repeat string n times in Kotlin

The built in CharSequence.repeat extension does this in an efficient way, see the source here.

val str: String = "*".repeat(100)

Of course, this will still require O(n) steps to create the string. However, using this built-in stdlib function has its advantages: it's cross-platform, easy to read, and can be improved in performance over time, if there's a more efficient solution. The loop inside it will probably be optimized by the compiler or the runtime anyway.

An alternative to the CharSequence.repeat is a CharArray with an init function:

CharArray(N, {i -> '*'}).joinToString(separator="")

This solution has the advantage that you can define prefix, postfix, and separator.

StringBuilder would improve the memory footprint here:

val sb = StringBuilder() 
val n = 100
for (j in 0 until n) {
  sb.append("*") 
}