Neighbour primes: Difference between revisions
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println(filter(isneiprime, primes(500)))
</lang>{{out}}<pre>[3, 5, 7, 13, 19, 67, 149, 179, 229, 239, 241, 269, 277, 307, 313, 397, 401, 419, 439, 487]</pre>
=={{header|Ksh}}==
<lang ksh>
#!/bin/ksh
# Find and show primes p such that p*q+2 is prime, where q is next prime after p and p<500
# # Variables:
#
integer MAX_PRIME=500
typeset -a parr
# # Functions:
#
# # Function _isprime(n) return 1 for prime, 0 for not prime
#
function _isprime {
typeset _n ; integer _n=$1
typeset _i ; integer _i
(( _n < 2 )) && return 0
for (( _i=2 ; _i*_i<=_n ; _i++ )); do
(( ! ( _n % _i ) )) && return 0
done
return 1
}
# # Function _neighbourprime(n) return p*q+2 if prime; 0 if not
#
function _neighbourprime {
typeset _indx ; integer _indx=$1
typeset _arr ; nameref _arr="$2"
typeset _neighbor
(( _neighbor = _arr[_indx] * _arr[_indx+1] + 2 ))
_isprime ${_neighbor}
(( $? )) && echo ${_neighbor} && return
echo 0
}
######
# main #
######
for ((i=2; i<MAX_PRIME; i++)); do
_isprime ${i} ; (( $? )) && parr+=( ${i} )
done
printf "%3s %3s %6s\n" p q p*q+2
printf "%3s %3s %6s\n" --- --- -----
for ((i=0; i<$((${#parr[*]}-1)); i++)); do
np=$(_neighbourprime ${i} parr)
(( np > 0 )) && printf "%3d %3d %6d\n" ${parr[i]} ${parr[i+1]} ${np}
done</lang>{{out}}
p q p*q+2
--- --- -----
3 5 17
5 7 37
7 11 79
13 17 223
19 23 439
67 71 4759
149 151 22501
179 181 32401
229 233 53359
239 241 57601
241 251 60493
269 271 72901
277 281 77839
307 311 95479
313 317 99223
397 401 159199
401 409 164011
419 421 176401
439 443 194479
487 491 239119</pre>
=={{header|Mathematica}}/{{header|Wolfram Language}}==
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