From bf5298b2d5f8b12d656c5387c64f73dca41b7be4 Mon Sep 17 00:00:00 2001
From: Dimitri Lozeve <dimitri@lozeve.com>
Date: Sat, 9 May 2020 12:15:04 +0200
Subject: [PATCH] Solve problem 9

---
 phase1.dyalog | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/phase1.dyalog b/phase1.dyalog
index 8cff9f3..00a66ea 100644
--- a/phase1.dyalog
+++ b/phase1.dyalog
@@ -14,3 +14,5 @@
  bits←{f←⍸∘⌽(2∘⊥⍣¯1)⋄∧/(f⍺)∊f⍵}
  ⍝ 8: Zigzag Numbers
  zigzag←∧/2=∘|2-/∘×2-/(10∘⊥⍣¯1)
+ ⍝ 9: Rise and Fall
+ risefall←{∧/(⍳∘≢≡⍋)¨(⊂((⊢⍳⌈/)↑⊢),⍵),⊂⌽((⊢⍳⌈/)↓⊢),⍵}