finished the Conditions.

Author: lzw-723

tutorials/learn-c.org/zh/Conditions.md

@@ -14,7 +14,7 @@ learn-c.org仍在建设中——如果你想贡献教程，请点击下方的`
 - [[变量和类型]]
 - [[数组]]
 - [[多维数组]]
-- [[Conditions]]
+- [[条件语句]]
 - [[Strings]]
 - [[For loops]]
 - [[While loops]]

tutorials/learn-c.org/zh/Welcome.md

@@ -0,0 +1,151 @@
+Tutorial
+--------
+
+### 决策
+
+生活中我们必须做出决定。要根据自己的需要来做出决定，程序也是如此。
+
+下面是C语言中决策结构的一般形式。
+
+
+    int target = 10;
+    if (target == 10) {
+        printf("Target is equal to 10");
+    }
+
+
+### `if`语句
+
+`if`语句检查一个表达式是`真`还是`假`，并根据结果执行不同的代码。
+
+使用了`==`运算符判断两个变量是否相等，就像上面的例子一样。
+
+也可以用不等式运算符来判断。例如——
+
+    int foo = 1;
+    int bar = 2;
+
+    if (foo < bar) {
+        printf("foo is smaller than bar.");
+    }
+
+    if (foo > bar) {
+        printf("foo is greater than bar.");
+    }
+
+使用`else`关键字在表达式为`false`时退出代码。
+
+    int foo = 1;
+    int bar = 2;
+
+    if (foo < bar) {
+        printf("foo is smaller than bar.");
+    } else {
+        printf("foo is greater than bar.");
+    }
+
+有两个以上的结果可供选择的情况下，可以将多个`if` `else`语句`连锁`。
+
+    int foo = 1;
+    int bar = 2;
+
+    if (foo < bar) {
+        printf("foo is smaller than bar.");
+    } else if (foo == bar) {
+        printf("foo is equal to bar.");
+    } else {
+        printf("foo is greater than bar.");
+    }
+
+也可以嵌套`if` `else`语句。
+
+    int peanuts_eaten = 22;
+    int peanuts_in_jar = 100;
+    int max_peanut_limit = 50;
+
+    if (peanuts_in_jar > 80) {
+        if (peanuts_eaten < max_peanut_limit) {
+            printf("Take as many peanuts as you want!\n");
+        }
+    } else {
+        if (peanuts_eaten > peanuts_in_jar) {
+            printf("You can't have anymore peanuts!\n");
+        }
+        else {
+            printf("Alright, just one more peanut.\n");
+        }
+    }
+
+
+两个或多个表达式可以使用逻辑运算符检查是否都为`真`，或是否其中一个为`真`。使用AND运算符`&&`检查两个表达式是否都为`真`。使用OR运算符`||`检查是否至少有一个表达式为`真`。
+
+    int foo = 1;
+    int bar = 2;
+    int moo = 3;
+
+    if (foo < bar && moo > bar) {
+        printf("foo is smaller than bar AND moo is larger than bar.");
+    }
+
+    if (foo < bar || moo > bar) {
+        printf("foo is smaller than bar OR moo is larger than bar.");
+    }
+
+NOT运算符`!`类似：
+
+    int target = 9;
+    if (target != 10) {
+        printf("Target is not equal to 10");
+    }
+
+
+Exercise
+--------
+
+练习，在`guessNumber`函数中构建一个`if`语句，检查数字`guess`是否等于555。如果是，在该函数内用`printf`打印出 "正确。你猜对了！"。如果`guess`小于555，用`printf`打印出 "你猜的低了"。如果`guess`大于555，用`printf`打印出 "你猜的高了"。
+
+* **重要**：不要忘记在要打印的字符串末尾添加一个换行符`\n`。
+
+Tutorial Code
+-------------
+
+    #include <stdio.h>
+
+    void guessNumber(int guess) {
+        // TODO：在这里写代码
+    }
+
+    int main() {
+        guessNumber(500);
+        guessNumber(600);
+        guessNumber(555);
+    }
+
+Expected Output
+---------------
+
+Your guess is too low.
+Your guess is too high.
+Correct. You guessed it!
+
+Solution
+--------
+
+    #include <stdio.h>
+
+    void guessNumber(int guess) {
+        // TODO：在这里写代码
+        if (guess < 555) {
+            printf("Your guess is too low.\n");
+        } else if (guess > 555) {
+            printf("Your guess is too high.\n");
+        } else {
+            printf("Correct. You guessed it!\n");
+        }
+    }
+
+    int main() {
+        guessNumber(500);
+        guessNumber(600);
+        guessNumber(555);
+    }