (e^(input))
   r   Reverse the stack

This program exits on an error with ln(input) on the stack.

Try it online! (note that I have put N to have visible output)

Then it's inverse:

Er^LE

This program exits on an error with e^(input) on the stack.

Try it online!

Addison Crump
sumber
2

Fuzzy Octo Guacamole, 7 bytes

non-competing, FOG is newer than the challenge

EZO@pZE

This is the equivalent of a function in FOG. It assumes the input is on the stack. This can be assigned to a function by the code "EZO@pZE""f"o, where f is any single-char name you want to assign. Then use it like any other command. Example: "EZO@pZE"'f'o^f.

Explanation:

EZO@pZE
E       # Push E (2.718....)
 Z      # Reverse stack (it is now [e, input])
  O     # log(x, y) which is ln(input)
   @    # Exit. (implicit output) Nothing after this gets run.
    p   # x^y (power function)
     Z  # Reverse stack
      E # Push E.

Reversed:

EZp@OZE
E       # Push E (2.718....)
 Z      # Reverse stack (it is now [e, input])
  O     # x^y (power function)
   @    # Exit. (implicit output) Nothing after this gets run.
    p   # log(x, y) which is ln(input)
     Z  # Reverse stack
      E # Push E.
Rɪᴋᴇʀ
sumber
1

Pyth, 12 bytes

Finds ln(input())

.lQ) " Q1n.^

Finds e^input()

^.n1Q " )Ql.

Spaces stop implicit printing of strings, each version calculates it then creates a string with the remaining characters.

ln(x) mode here

e^x mode here

Blue
sumber
1

Jolf, 9 bytes

Program 1: exp of input

amoj"jOma
a         print
 moj      e^j
    "jOma  the rest of the line is captured as a string; implicit printing is restricted.

Program 2: ln of input

amOj"joma
a         print
 mOj      ln(j)
    "joma  the rest of the line is captured as a string; implicit printing is restricted.

Bonus points for being a case-insensitive palindrome? Try it here!

Conor O'Brien
sumber
1

J, 8 bytes

The natural logarithm is ^., and exponential ^. The problem is, . can only modify a valid verb, otherwise, a spelling error will occur. Thus, we can't use the left argument trick in the APL answer, becuase ^.[^ would cause an error when reversed, as ^[.^ creates an invalid verb. So, we must use comments; but NB. is so long :( Fortunately, they both end with ., so&ldots; there's that.

Logarithm:

^.NB.BN^

Exponential:

^NB.BN.^

You can enter them for yourself online!

Conor O'Brien
sumber
0

Java 8, 198 182 30 bytes

d->Math.log(d)//)d(pxe.htaM<-d

Try it online.

and reversed:

d->Math.exp(d)//)d(gol.htaM<-d

Try it online.

Uses the comment trick (//) with built-ins for Math.log and Math.exp.

Kevin Cruijssen
sumber
0

Runic Enchantments, 9 bytes

i'lA@Ae'i

Try it online!

An ungodly uninteresting program. @ insures termination of the implied entry point at the left, everything after is unexecuted. I tried really hard to re-use the ' or A instructions, but to no avail, even at larger program sizes. The required explicit entry point for multi-line programs essentially precludes it.

Draco18s
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