How to Solve Diagonal Print

Given a string and a number, and print the string diagonally filling in empty space with periods and going down as many lines as the given number. So "Peter piper picked a peck of pickled peppers" with n = 10 becomes

P.........r......... .........i.........p...
.e......... .........p.........c.........e..
..t.........p.........e.........k.........r.
...e.........i.........c.........l.........s
....r.........c.........k.........e.........
..... .........k......... .........d........
......P.........e.........o......... .......
.......i.........d.........f.........P...... 
........p......... ......... .........e.....
.........e.........a.........p.........p....

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package main   import (     "fmt"     "os"     "strconv" )   func main() {     str := os.Args[1]     n, _ := strconv.Atoi(os.Args[2])     for i := 0; i < n; i++ {         for k := 0; k < i; k++ {             fmt.Print(".")         }         for j := 0; j < len(str)-i; j++ {             if (j % n == 0 && j+1 < len(str)) {                 fmt.Print(string(str[j+i]))             } else {                 fmt.Print(".")             }         }         fmt.Println()     } }