lecture 3 stuff

lectures/03/find.rs

@@ -0,0 +1,71 @@
+// motivation for closures
+fn sum<T: std::ops::AddAssign + Copy>(s: &[T]) -> T {
+    let mut total = s[0];
+
+    for i in 1..s.len() {
+        total += s[i];
+    }
+
+    total
+}
+
+fn find<T>(s: &[T], f: fn(&T) -> bool) -> Option<&T> {  // only works for functions f
+    for x in s {
+        if f(x) {
+            return Some(x);
+        }
+    }
+    None
+}
+
+fn find2<T, F>(s: &[T], f: F) -> Option<&T>  // works for both functions & closures
+where F: Fn(&T) -> bool {
+    for x in s {
+        if f(x) {
+            return Some(x);
+        }
+    }
+    None
+}
+
+fn main() {
+    let v = vec![3, 2, 7, 6, 8];
+    println!("{}", sum(&v));
+
+    let v = vec![3.1, 2.0, 7.3, 6.1, 8.0];
+    println!("{}", sum(&v));
+
+    let v = vec![3, 2, 7, 6, 8];
+    fn is_even(x: &i32) -> bool {
+        x % 2 == 0
+    }
+
+    if let Some(r) = find(&v, is_even) {
+        println!("{r}");
+    }
+
+    let divisor = 2;
+    /* 
+    fn divisible(x: &i32) -> bool {
+        x % divisor == 0    // ERROR: can't capture divisor
+    }
+    */
+
+    // let divisible = |x: &i32| -> bool { x % divisor == 0 };  // closure; can capture environment
+    let divisible = |x: &i32| { x % divisor == 0 };  // closure; can capture environment
+
+    /*
+    if let Some(r) = find(&v, divisible) {  // ERROR: A closure is not regular fn
+        println!("{r}");
+    }
+    */
+
+    if let Some(r) = find2(&v, divisible) {  // OK
+        println!("{r}");
+    }
+/*
+    if let Some(r) = find2(&v, is_even) {  // OK, also works for regular function
+        println!("{r}");
+    }
+*/
+}