From 9fb97f548f83b4406f5315891c2f29f53f1940d4 Mon Sep 17 00:00:00 2001
From: kennethcheng <a1478925630@163.com>
Date: Mon, 14 Sep 2026 09:00:54 +0800
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---
 .../comprehensive/Comprehensive.scala         | 11 +++++++
 src/main/scala/practise7/demo/Demo.scala      | 25 +++++++++++++++
 .../scala/practise7/demo/Demo1Foreach.scala   | 19 ++++++++++++
 .../practise7/demo/Demo2ForeachMap.scala      | 17 ++++++++++
 .../demo/Demo3ForeachUnderscore.scala         | 13 ++++++++
 .../scala/practise7/demo/Demo4Reduce.scala    | 19 ++++++++++++
 .../demo/Demo5ReduceUnderscore.scala          | 11 +++++++
 .../scala/practise7/demo/Demo6ReduceMax.scala | 21 +++++++++++++
 src/main/scala/practise7/demo/Demo7Fold.scala | 13 ++++++++
 .../practise7/demo/Demo8FoldMultiply.scala    | 12 +++++++
 .../practise7/exercises/Exercises1.scala      | 27 ++++++++++++++++
 .../practise7/exercises/Exercises2.scala      | 31 +++++++++++++++++++
 .../practise7/exercises/Exercises3.scala      | 25 +++++++++++++++
 .../practise7/exercises/Exercises4.scala      | 20 ++++++++++++
 .../practise7/exercises/Exercises5.scala      | 22 +++++++++++++
 .../practise7/exercises/Exercises6.scala      | 31 +++++++++++++++++++
 16 files changed, 317 insertions(+)
 create mode 100644 src/main/scala/practise7/comprehensive/Comprehensive.scala
 create mode 100644 src/main/scala/practise7/demo/Demo.scala
 create mode 100644 src/main/scala/practise7/demo/Demo1Foreach.scala
 create mode 100644 src/main/scala/practise7/demo/Demo2ForeachMap.scala
 create mode 100644 src/main/scala/practise7/demo/Demo3ForeachUnderscore.scala
 create mode 100644 src/main/scala/practise7/demo/Demo4Reduce.scala
 create mode 100644 src/main/scala/practise7/demo/Demo5ReduceUnderscore.scala
 create mode 100644 src/main/scala/practise7/demo/Demo6ReduceMax.scala
 create mode 100644 src/main/scala/practise7/demo/Demo7Fold.scala
 create mode 100644 src/main/scala/practise7/demo/Demo8FoldMultiply.scala
 create mode 100644 src/main/scala/practise7/exercises/Exercises1.scala
 create mode 100644 src/main/scala/practise7/exercises/Exercises2.scala
 create mode 100644 src/main/scala/practise7/exercises/Exercises3.scala
 create mode 100644 src/main/scala/practise7/exercises/Exercises4.scala
 create mode 100644 src/main/scala/practise7/exercises/Exercises5.scala
 create mode 100644 src/main/scala/practise7/exercises/Exercises6.scala

diff --git a/src/main/scala/practise7/comprehensive/Comprehensive.scala b/src/main/scala/practise7/comprehensive/Comprehensive.scala
new file mode 100644
index 0000000..06de2dd
--- /dev/null
+++ b/src/main/scala/practise7/comprehensive/Comprehensive.scala
@@ -0,0 +1,11 @@
+package practise7.comprehensive
+
+object Comprehensive {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
+    val result = nums.filter(_ % 2 == 0)
+      .map(_ * 10)
+      .reduce(_ + _)
+    println(result)
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo.scala b/src/main/scala/practise7/demo/Demo.scala
new file mode 100644
index 0000000..ae0a757
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo.scala
@@ -0,0 +1,25 @@
+package practise7.demo
+
+/*
+* 第 7 课：foreach、reduce、fold，开始学习“遍历”和“把多个元素聚合成一个结果
+* 这一课很重要，因为前面：
+*map
+*filter
+*主要是在“转换和筛选数据”。
+*
+*这一课开始学习：
+*遍历数据
+*↓
+*执行某个操作
+*
+*多个数据
+*↓
+*逐步计算
+*↓
+*最终得到一个结果
+*
+*这也是后面学习 Spark 聚合操作的基础。
+*/
+object Demo {
+
+}
diff --git a/src/main/scala/practise7/demo/Demo1Foreach.scala b/src/main/scala/practise7/demo/Demo1Foreach.scala
new file mode 100644
index 0000000..4d41aa2
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo1Foreach.scala
@@ -0,0 +1,19 @@
+package practise7.demo
+
+/*
+foreach 基本用法
+*/
+object Demo1Foreach {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    nums.foreach(x => println(x)) // 把 List 中的每一个元素依次交给这个函数。
+
+    /*
+      *1 → println(1)
+      *2 → println(2)
+      *3 → println(3)
+      *4 → println(4)
+      *5 → println(5)
+    */
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo2ForeachMap.scala b/src/main/scala/practise7/demo/Demo2ForeachMap.scala
new file mode 100644
index 0000000..77acb97
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo2ForeachMap.scala
@@ -0,0 +1,17 @@
+package practise7.demo
+
+/*
+foreach 和 map 区别
+*/
+object Demo2ForeachMap {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    val resultMap = nums.map(_ * 2)
+    // map👉处理每个元素👉产生一个新的集合
+    println(resultMap)
+
+    val resultForeach = nums.foreach(x => println(x))
+    // foreach👉处理每个元素👉执行某个动作👉通常不关心返回集合
+    println(resultForeach)
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo3ForeachUnderscore.scala b/src/main/scala/practise7/demo/Demo3ForeachUnderscore.scala
new file mode 100644
index 0000000..89100e2
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo3ForeachUnderscore.scala
@@ -0,0 +1,13 @@
+package practise7.demo
+
+/*
+foreach 也可以使用 _
+*/
+object Demo3ForeachUnderscore {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    println(nums.foreach(x => println(x)))
+    println(nums.foreach(println(_)))
+    println(nums.foreach(println))
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo4Reduce.scala b/src/main/scala/practise7/demo/Demo4Reduce.scala
new file mode 100644
index 0000000..9edd610
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo4Reduce.scala
@@ -0,0 +1,19 @@
+package practise7.demo
+
+/*
+reduce：把多个元素变成一个结果
+*/
+object Demo4Reduce {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    val result = nums.reduce((a, b) => a + b)
+    // 不断把两个元素合并，最后得到一个结果。
+    println(result)
+    /*
+    1 + 2 = 3
+    3 + 3 = 6
+    6 + 4 = 10
+    10 + 5 = 15
+     */
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo5ReduceUnderscore.scala b/src/main/scala/practise7/demo/Demo5ReduceUnderscore.scala
new file mode 100644
index 0000000..262b7d0
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo5ReduceUnderscore.scala
@@ -0,0 +1,11 @@
+package practise7.demo
+
+/*
+* reduce(_ + _)
+*/
+object Demo5ReduceUnderscore {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    println(nums.reduce(_ + _))
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo6ReduceMax.scala b/src/main/scala/practise7/demo/Demo6ReduceMax.scala
new file mode 100644
index 0000000..ae6ed2f
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo6ReduceMax.scala
@@ -0,0 +1,21 @@
+package practise7.demo
+
+/*
+* reduceMax
+* reduce是把集合中的元素按照某种规则不断合并成一个结果。
+*/
+object Demo6ReduceMax {
+  def main(args: Array[String]): Unit = {
+    val nums = List(10, 5, 30, 20)
+
+    val result = nums.reduce((a, b) => {
+      if (a > b) a else b
+    })
+
+    println(result)
+
+    // 也可以写成
+    val result2 = nums.reduce((a, b) => if (a > b) a else b)
+    println(result2)
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo7Fold.scala b/src/main/scala/practise7/demo/Demo7Fold.scala
new file mode 100644
index 0000000..f3c6d2b
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo7Fold.scala
@@ -0,0 +1,13 @@
+package practise7.demo
+
+/*
+* fold 基本用法
+* fold 可以提供一个初始值。
+*/
+object Demo7Fold {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    val result = nums.fold(5)(_ + _) // 5 + 1 + 2 + 3 + 4 + 5 = 20，初始值为5
+    println(result)
+  }
+}
diff --git a/src/main/scala/practise7/demo/Demo8FoldMultiply.scala b/src/main/scala/practise7/demo/Demo8FoldMultiply.scala
new file mode 100644
index 0000000..fbe1f34
--- /dev/null
+++ b/src/main/scala/practise7/demo/Demo8FoldMultiply.scala
@@ -0,0 +1,12 @@
+package practise7.demo
+
+/*
+* fold 也不只是求和
+*/
+object Demo8FoldMultiply {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    val result = nums.fold(2)(_ * _) // 2 * 1 * 2 * 3 * 4 * 5 = 240
+    println(result)
+  }
+}
diff --git a/src/main/scala/practise7/exercises/Exercises1.scala b/src/main/scala/practise7/exercises/Exercises1.scala
new file mode 100644
index 0000000..7bd5be5
--- /dev/null
+++ b/src/main/scala/practise7/exercises/Exercises1.scala
@@ -0,0 +1,27 @@
+package practise7.exercises
+
+/*
+* 练习 1：foreach
+val nums = List(1, 2, 3, 4, 5)
+
+使用 foreach，依次打印：
+1
+2
+3
+4
+5
+
+尝试两种写法：
+nums.foreach(x => println(x))
+
+以及：
+nums.foreach(println)
+观察两者是否一样。
+*/
+object Exercises1 {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    nums.foreach(x => println(x))
+    nums.foreach(println)
+  }
+}
diff --git a/src/main/scala/practise7/exercises/Exercises2.scala b/src/main/scala/practise7/exercises/Exercises2.scala
new file mode 100644
index 0000000..f79d174
--- /dev/null
+++ b/src/main/scala/practise7/exercises/Exercises2.scala
@@ -0,0 +1,31 @@
+package practise7.exercises
+
+/*
+练习 2：foreach + if
+val nums = List(1, 2, 3, 4, 5, 6)
+
+使用 foreach：
+只打印偶数
+
+结果：
+2
+4
+6
+
+提示：
+if (x % 2 == 0) {
+  println(x)
+}
+*/
+
+object Exercises2 {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5, 6)
+    val result = nums.foreach(x =>
+      if (x % 2 == 0) {
+        println(x)
+      }
+    )
+    println(result)
+  }
+}
diff --git a/src/main/scala/practise7/exercises/Exercises3.scala b/src/main/scala/practise7/exercises/Exercises3.scala
new file mode 100644
index 0000000..5740401
--- /dev/null
+++ b/src/main/scala/practise7/exercises/Exercises3.scala
@@ -0,0 +1,25 @@
+package practise7.exercises
+
+/*
+* 练习 3：reduce 求和
+val nums = List(1, 2, 3, 4, 5)
+
+使用：
+
+reduce
+
+得到：
+
+15
+
+要求尽量使用：
+
+_
+* */
+object Exercises3 {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    val result = nums.reduce(_ + _)
+    println(result)
+  }
+}
diff --git a/src/main/scala/practise7/exercises/Exercises4.scala b/src/main/scala/practise7/exercises/Exercises4.scala
new file mode 100644
index 0000000..d698df4
--- /dev/null
+++ b/src/main/scala/practise7/exercises/Exercises4.scala
@@ -0,0 +1,20 @@
+package practise7.exercises
+
+/*
+练习 4：reduce 求最大值
+val nums = List(10, 20, 5, 30, 15)
+
+使用 reduce 找出最大值：
+30
+
+这里可以继续使用：
+if (a > b) a else b
+*/
+object Exercises4 {
+  def main(args: Array[String]): Unit = {
+    val nums = List(10, 20, 5, 30, 15)
+    val result = nums.reduce((a, b) => if (a > b) a else b)
+    println(result)
+  }
+
+}
diff --git a/src/main/scala/practise7/exercises/Exercises5.scala b/src/main/scala/practise7/exercises/Exercises5.scala
new file mode 100644
index 0000000..464bcb4
--- /dev/null
+++ b/src/main/scala/practise7/exercises/Exercises5.scala
@@ -0,0 +1,22 @@
+package practise7.exercises
+
+/*
+练习 5：fold 求和
+val nums = List(1, 2, 3, 4, 5)
+
+使用：
+fold
+
+并指定初始值：
+0
+
+最终得到：
+15
+*/
+object Exercises5 {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    val result = nums.fold(0)(_ + _)
+    println(result)
+  }
+}
diff --git a/src/main/scala/practise7/exercises/Exercises6.scala b/src/main/scala/practise7/exercises/Exercises6.scala
new file mode 100644
index 0000000..6e3a0fa
--- /dev/null
+++ b/src/main/scala/practise7/exercises/Exercises6.scala
@@ -0,0 +1,31 @@
+package practise7.exercises
+
+/*
+练习 6 ⭐
+
+给：
+val nums = List(1, 2, 3, 4, 5)
+
+分别使用：
+reduce
+fold
+
+计算乘积：
+120
+
+也就是：
+1 × 2 × 3 × 4 × 5
+
+fold 使用初始值：
+1
+*/
+object Exercises6 {
+  def main(args: Array[String]): Unit = {
+    val nums = List(1, 2, 3, 4, 5)
+    val resultFold = nums.fold(1)(_ * _)
+    println(resultFold)
+
+    val resultReduce = nums.reduce(_ * _)
+    println(resultReduce)
+  }
+}