Programs for printing reverse triangle patterns with * in scala - scala

Question

I am trying to explore Scala. I am new to Scala. This might be a simple question and searched in google to get below scenario to solve. But couldn't get answers. Instead of Scala I am getting Java related things.
My requirement to print format like below.
* * * * *
* * * *
* * *
*
Can someone suggest me how to get this format.
Thanks in advance.
Kanti

Answer 1

Just for the sake of illustration, here are two possible solution to the problem.
The first one is completely imperative, while the second one is more functional.
The idea is that this serves as an example to help you think how to solve problems in a programmatic way.
As many of us have already commented, if you do not understand the basic ideas behind the solution, then this code will be useless in the long term.
Here is the imperative solution, the idea is simple, we need to print n lines, each line contains n - i starts (where i is the number of the line, starting at 0). The starts are separated by an empty space.
Finally, before printing the starts, we need some padding, looking at example inputs, you can see that the padding starts at 0 and increases by 1 for each line.
def printReverseTriangle(n: Int): Unit = {
var i = 0
var padding = 0
while (i < n) {
var j = padding
while (j > 0) {
print(" ")
j -= 1
}
var k = n - i
while (k > 0) {
print("* ")
k -= 1
}
println()
i += 1
padding += 1
}
}
And here is a more functional approach.
As you can see, in this case we do not need to mutate anything, all the high level operators do that for us. And we only need to focus on the description of the solution.
def printReverseTriangle(size: Int): Unit = {
def makeReverseTriangle(size: Int): List[String] =
List.tabulate(size) { i =>
(" " * (size - i)) + ("* " * i)
}.reverse
println(makeReverseTriangle(size).mkString("\n"))
}

Answer 2

To add an alternative to Luis's answer, here's a recursive solution:
import scala.annotation.tailrec
def printStars(i: Int): Unit = {
#tailrec
def loop(j: Int): Unit = {
if(j > 0) {
val stars = Range(0, j).map(_ => "*").mkString(" ") // make stars
if(i == j) println(stars) // no need for spaces
else println((" " * (i - j)) + stars) // spaces before the stars
loop(j - 1)
}
}
loop(i)
}
printStars(3)
// * * *
// * *
// *
This function will take a maximum triangle size (i), and for that size until i is no longer greater than 0 it will print out the correct number of stars (and spaces), then decrement by 1.
Note: Range(0, j).map(_ => "*").mkString(" ") can be replaced with List.tabulate(j)(_ => "*").mkString(" ") per Luis's answer - I'm not sure which is faster (I've not tested it).