## digitalmars.D.learn - [BEGINNER] reccurence! and sequence!

- helxi (10/14) Jun 26 2017 /** okay, it's a triangular number array
- ag0aep6g (27/41) Jun 26 2017 `a` is a tuple of the run-time arguments you pass to `sequence`. In this...
- helxi (7/40) Jul 05 2017 1. In the last example of reccurence, what does n in (a,n) refer
- =?UTF-8?Q?Ali_=c3=87ehreli?= (15/29) Jul 05 2017 n is "the index of the current value". Each time the lambda is called,

Can someone give me a very watered-down explanation of what std.range's recurrence! and sequence! do?auto tri = sequence!((a,n) => n*(n+1)/2)();/** okay, it's a triangular number array * I understand n is the index number, the nth term * However where does this 'a' go? */auto odds = sequence!("a[0] + n * a[1]")(1, 2);/** okay, this is a range of odd numbers / where and how do I plug (1, 2) into ""a[0] + n * a[1]"sequence!("n+2")(1).take(10).writeln;//[2, 3, 4, 5, 6, 7, 8, 9, 10, 11]recurrence!("n+2")(1).take(10).writeln;//[1, 3, 4, 5, 6, 7, 8, 9, 10, 11]

Jun 26 2017

On 06/26/2017 11:51 AM, helxi wrote:`a` is a tuple of the run-time arguments you pass to `sequence`. In this example, no arguments are passed (empty parens at the end of the call), so `a` is empty.auto tri = sequence!((a,n) => n*(n+1)/2)();/** okay, it's a triangular number array * I understand n is the index number, the nth term * However where does this 'a' go? */a[0] = 1 a[1] = 2auto odds = sequence!("a[0] + n * a[1]")(1, 2);/** okay, this is a range of odd numbers / where and how do I plug (1, 2) into ""a[0] + n * a[1]"`a` isn't used in the string lambda, and it's not considered an element of the range. So n starts at 0 and this just prints 0+2, 1+2, 2+2, etc.sequence!("n+2")(1).take(10).writeln;//[2, 3, 4, 5, 6, 7, 8, 9, 10, 11]`a` is still not used in the string lambda, but `recurrence` uses the values in `a` as the first elements of the range. `n` is incremented accordingly (to 1), so this prints: 1 = a[0], 3 = (n = 1) + 2, 4 = (n = 2) + 2, etc. Another difference between `sequence` and `recurrence` is that `a` always refers to the same initial value(s) in `sequence`, while in `recurrence` it gets updated and refers to the previous element(s) of the range: ---- 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 ----recurrence!("n+2")(1).take(10).writeln;//[1, 3, 4, 5, 6, 7, 8, 9, 10, 11]

Jun 26 2017

On Monday, 26 June 2017 at 10:34:22 UTC, ag0aep6g wrote:On 06/26/2017 11:51 AM, helxi wrote:Oh thank you. Just 2 follow-up questions:[...]`a` is a tuple of the run-time arguments you pass to `sequence`. In this example, no arguments are passed (empty parens at the end of the call), so `a` is empty.[...]a[0] = 1 a[1] = 2[...]`a` isn't used in the string lambda, and it's not considered an element of the range. So n starts at 0 and this just prints 0+2, 1+2, 2+2, etc.[...]`a` is still not used in the string lambda, but `recurrence` uses the values in `a` as the first elements of the range. `n` is incremented accordingly (to 1), so this prints: 1 = a[0], 3 = (n = 1) + 2, 4 = (n = 2) + 2, etc. Another difference between `sequence` and `recurrence` is that `a` always refers to the same initial value(s) in `sequence`, while in `recurrence` it gets updated and refers to the previous element(s) of the range: ---- 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 ----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? 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)?

Jul 05 2017

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

Jul 05 2017

On Thursday, 6 July 2017 at 00:21:44 UTC, Ali Çehreli wrote:On 07/05/2017 04:38 PM, helxi wrote:Hmm, I get it now. Thank you very much.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)[...]Oh thank you. Just 2 follow-up questions:[...]1. In the last example of reccurence, what does n in (a,n)2. How would you chain until! with reccurence? For example Iwant tocompute 1, 10, 100, ..., (until the value remains smallerthan 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

Jul 06 2017

On 07/06/2017 02:21 AM, Ali Çehreli wrote:On 07/05/2017 04:38 PM, helxi wrote:[...]>> 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)Looks like I was wrong when I stated that "a[0] refers to the previous value". `a[0]` is actually invalid for n > 1. The documentation for `recurrence` says you have to index relative to n, and you can only go back as many values as you gave initially. I should have used `a[n - 1]`.

Jul 06 2017