From ab5d1c699755f51192d5cab00bf0e601032018b3 Mon Sep 17 00:00:00 2001
From: Aryadev Chavali <aryadev@aryadevchavali.com>
Date: Mon, 22 Nov 2021 06:08:52 +0000
Subject: (list)+filter function

This generates a new list, and appends to it every time the current
list value satisfies the condition.  It then recursively calls the
filter on the next value.
---
 list.cpp | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/list.cpp b/list.cpp
index b83b018..caa4fbc 100644
--- a/list.cpp
+++ b/list.cpp
@@ -75,6 +75,16 @@ T reduce(List<T> *lst, T (*reducer) (T, T), T init = 0)
   return reduce(lst->next, reducer, init);
 }
 
+template <typename T>
+List<T> *filter(List<T> *lst, bool (*f)(T), List<T> *new_lst = nullptr)
+{
+  if (!lst)
+    return new_lst;
+  if (f(lst->value))
+    new_lst = append(new_lst, lst->value);
+  return filter(lst->next, f, new_lst);
+}
+
 template <typename T>
 std::ostream& operator<<(std::ostream& ostream, const List<T> *lst)
 {
-- 
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