Please Read Primes

Question

Do you know the book Murderous Maths, in the Horrible Science series? In the book, the Laplace Princess' "ridiculous" cipher might be interesting enough and simple enough to be a code-golf challenge.

Task

The original cipher looks like this:

Please read primes elephant help you me like cold

The nth word is only meaningful if it is a prime. So, the cipher becomes:

read primes help me

Your task is to write a program that inputs the cipher and outputs the plaintext.

Rules

• You can take the input in any acceptable forms, e. g. an list of words.

• The input will only consist of printable-ASCII characters.

• You can use 0-based index or 1-based index.

• A "Word" includes any punctuation. For example:

  This is, "a" great test! I think.
  

  should output

  is, "a" test! think.
  

Answer 1

Japt -f, 6 2 bytes

I/O as arrays of words with 0-based indexing

Vj

Try it

Vj     :Implicit filter of input array
V      :0-based index of current word
 j     :Is prime?

---

Japt -S, 4 bytes

I/O as space-delimited strings, with 0-based indexing

¸fÏj

Try it

¸fÏj     :Implicit input of string
¸        :Split on spaces
 f       :Filter by
  Ï      :  0-based index
   j     :  Is prime?
         :Implicit output, joined by spaces

---

Japt -S, 6 bytes

I/O as space-delimited strings, with 1-based indexing

¸fÏÄ j

Try it

Same as above using Ä to add 1 to the index.

Answer 2

05AB1E, 6 bytes

#āpÏðý

Try it online!

#         # split the input on spaces
 ā        # push [1..length of the list of words]
  p       # is prime?
   Ï      # keep only words where the above check is true
    ðý    # join with spaces

Answer 3

Perl 6, 22 bytes

*[grep &is-prime,2..*]

Try it online!

Takes input as a list of words and outputs the zero indexed list.

Answer 4

Kotlin, 201 bytes

Input is zero indexed.

fun p(s:Int):MutableList<Int>{val l=MutableList(0){0}
o@
for(v in 2..s){for(e in l)if(v%e<1)continue@o
l.add(v)}
return l}
fun main(){var w=readLine()!!.split(" ")
for(i in p(w.size))print("${w[i]} ")}

Try it online!

Answer 5

Jelly, 4 bytes

LÆRị

A monadic Link accepting a list of words which yields a list of words.

Try it online! (the footer calls the Link then formats, since a full-program implicitly smashes output)

How?

LÆRị - Link: list, W
L    - length (W)
 ÆR  - inclusive prime range = [2,3,5,7,...,Prime <= length(W)]
   ị - index into (W)

---

Also 4:

JẒƇị

Answer 6

PHP, 167 bytes

<?php $i=explode(' ',$argv[1]);$l=count($i);unset($i[0]);for($x=1;$x<$l;$x++){$y=$x+1;$z=2;while($z<$y){$t=$y/$z;if($t==(int)$t)unset($i[$x]);$z++;}}echo join(' ',$i);

Try it online!

Ungolfed:

<?php

$input = explode(' ', $argv[1]);
$length = count($input);
unset($input[0]);

for($x = 1; $x < $length; $x++) {
    $y = $x + 1;
    $z = 2;

    while($z < $y) {
        $test = $y / $z;

        if($test == (int) $test) {
            unset($input[$x]);
        }

        $z++;
    }
}

echo join(' ', $input);

?>

Answer 7

Python 3, 125 107 bytes

107 bytes thanks to Black Owl Kai

i=input().split();j=k=1
for c in i:
 if j>1:
  while k>2:
   k-=1
   if j%k<1:k=0
  if k:print(c)
 j+=1;k=j

Try it online!

120 bytes if it strictly follows rules. -25 bytes thanks to Black Owl Kai

Python 3, 145 120 bytes

i=input().split();j=k=1;l=[]
for c in i:
 if j>1:
  while k>2:
   k-=1
   if j%k<1:k=0
  if k:l+=[c]
 j+=1;k=j
print(*l)

Try it online!

Answer 8

Python 3, 90 bytes

print(*[w for n,w in enumerate(input().split()[1:])if all((n+2)%i for i in range(2,n+2))])

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Answer 9

Rust, 339 bytes

First ever submission to Code Golf, but it works!

use std::io;use std::io::BufRead;fn main(){println!("{}",io::BufReader::new(io::stdin().lock()).lines().flat_map(|s|s.unwrap().split_whitespace().map(|s|s.to_owned()).collect::<Vec<String>>()).enumerate().filter(|(x,_)|{if *x<2{return false}for i in 2..*x{if x%i==0{return false}};true}).map(|(_,y)|y).collect::<Vec<String>>().join(" "));}

This code is compressed down from this:

use std::io;
use std::io::BufRead;
use std::io::BufReader;

fn main() {
    let s = BufReader::new(io::stdin().lock())
        .lines()
        .flat_map(|s| {
            s.unwrap()
                .split_whitespace()
                .map(|s| s.to_owned())
                .collect::<Vec<String>>()
        })
        .enumerate()
        .filter(|(x,_)| {
            if *x<2 {
                return false
            }
            for i in 2..*x {
                if x % i == 0 {
                    return false
                }
            };
            true
        })
        .map(|(_,y)| y)
        .collect::<Vec<String>>()
        .join(" ");
    println!("{}", &s);
}

Answer 10

Ruby -p -rprime, 36 bytes

Replaces every word (and the space after it, if present) with the empty string if it is a non-prime word.

i=0
gsub(/\S+ ?/){$&if(i+=1).prime?}

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Answer 11

C (gcc), 95 bytes

#define f(w,n)for(int i=0,j,p[n]={};++i<n;p[i]||printf("%s ",w[i]))for(j=i*i;j<n;j+=i)p[j]=i>1;

Try it online!

Sieve of E.

Answer 12

R + gmp, 48 bytes

(W=scan(,'',,,,''))[gmp::isprime(1:length(W))>0]

Try it online!

This use gmp for the isprime builtin. This returns 0, 1 or 2, hence the test for > 0. scan takes the input as characters split on whitespace and quotes set to empty string.

Answer 13

Vyxal, 8 bytes

⌈'&›¥æ;Ṅ

Try it Online!

⌈        # Split on spaces
 '    ;  # Filter by 
  &›     # Increment register
    ¥æ   # Register prime?
       Ṅ # Join on spaces