Task: Strong and weak primes
C: #include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>

const int PRIMES[] = {
    2, 3, 5, 7,
    11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97,
    101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293,
    307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523,
    541, 547, 557, 563, 569, 571, 577, 587, 593, 599, 601, 607, 613, 617, 619, 631, 641, 643, 647, 653, 659, 661, 673, 677, 683, 691, 701, 709, 719, 727, 733, 739, 743, 751, 757, 761, 769,
    773, 787, 797, 809, 811, 821, 823, 827, 829, 839, 853, 857, 859, 863, 877, 881, 883, 887, 907, 911, 919, 929, 937, 941, 947, 953, 967, 971, 977, 983, 991, 997,
    1009, 1013, 1019, 1021, 1031, 1033, 1039, 1049, 1051, 1061, 1063, 1069, 1087, 1091, 1093, 1097, 1103, 1109, 1117, 1123, 1129, 1151, 1153, 1163, 1171, 1181, 1187, 1193, 1201, 1213, 1217,
    1223, 1229, 1231, 1237, 1249, 1259, 1277, 1279, 1283, 1289, 1291, 1297, 1301, 1303, 1307, 1319, 1321, 1327, 1361, 1367, 1373, 1381, 1399, 1409, 1423, 1427, 1429, 1433, 1439, 1447, 1451,
    1453, 1459, 1471, 1481, 1483, 1487, 1489, 1493, 1499, 1511, 1523, 1531, 1543, 1549, 1553, 1559, 1567, 1571, 1579, 1583, 1597, 1601, 1607, 1609, 1613, 1619, 1621, 1627, 1637, 1657, 1663,
    1667, 1669, 1693, 1697, 1699, 1709, 1721, 1723, 1733, 1741, 1747, 1753, 1759, 1777, 1783, 1787, 1789, 1801, 1811, 1823, 1831, 1847, 1861, 1867, 1871, 1873, 1877, 1879, 1889, 1901, 1907,
    1913, 1931, 1933, 1949, 1951, 1973, 1979, 1987, 1993, 1997, 1999, 2003, 2011, 2017, 2027, 2029, 2039, 2053, 2063, 2069, 2081, 2083, 2087, 2089, 2099, 2111, 2113, 2129, 2131, 2137, 2141,
    2143, 2153, 2161, 2179, 2203, 2207, 2213, 2221, 2237, 2239, 2243, 2251, 2267, 2269, 2273, 2281, 2287, 2293, 2297, 2309, 2311, 2333, 2339, 2341, 2347, 2351, 2357, 2371, 2377, 2381, 2383,
    2389, 2393, 2399, 2411, 2417, 2423, 2437, 2441, 2447, 2459, 2467, 2473, 2477, 2503, 2521, 2531, 2539, 2543, 2549, 2551, 2557, 2579, 2591, 2593, 2609, 2617, 2621, 2633, 2647, 2657, 2659,
    2663, 2671, 2677, 2683, 2687, 2689, 2693, 2699, 2707, 2711, 2713, 2719, 2729, 2731, 2741, 2749, 2753, 2767, 2777, 2789, 2791, 2797, 2801, 2803, 2819, 2833, 2837, 2843, 2851, 2857, 2861,
    2879, 2887, 2897, 2903, 2909, 2917, 2927, 2939, 2953, 2957, 2963, 2969, 2971, 2999, 3001, 3011, 3019, 3023, 3037, 3041, 3049, 3061, 3067, 3079, 3083, 3089, 3109, 3119, 3121, 3137, 3163
};
#define PRIME_LENGTH (sizeof(PRIMES) / sizeof(int))

bool isPrime(int n) {
    int i;

    if (n < 2) {
        return false;
    }

    for (i = 0; i < PRIME_LENGTH; ++i) {
        if (n == PRIMES[i]) {
            return true;
        }
        if (n % PRIMES[i] == 0) {
            return false;
        }
        if (n < PRIMES[i] * PRIMES[i]) {
            break;
        }
    }

    return true;
}

int main() {
    const int MAX_LENGTH = 700000;
    int i, n, c1, c2;

    int *primePtr = calloc(MAX_LENGTH, sizeof(int));
    if (primePtr == 0) {
        return EXIT_FAILURE;
    }

    for (i = 0; i < PRIME_LENGTH; i++) {
        primePtr[i] = PRIMES[i];
    }

    i--;
    for (n = PRIMES[i] + 4; n < 10000100;) {
        if (isPrime(n)) {
            primePtr[i++] = n;
        }
        n += 2;

        if (isPrime(n)) {
            primePtr[i++] = n;
        }
        n += 4;

        if (i >= MAX_LENGTH) {
            printf("Allocate more memory.");
            return EXIT_FAILURE;
        }
    }

    
    printf("First 36 strong primes:");
    c1 = 0;
    c2 = 0;
    for (n = 0, i = 1; i < MAX_LENGTH - 1; i++) {
        if (2 * primePtr[i] > primePtr[i - 1] + primePtr[i + 1]) {
            if (n < 36) {
                printf("  %d", primePtr[i]);
                n++;
            }
            if (primePtr[i] < 1000000) {
                c1++;
                c2++;
            } else if (primePtr[i] < 10000000) {
                c2++;
            } else break;
        }
    }
    printf("\nThere are %d strong primes below 1,000,000", c1);
    printf("\nThere are %d strong primes below 10,000,000\n\n", c2);

    
    printf("First 37 weak primes:");
    c1 = 0;
    c2 = 0;
    for (n = 0, i = 1; i < MAX_LENGTH - 1; i++) {
        if (2 * primePtr[i] < primePtr[i - 1] + primePtr[i + 1]) {
            if (n < 37) {
                printf("  %d", primePtr[i]);
                n++;
            }
            if (primePtr[i] < 1000000) {
                c1++;
                c2++;
            } else if (primePtr[i] < 10000000) {
                c2++;
            } else break;
        }
    }
    printf("\nThere are %d weak primes below 1,000,000", c1);
    printf("\nThere are %d weak primes below 10,000,000\n\n", c2);

    free(primePtr);
    return EXIT_SUCCESS;
}

Go: package main

import "fmt"

func sieve(limit int) []bool {
    limit++
    
    
    c := make([]bool, limit)
    c[0] = true
    c[1] = true

    p := 3 
    for {
        p2 := p * p
        if p2 >= limit {
            break
        }
        for i := p2; i < limit; i += 2 * p {
            c[i] = true
        }
        for {
            p += 2
            if !c[p] {
                break
            }
        }
    }
    return c
}

func commatize(n int) string {
    s := fmt.Sprintf("%d", n)
    le := len(s)
    for i := le - 3; i >= 1; i -= 3 {
        s = s[0:i] + "," + s[i:]
    }
    return s
}

func main() {
    
    const limit = 1e7 + 19
    sieved := sieve(limit)
    
    var primes = []int{2}
    for i := 3; i <= limit; i += 2 {
        if !sieved[i] {
            primes = append(primes, i)
        }
    }
    
    var strong []int
    var weak = []int{3}                  
    for i := 2; i < len(primes)-1; i++ { 
        if primes[i] > (primes[i-1]+primes[i+1])/2 {
            strong = append(strong, primes[i])
        } else if primes[i] < (primes[i-1]+primes[i+1])/2 {
            weak = append(weak, primes[i])
        }
    }

    fmt.Println("The first 36 strong primes are:")
    fmt.Println(strong[:36])
    count := 0
    for _, p := range strong {
        if p >= 1e6 {
            break
        }
        count++
    }
    fmt.Println("\nThe number of strong primes below 1,000,000 is", commatize(count))
    fmt.Println("\nThe number of strong primes below 10,000,000 is", commatize(len(strong)))

    fmt.Println("\nThe first 37 weak primes are:")
    fmt.Println(weak[:37])
    count = 0
    for _, p := range weak {
        if p >= 1e6 {
            break
        }
        count++
    }
    fmt.Println("\nThe number of weak primes below 1,000,000 is", commatize(count))
    fmt.Println("\nThe number of weak primes below 10,000,000 is", commatize(len(weak)))
}