[BEGINNER] reccurence! and sequence!

[Ali Çehreli via Digitalmars-d-learn]

On 07/05/2017 04:38 PM, helxi wrote:

 >> ----
 >> sequence!((a, n) => a[0] + 1)(1).take(10).writeln;
 >>     // [2, 2, 2, 2, 2, 2, 2, 2, 2, 2]
 >>     // because a[0] is always 1
 >>
 >> recurrence!((a, n) => a[0] + 1)(1).take(10).writeln;
 >>     // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
 >>     // because a[0] refers to the previous value
 >> ----
 >
 > Oh thank you. Just 2 follow-up questions:
 >> recurrence!((a, n) => a[0] + 1)(1).take(10).writeln;
 > 1. In the last example of reccurence, what does n in (a,n) refer to?

n is "the index of the current value". Each time the lambda is called,

   a[n] is what is being generated
   a[n-1] is the previous value
   a[0] is the same as a[n-1]? (I find this confusing)

 > 2. How would you chain until! with reccurence? For example I want to
 > compute 1, 10, 100, ..., (until the value remains smaller than 1000_000)?

import std.stdio;
import std.algorithm;
import std.range;

void main() {
     auto r = recurrence!((a, n) => a[n-1] * 10)(1);
     auto u = r.until!(a => a >= 1_000_000);
     writeln(u);
}

[1, 10, 100, 1000, 10000, 100000]

Ali