uva10924.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int primes[52*20];

void init_primes() {
    int i, j;
    for(i=0; i<52*20; i++)
        primes[i] = 1;

    primes[0] = 0;
    primes[1] = 0;
    primes[2] = 1;

    for(i=4; i<52*20; i+=2)
        primes[i] = 0;

    for(i=3; i<52*20; i+=2)
        if (primes[i] == 1)
            for(j=i*i; j<52*20; j+=i)
                primes[j] = 0;
}

int main() {
    init_primes();

    char input[21];
    while (scanf("%s", input) != EOF) {
        int value = 0;

        int len = strlen(input);
        int i;
        for(i=0; i<len; i++) {
            if ('a' <= input[i] && input[i] <= 'z')
                value += (int) (input[i] - 'a' + 1);
            else
                value += (int) (input[i] - 'A' + 27);
        }

        if (value == 1 || primes[value])
            printf("It is a prime word.\n");
        else
            printf("It is not a prime word.\n");
    }

    return 0;
}

uva10924_2.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int primes[500];
int p_idx = 0;

int is_prime(const int N) {
    int i;
    for (i=0; i<p_idx && primes[i]*primes[i] <= N; i++)
        if (N % primes[i] == 0)
            return 0;

    return 1;
}

void init_primes() {
    primes[0] = 2;
    primes[1] = 3;
    primes[2] = 5;

    p_idx = 3;

    int i;
    for (i=7; i < 52*20; i+=2)
        if (is_prime(i))
            primes[p_idx++] = i;
}

int binary_search(const int n) {
    int top = p_idx-1;
    int down = 0;

    while (down <= top) {
        int mid = (top + down) / 2;

        if (primes[mid] == n)
            return 1;
        else if (n < primes[mid])
            top = mid-1;
        else
            down = mid+1;
    }

    return 0;
}

int main() {
    init_primes();

    char input[21];
    while (scanf("%s", input) != EOF) {
        int value = 0;

        int len = strlen(input);
        int i;
        for(i=0; i<len; i++) {
            if ('a' <= input[i] && input[i] <= 'z')
                value += (int) (input[i] - 'a' + 1);
            else
                value += (int) (input[i] - 'A' + 27);
        }

        if (value == 1 || binary_search(value))
            printf("It is a prime word.\n");
        else
            printf("It is not a prime word.\n");
    }

    return 0;
}