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;
}

VB: Function build_spiral(n)
	
	Dim matrix()
	ReDim matrix(n-1,n-1)
	
	x = (n-1)/2 : y = (n-1)/2
	
	x_max = 1 : y_max = 1 : count = 1
	
	dir = "R"
	
	For i = 1 To n*n
		l = Len(n*n)
		If IsPrime(i) Then
			matrix(x,y) = Right("000" & i,l)
		Else
			matrix(x,y) = String(l,"-")
		End If
		Select Case dir
			Case "R"
				If x_max > 0 Then
					x = x + 1 : x_max = x_max - 1
				Else
					dir = "U" : y_max = count
					y = y - 1 : y_max = y_max - 1
				End If
			Case "U"
				If y_max > 0 Then
					y = y - 1 : y_max = y_max - 1
				Else
					dir = "L" : count = count + 1 : x_max = count
					x = x - 1 : x_max = x_max - 1
				End If
			Case "L"
				If x_max > 0 Then
					x = x - 1 : x_max = x_max - 1
				Else
					dir = "D" : y_max = count
					y = y + 1 : y_max = y_max - 1
				End If
			Case "D"
				If y_max > 0 Then
					y = y + 1 : y_max = y_max - 1
				Else
					dir = "R" : count = count + 1 : x_max = count
					x = x + 1 : x_max = x_max - 1
				End If
		End Select
	Next
	
	For y = 0 To n - 1
		For x = 0 To n - 1
			If x = n - 1 Then
				WScript.StdOut.Write matrix(x,y)
			Else
				WScript.StdOut.Write matrix(x,y) & vbTab
			End If
		Next
		WScript.StdOut.WriteLine
	Next
End Function

Function IsPrime(n)
	If n = 2 Then
		IsPrime = True
	ElseIf n <= 1 Or n Mod 2 = 0 Then
		IsPrime = False
	Else
		IsPrime = True
		For i = 3 To Int(Sqr(n)) Step 2
			If n Mod i = 0 Then
				IsPrime = False
				Exit For
			End If
		Next
	End If
End Function


build_spiral(9)