Main.kt

/**
 *  Let's make a program that accepts a matrix and if
 *  the elements below the main diagonal are less than
 *  zero replace them with zero ...
 */
fun main() {
    """
     1  2 -1
    -1 -1  6
    -1 -1  9
    """.trimIndent().split("\\n+".toRegex()).map { s ->
        s.trim().split("\\s+".toRegex()).map {
            it.toInt()
        }
    }.mapIndexed { i, ints ->
        ints.mapIndexed { j, joints ->
            if (j < i && joints < 0) {
                0
            } else {
                joints
            }
        }
    }.forEach { ints ->
        ints.forEach {
            print(it.toString().padStart(3, " ".first()))
        }
        println()
    }
}