Some number tricks (golang)

// number_tricks2.go
//
// a couple of number tricks for the fun of it
//
// 'var x int = 123' is the same as (shorthand) 'x := 123'
//
// online play at:
// https://play.golang.org/p/Vfwg6cfZhd
//
// tested with Go version 1.4.2   by vegaseat (dns)  15may2015

package main

import "fmt"

// sum integers 0 to n (inclusive)
func sum(n uint64) uint64 {
    sum := uint64(0)
    for x := uint64(0); x <= n; x++ {
        sum += x
    }
    return sum
}

func main() {
    fmt.Println("Old programming humor:")
    fmt.Printf("Oct %o == Dec %d\n", 25, 25)
    
    fmt.Println("-------------------------")
	fmt.Println("Nine ones squared ...")
	// int or int32 goes to 2147483647
	// int64 goes to 9223372036854775807
	var n9 int64 = 111111111
	fmt.Printf("%v * %v = %v\n", n9, n9, n9*n9)
	fmt.Println("(Result is 1 to 9 and down again)")

	fmt.Println("-------------------------")
	// magic prime 37
	n := 37
	for k := 3; k < 28; k += 3 {
		fmt.Printf("%v * %2d = %v\n", n, k, n*k)
	}

	fmt.Println("-------------------------")
	// 1 through 9 without the 8
	mm := 12345679
	// note that the digits in k sum to 9
	for k := 9; k < 82; k += 9 {
		fmt.Printf("%v * %2d = %v\n", mm, k, mm*k)
	}

	fmt.Println("-------------------------")
	fmt.Println("The whole thing on steroids:")
	steroid := uint64(1000000001)
	for k := 9; k < 82; k += 9 {
		fmt.Printf("%v\n", steroid * uint64(mm) * uint64(k))
	}    

	fmt.Println("-------------------------")
	fmt.Println("Sum 0 to a multiple of 10:")
	// multiples of 10
	tens := []uint64{10, 100, 1000, 10000, 100000, 1000000, 10000000}
	for _, ten := range tens {
		fmt.Printf("sum(%d) = %v\n", ten, sum(ten))
	}

	fmt.Println("-------------------------")
	// pick any 2 digit integer eg. 57
	p := 57
	q := p
	mp := []int{3, 7, 13, 37} // all primes
	for _, k := range mp {
		q *= k
	}
	fmt.Printf("You picked %d and got %d\n", p, q)

	fmt.Println("-------------------------")
	// pick any 5 digit integer eg. 11457
	p2 := 11475
	q2 := p2
	mp2 := []int{11, 9091} // primes again
	for _, k := range mp2 {
		q2 *= k
	}
	fmt.Printf("You picked %d and got %d\n", p2, q2)

	fmt.Println("-------------------------")
	fmt.Println("Grains of rice on a chessboard if doubled each square:")
	// has to be an unsigned integer for left shift to work properly
	var square_number uint64 = 64

	grains_on_square := uint64(1) << (square_number - 1)
	sf := "..... number of grains on square number %v = %v\n"
	fmt.Printf(sf, square_number, grains_on_square)

	grains_total := (uint64(1) << square_number) - 1
	sf2 := "total number of grains by square number %v = %v\n"
	fmt.Printf(sf2, square_number, grains_total)

	fmt.Printf("calculated with a loop for comparison      = %v\n",
		uint64(18446744073709551615))
}

/* result:
Old programming humor:
Oct 31 == Dec 25
-------------------------
Nine ones squared ...
111111111 * 111111111 = 12345678987654321
(Result is 1 to 9 and down again)
-------------------------
37 *  3 = 111
37 *  6 = 222
37 *  9 = 333
37 * 12 = 444
37 * 15 = 555
37 * 18 = 666
37 * 21 = 777
37 * 24 = 888
37 * 27 = 999
-------------------------
12345679 *  9 = 111111111
12345679 * 18 = 222222222
12345679 * 27 = 333333333
12345679 * 36 = 444444444
12345679 * 45 = 555555555
12345679 * 54 = 666666666
12345679 * 63 = 777777777
12345679 * 72 = 888888888
12345679 * 81 = 999999999
-------------------------
The whole thing on steroids:
111111111111111111
222222222222222222
333333333333333333
444444444444444444
555555555555555555
666666666666666666
777777777777777777
888888888888888888
999999999999999999
-------------------------
Sum 0 to a multiple of 10:
sum(10) = 55
sum(100) = 5050
sum(1000) = 500500
sum(10000) = 50005000
sum(100000) = 5000050000
sum(1000000) = 500000500000
sum(10000000) = 50000005000000
-------------------------
You picked 57 and got 575757
-------------------------
You picked 11475 and got 1147511475
-------------------------
Grains of rice on a chessboard if doubled each square:
..... number of grains on square number 64 = 9223372036854775808
total number of grains by square number 64 = 18446744073709551615
calculated with a loop for comparison      = 18446744073709551615
*/