Task: Ulam spiral (for primes)
C: #include <stdio.h>
#include <stdint.h>
#include <stdlib.h>
#include <math.h>

typedef uint32_t bitsieve;

unsigned sieve_check(bitsieve *b, const unsigned v)
{
    if ((v != 2 && !(v & 1)) || (v < 2))
        return 0;
    else
        return !(b[v >> 6] & (1 << (v >> 1 & 31)));
}

bitsieve* sieve(const unsigned v)
{
    unsigned i, j;
    bitsieve *b = calloc((v >> 6) + 1, sizeof(uint32_t));

    for (i = 3; i <= sqrt(v); i += 2)
        if (!(b[i >> 6] & (1 << (i >> 1 & 31))))
            for (j = i*i; j < v; j += (i << 1))
                b[j >> 6] |= (1 << (j >> 1 & 31));

    return b;
}

#define max(x,y) ((x) > (y) ? (x) : (y))


int ulam_get_map(int x, int y, int n)
{
    x -= (n - 1) / 2;
    y -= n / 2;

    int mx = abs(x), my = abs(y);
    int l = 2 * max(mx, my);
    int d = y >= x ? l * 3 + x + y : l - x - y;

    return pow(l - 1, 2) + d;
}


void output_ulam_spiral(int n, const char glyph)
{
    
    n -= n % 2 == 0 ? 1 : 0;

    const char *spaces = ".................";
    int mwidth = log10(n * n) + 1;

    bitsieve *b = sieve(n * n + 1);
    int x, y;

    for (x = 0; x < n; ++x) {
        for (y = 0; y < n; ++y) {
            int z = ulam_get_map(y, x, n);

            if (glyph == 0) {
                if (sieve_check(b, z))
                    printf("%*d ", mwidth, z);
                else
                    printf("%.*s ", mwidth, spaces);
            }
            else {
                printf("%c", sieve_check(b, z) ? glyph : spaces[0]);
            }
        }
        printf("\n");
    }

    free(b);
}

int main(int argc, char *argv[])
{
    const int n = argc < 2 ? 9 : atoi(argv[1]);

    output_ulam_spiral(n, 0);
    printf("\n");

    output_ulam_spiral(n, '#');
    printf("\n");

    return 0;
}

Java: import java.util.Arrays;

public class Ulam{
	enum Direction{
		RIGHT, UP, LEFT, DOWN;
	}
	
	private static String[][] genUlam(int n){
		return genUlam(n, 1);
	}

	private static String[][] genUlam(int n, int i){
		String[][] spiral = new String[n][n];
		Direction dir = Direction.RIGHT;
		int j = i;
		int y = n / 2;
		int x = (n % 2 == 0) ? y - 1 : y; 
		while(j <= ((n * n) - 1 + i)){
			spiral[y][x] = isPrime(j) ? String.format("%4d", j) : " ---";

			switch(dir){
			case RIGHT:
				if(x <= (n - 1) && spiral[y - 1][x] == null && j > i) dir = Direction.UP; break;
			case UP:
				if(spiral[y][x - 1] == null) dir = Direction.LEFT; break;
			case LEFT:
				if(x == 0 || spiral[y + 1][x] == null) dir = Direction.DOWN; break;
			case DOWN:
				if(spiral[y][x + 1] == null) dir = Direction.RIGHT; break;
			}
			
			switch(dir){
				case RIGHT:	x++; break;
				case UP: 	y--; break;
				case LEFT:	x--; break;
				case DOWN:	y++; break;
			}
			j++;
		}
		return spiral;
	}
	
	public static boolean isPrime(int a){
		   if(a == 2) return true;
		   if(a <= 1 || a % 2 == 0) return false;
		   long max = (long)Math.sqrt(a);
		   for(long n = 3; n <= max; n += 2){
		      if(a % n == 0) return false;
		   }
		   return true;
		}
	
	public static void main(String[] args){
		String[][] ulam = genUlam(9);
		for(String[] row : ulam){
			System.out.println(Arrays.toString(row).replaceAll(",", ""));
		}
		System.out.println();
		
		for(String[] row : ulam){
			System.out.println(Arrays.toString(row).replaceAll("\\[\\s+\\d+", "[  * ").replaceAll("\\s+\\d+", "   * ").replaceAll(",", ""));
		}
	}
}