Truncatable primes: Difference between revisions

=={{header|C}}==

<lang C>#include<stdio.h>

#include<string.h>

#include<stdlib.h>

#include<math.h>

int isprime(int n){

int isprm[]={0,0,1,1,0,1,0,1,0,0};

int a=7,h;

if(n<10)return isprm[n];

if(!(n&1)||!(n%3))return 0;

h=sqrt(n)+3;

while(a<h){

if(!(n%a)||!(n%(a-2)))return 0;

a+=6;

}

return 1;

}

void leftTrunc(char*s,int l){

memmove(s,s+1,l);

}

void rightTrunc(char*s,int l){

s[l-1]=0;

}

int isTruncPrime(int n,void(*func)(char*,int)){

char s[32];

while(isprime(n)){

ltoa(n,s,10);

if(strchr(s,'0'))break;

func(s,strlen(s));

if(!(n=atol(s)))return 1;

}

return 0;

}

void getHiTruncPrime(int n,void(*func)(char*,int)){

while(n--){

if(isTruncPrime(n,func)){

printf("%d\n",n);

break;

}

}

}

int main(){

puts("Highest left- and right-truncatable primes under 1e6:");

getHiTruncPrime(1000000,leftTrunc);

getHiTruncPrime(1000000,rightTrunc);

puts("Press Enter");

getchar();

return 0;

}</lang>

Output:

<pre>Highest left- and right-truncatable primes under 1e6:

998443

739399

Press Enter</pre>