Task: Ulam spiral (for primes)
C++: #include <cmath>
#include <iostream>
#include <string>
#include <iomanip>
#include <vector>

class ulamSpiral {
public:
    void create( unsigned n, unsigned startWith = 1 ) {
        _lst.clear();
        if( !( n & 1 ) ) n++;
        _mx = n;
        unsigned v = n * n;
        _wd = static_cast<unsigned>( log10( static_cast<long double>( v ) ) ) + 1;
        for( unsigned u = 0; u < v; u++ )
            _lst.push_back( -1 );

        arrange( startWith );

    }
    void display( char c ) {
        if( !c ) displayNumbers();
        else displaySymbol( c );
    }

private:
    bool isPrime( unsigned u ) {
        if( u < 4 ) return u > 1;
        if( !( u % 2 ) || !( u % 3 ) ) return false;
 
        unsigned q = static_cast<unsigned>( sqrt( static_cast<long double>( u ) ) ),
                 c = 5;
        while( c <= q ) {
            if( !( u % c ) || !( u % ( c + 2 ) ) ) return false;
            c += 6;
        }
        return true;
    }
    void arrange( unsigned s ) {
        unsigned stp = 1, n = 1, posX = _mx >> 1, 
                 posY = posX, stC = 0;
        int dx = 1, dy = 0;
    
        while( posX < _mx && posY < _mx ) {
            _lst.at( posX + posY * _mx ) =  isPrime( s ) ? s : 0;
            s++;
        
            if( dx ) {
                posX += dx;
                if( ++stC == stp ) {
                    dy = -dx;
                    dx = stC = 0;
                }
            } else {
                posY += dy;
                if( ++stC == stp ) {
                    dx = dy;
                    dy = stC = 0;
                    stp++;
                }
            }
        }
    }
    void displayNumbers() {
        unsigned ct = 0;
        for( std::vector<unsigned>::iterator i = _lst.begin(); i != _lst.end(); i++ ) {
            if( *i ) std::cout << std::setw( _wd ) << *i << " ";
            else std::cout << std::string( _wd, '*' ) << " ";
            if( ++ct >= _mx ) {
                std::cout << "\n";
                ct = 0;
            }
        }
        std::cout << "\n\n";
    }
    void displaySymbol( char c ) {
        unsigned ct = 0;
        for( std::vector<unsigned>::iterator i = _lst.begin(); i != _lst.end(); i++ ) {
            if( *i ) std::cout << c;
            else std::cout << " ";
            if( ++ct >= _mx ) {
                std::cout << "\n";
                ct = 0;
            }
        }
        std::cout << "\n\n";
    }

    std::vector<unsigned> _lst;
    unsigned _mx, _wd;
};

int main( int argc, char* argv[] )
{
    ulamSpiral ulam;
    ulam.create( 9 );
    ulam.display( 0 );
    ulam.create( 35 );
    ulam.display( '#' );
    return 0;
}

Go: package main

import (
	"math"
	"fmt"
)

type Direction byte

const (
	RIGHT Direction = iota
	UP
	LEFT
	DOWN
)

func generate(n,i int, c byte) {
	s := make([][]string, n)
	for i := 0; i < n; i++ { s[i] = make([]string, n) }
	dir := RIGHT
	y := n / 2
	var x int
	if (n % 2 == 0) { x = y - 1 } else { x = y } 

	for j := i; j <= n * n - 1 + i; j++ {
		if (isPrime(j)) {
			if (c == 0) { s[y][x] = fmt.Sprintf("%3d", j) } else { s[y][x] = fmt.Sprintf("%2c ", c) }
		} else { s[y][x] = "---" }

		switch dir {
		case RIGHT : if (x <= n - 1 && s[y - 1][x] == "" && j > i) { dir = UP }
		case UP : if (s[y][x - 1] == "") { dir = LEFT }
		case LEFT : if (x == 0 || s[y + 1][x] == "") { dir = DOWN }
		case DOWN : if (s[y][x + 1] == "") { dir = RIGHT }
		}

		switch dir {
		case RIGHT : x += 1
		case UP : y -= 1
		case LEFT : x -= 1
		case DOWN : y += 1
		}
	}

	for _, row := range s { fmt.Println(fmt.Sprintf("%v", row)) }
	fmt.Println()
}

func isPrime(a int) bool {
	if (a == 2) { return true }
	if (a <= 1 || a % 2 == 0) { return false }
	max := int(math.Sqrt(float64(a)))
	for n := 3; n <= max; n += 2 { if (a % n == 0) { return false } }
	return true
}

func main() {
	generate(9, 1, 0) 
	generate(9, 1, '*') 
}