Output nearby primes

Question

Write a program which takes an input (which may or may not be prime), and lists the immediate prime following and preceding it.

Example input:

1259

Example output:

1249 1277

Shortest program wins. Must execute within 10 seconds on a modern desktop PC. Inputs will be limited to 10,000 maximum.

Answer 1

Mathematica, 19

#~NextPrime~{-1,1}&

Answer 2

Mathematica: 28 chars

(k=NextPrime;{k[#,-1],k@#})&  

Usage

%[1259]
{1249, 1277}  

%[121231313159]  
{121231313129, 121231313191}

Answer 3

Perl 5.10 (perl -E), 65 chars

Half the credit (at least) should go to @J B.

$m=<>;for(-1,1){$n=$m;0while(1x($n+=$_))=~/^1$|(^11+)\1+$/;say$n}

Answer 4

Python - 93

Based on answer by fR0DDY. I basically merged lines 4 and 5, and shortened line 2 by using a different method.

n=input()-1
m=n+2
f=lambda n:any(n%x<1for x in range(2,n))
exec"n-=f(n);m+=f(m);"*m
print n,m

Answer 5

J, 22 characters

(_4&p:,4&p:)(".stdin)_

Answer 6

Python 116 111 109 Characters

n=input()-1
m=n+2
f=lambda n:any(pow(b,n-1,n)>1for b in(3,5,7,13))
while f(n):n-=1
while f(m):m+=1
print n,m

Answer 7

APL (Dyalog Extended), 6 bytes

¯4 4⍭⊢

Try it online!

I wish I knew a way to shorten this further.

¯4 and 4 when passed to ⍭ give the closest primes to the right argument(⊢).

Answer 8

Jelly, 5 bytes

Æp,Æn

Try it online!

Answer 9

Swift 190 187 185 110

Swift is very bad in code-golf, but I tried it anyway :D
It's getting shorter and shorter...(Thanks to @HermanLauenstein for removing 75 bytes)

var a=Int(readLine()!)!;for b in[-1,1]{var n=a,c=0;while c<1{n+=b;c=1;for i in 2..<n{if n%i<1{c=0}}};print(n)}

Answer 10

Haskell: 99

s(z:y)=z:s[x|x<-y,mod x z>0];f(x:y:z:w)=(x,z):f(y:z:w);p x=(head.filter(\(c,v)->c<x&&v>x).f.s)[2..]

Example

Main> p 1259
(1249,1277)

Answer 11

Python, 116 139 chars (double indent is tab-char)

Uses good ole Sieve of Eratosthenes

Edits and (thanks a TON @JPvdMerwe). Should work with primes now.

l=n=input();a=range(n*2)
for i in a[2:]:a=[k for k in a if k==i or k%i]
for g in a:
 if g>n:print l,g;break
 if i!=n:l=g

Original

a=range(9999)
j=lambda a,n:[i for i in a if i==n or i%n]
for i in a[2:]:a=j(a,i)
o=n=input();
for i in a:
 if o<n and i>n: 
  print o,i
 o=i

Answer 12

Scala 119:

def p(n:Int)=(2 to n-1).exists(n%_==0)
def i(n:Int,v:Int):Int=if(!p(n+v))n+v else i(n+v,v)
Seq(-1,1).map(i(readInt,_))

ungolfed:

def notPrime (n:Int) = 
    (2 to n-1).exists (n % _ == 0)

def itPrime (n: Int, vector:Int) : Int =
    if (! notPrime (n+vector)) n+vector
    else itPrime (n+vector, vector)

def nearbyPrime (value: Int) =
    Seq (-1, 1).map (sign => itPrime (value, sign))

21.2s to run all 9998 ints from 3 to 10.000

Answer 13

Python (123)

import Primes as p
j=i=int(input())
n=p.primes(i*2)
while not i in n[:]:
 i+=1
print(i)
while not j in n[:]:
 j-=1
print(j)

NOTE: The Primes module was written by me but it existed before this question was asked. It was NOT written for this. Nevertheless, this was deemed unfair, so here is the updated version.

Python(215)

j=i=int(input())
import math as m
s=list(range(i*2))
for n in s[:]:
 for l in range(1,int(m.ceil(m.sqrt(n)))):
  if(n%l)==0and l!=1and n in s:s.remove(n)
while not i in s:i+=1
print(i)
while not j in s:j-=1
print(j)

Answer 14

Stax, 7 bytes

èëïy▌Ü╚

Run and debug it

ASCII equivalent:

^:Px:pLJ

Built-ins.

Answer 15

JavaScript (Node.js), 79 bytes

f=(n,d)=>d?p(n)?n:f(n+d,d):[f(n-1,-1),f(n+1,1)]
p=(n,i=1)=>++i*i>n||n%i&&p(n,i)

Try it online!

Answer 16

Arn, 22 bytes

ét█ï▀¢│B¬Ÿ6├¼òa¡b←Æ╚ÑÕ

Try it!

Explained

Unpacked: k:((v:#.^2):i)[v?--k v?(k+2

One of the major weaknesses of Arn (along with operator precedence lmao)

  :                     Assign variable
k                       with name of k
    (                   Begin expression
        (
            :
          v
              #.        List of primes from 0 to
                  _     Variable initialized to STDIN; implied
                ^       exponentiated by
                  2     Literal two
        )               End expression
      :i                Index of
        _               implied
    )
[                       Begin sequence
    v
  ?                     Indexed by
    --k                 k minus 1
    v
  ?
    (
        k
      +                 Plus
        2
    )                   implied
]                       End of sequence; implied

Answer 17

C (gcc), 98 89 87 bytes

-9 bytes thanks to ceilingcat

N,i;g(n,d){for(i=N=n+=d;--i>1;)N*=N%i>0;N?printf("%d ",n):g(n,d);}f(n){g(n,-1);g(n,1);}

Try it online!

Full program version, C (gcc), 116 107 105 bytes

-9 bytes thanks to ceilingcat

N,i;g(n,d){for(i=N=n+=d;--i>1;)N*=N%i>0;N?printf("%d ",n):g(n,d);}main(n){scanf("%d",&n);g(n,-1);g(n,1);}

Try it online!

Answer 18

05AB1E, 6 bytes

ÅMsÅN»

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ÅMsÅN»  # full program
     »  # join...
ÅM      # nearest prime less than...
        # implicit input...
     »  # with...
   ÅN   # nearest prime greater than...
  s     # implicit input...
     »  # by a newline
        # implicit output