添加斐波那契

data/1..NET初阶/2.C#语法/7.C# 数组和 foreach 语句/Loop.md

+# 斐波那契数列
+
+C# 的 foreach 可以遍历实现了迭代器接口的数据。以下哪个选项的代码不对？
+
+## 答案
+
+```csharp
+int FibNumbers(int n){
+    if(n==0) return 0;
+    if(n==1) return 1;
+    return FibNumbers(n-1) + FibNumbers(n-2);
+}
+
+foreach (int number in FibNumbers(7)) {
+    Console.WriteLine($"{number}");
+}
+```
+
+## 选项
+
+### A
+
+```csharp
+int[] fibNumbers = new int[] { 0, 1, 1, 2, 3, 5, 8, 13 };
+foreach (int number in fibNumbers) {
+    Console.WriteLine($"{number}");
+}
+```
+
+### B
+
+```csharp
+IEnumerable<int> FibNumbers(int n){
+    yield return 0;
+    yield return 1;
+
+    int a = 0;
+    int b = 1;
+    for(int i=1;i<n;i++){
+        int next = a+b;
+        yield return next;
+        a = b;
+        b = next;
+    }
+}
+
+foreach (int number in FibNumbers(7)) {
+    Console.WriteLine($"{number}");
+}
+```
+
+### C
+
+```csharp
+var fibNumbers = new FibNumbers(7);
+foreach(int number in fibNumbers){
+    Console.WriteLine($"{number}");
+}
+
+class FibNumbers : IEnumerable<int>{
+    int n;
+    FibNumbersEnumerator e;
+    public FibNumbers(int n){
+        this.n = n;
+        this.e = new FibNumbersEnumerator(this.n);
+    }
+    public IEnumerator<int> GetEnumerator(){
+        return this.e;
+    }
+    IEnumerator IEnumerable.GetEnumerator(){
+        return this.GetEnumerator();
+    }
+}
+
+class FibNumbersEnumerator : IEnumerator<int>{
+    int n;
+    int a;
+    int b;
+    int index;
+    public FibNumbersEnumerator(int n){
+        this.n = n;
+        this.Reset();
+    }
+
+    public int Current{
+       get { return this.a; }
+    }
+
+    object IEnumerator.Current{
+       get { return this.Current; }
+    }
+
+    public bool MoveNext(){
+       if(this.index<1){
+            this.a = 0;
+            this.b = 1;
+       }else{
+            int next = this.a+this.b;
+            this.a = this.b;
+            this.b = next;
+            
+       }
+       
+       this.index +=1;
+       if(this.index==this.n+2){
+           return false;
+       }else{
+           return true;
+       }
+    }
+
+    public void Reset(){
+        this.a = 0;
+        this.b = 0;
+        this.index = 0;
+    }
+
+    public void Dispose(){}
+}
+```

data/1..NET初阶/2.C#语法/7.C# 数组和 foreach 语句/sample/Program.cs

-// See https://aka.ms/new-console-template for more information
-Console.WriteLine("Hello, World!");
+
+using System.Collections;
+
+int[] fibNumbers = new int[] { 0, 1, 1, 2, 3, 5, 8, 13 };
+foreach (int number in fibNumbers) {
+    Console.WriteLine($"{number}");
+}
+
+IEnumerable<int> FibNumbers(int n){
+    yield return 0;
+    yield return 1;
+
+    int a = 0;
+    int b = 1;
+    for(int i=1;i<n;i++){
+        int next = a+b;
+        yield return next;
+        a = b;
+        b = next;
+    }
+}
+
+foreach (int number in FibNumbers(7)) {
+    Console.WriteLine($"{number}");
+}
+
+var fibNumbers = new FibNumbers(7);
+foreach(int number in fibNumbers){
+    Console.WriteLine($"{number}");
+}
+
+class FibNumbers : IEnumerable<int>{
+    int n;
+    FibNumbersEnumerator e;
+    public FibNumbers(int n){
+        this.n = n;
+        this.e = new FibNumbersEnumerator(this.n);
+    }
+    public IEnumerator<int> GetEnumerator(){
+        return this.e;
+    }
+    IEnumerator IEnumerable.GetEnumerator(){
+        return this.GetEnumerator();
+    }
+}
+
+class FibNumbersEnumerator : IEnumerator<int>{
+    int n;
+    int a;
+    int b;
+    int index;
+    public FibNumbersEnumerator(int n){
+        this.n = n;
+        this.Reset();
+    }
+
+    public int Current{
+       get { return this.a; }
+    }
+
+    object IEnumerator.Current{
+       get { return this.Current; }
+    }
+
+    public bool MoveNext(){
+       if(this.index<1){
+            this.a = 0;
+            this.b = 1;
+       }else{
+            int next = this.a+this.b;
+            this.a = this.b;
+            this.b = next;
+            
+       }
+       
+       this.index +=1;
+       if(this.index==this.n+2){
+           return false;
+       }else{
+           return true;
+       }
+    }
+
+    public void Reset(){
+        this.a = 0;
+        this.b = 0;
+        this.index = 0;
+    }
+
+    public void Dispose(){}
+}
+
+
+
+int FibNumbers(int n){
+    if(n==0) return 0;
+    if(n==1) return 1;
+    return FibNumbers(n-1) + FibNumbers(n-2);
+}
+
+foreach (int number in FibNumbers(7)) {
+    Console.WriteLine($"{number}");
+}
\ No newline at end of file