68 lines
1.5 KiB
C
68 lines
1.5 KiB
C
/* We shall say that an n-digit number is pandigital if it makes use of all the digits 1 to n exactly once. For example, 2143 is a 4-digit pandigital
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* and is also prime.
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*
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* What is the largest n-digit pandigital prime that exists?*/
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#define _POSIX_C_SOURCE 199309L
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#include <stdio.h>
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#include <stdlib.h>
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#include <math.h>
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#include <time.h>
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#include "projecteuler.h"
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int count_digits(int n);
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int main(int argc, char **argv)
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{
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/* 8- and 9-digit pandigital numbers can't be prime, because
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* 1+2+...+8=36, which is divisible by 3, and 36+9=45 which is
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* also divisible by 3, and therefore the whole number is divisible
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* by 3. So we can start from the largest 7-digit pandigital number,
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* until we find a prime.*/
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int i = 7654321;
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double elapsed;
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struct timespec start, end;
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clock_gettime(CLOCK_MONOTONIC, &start);
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while(i > 0)
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{
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if(is_pandigital(i, count_digits(i)) && is_prime(i))
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{
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break;
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}
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/*Skipping the even numbers.*/
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i -= 2;
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}
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clock_gettime(CLOCK_MONOTONIC, &end);
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elapsed = (end.tv_sec - start.tv_sec) + (double)(end.tv_nsec - start.tv_nsec) / 1000000000;
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printf("Project Euler, Problem 41\n");
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printf("Answer: %d\n", i);
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printf("Elapsed time: %.9lf seconds\n", elapsed);
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return 0;
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}
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int count_digits(int n)
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{
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int i = 0;
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if(n == 0)
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{
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return 1;
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}
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while(n > 0)
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{
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i++;
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n /= 10;
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}
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return i;
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}
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