4.5 if

1.6/ja/book/if.md

@@ -1,54 +1,73 @@
 % if
+<!-- % if -->
 
-Rust’s take on `if` is not particularly complex, but it’s much more like the
-`if` you’ll find in a dynamically typed language than in a more traditional
-systems language. So let’s talk about it, to make sure you grasp the nuances.
+<!-- Rust’s take on `if` is not particularly complex, but it’s much more like the -->
+<!-- `if` you’ll find in a dynamically typed language than in a more traditional -->
+<!-- systems language. So let’s talk about it, to make sure you grasp the nuances. -->
+Rustにおける `if` の扱いはさほど複雑ではありませんが、伝統的なシステムプログラミング言語のそれに比べて、
+動的型付け言語でみられる `if` にずっと近いものになっています。そのニュアンスをしっかり理解できるよう、
+さっそく説明していきましょう。
 
-`if` is a specific form of a more general concept, the ‘branch’. The name comes
-from a branch in a tree: a decision point, where depending on a choice,
-multiple paths can be taken.
+<!-- `if` is a specific form of a more general concept, the ‘branch’. The name comes -->
+<!-- from a branch in a tree: a decision point, where depending on a choice, -->
+<!-- multiple paths can be taken. -->
+`if` は一般化されたコンセプト、「分岐(branch)」の特別な形式です。この名前は木の枝(branch)を由来とし:
+取りうる複数のパスから、選択の決定を行うポイントを表します。
 
-In the case of `if`, there is one choice that leads down two paths:
+<!-- In the case of `if`, there is one choice that leads down two paths: -->
+`if` の場合は、続く2つのパスから1つを選択します。
 
 ```rust
 let x = 5;
 
 if x == 5 {
-    println!("x is five!");
+# // println!("x is five!");
+    println!("x は 5 です!");
 }
 ```
 
-If we changed the value of `x` to something else, this line would not print.
-More specifically, if the expression after the `if` evaluates to `true`, then
-the block is executed. If it’s `false`, then it is not.
+<!-- If we changed the value of `x` to something else, this line would not print. -->
+<!-- More specifically, if the expression after the `if` evaluates to `true`, then -->
+<!-- the block is executed. If it’s `false`, then it is not. -->
+仮に `x` を別の値へと変更すると、この行は表示されません。より正確に言うなら、
+`if` のあとにくる式が `true` に評価された場合に、ブロックが実行されます。
+`false` の場合、ブロックは実行されません。
 
-If you want something to happen in the `false` case, use an `else`:
+<!-- If you want something to happen in the `false` case, use an `else`: -->
+`false` の場合にも何かをしたいなら、 `else` を使います:
 
 ```rust
 let x = 5;
 
 if x == 5 {
-    println!("x is five!");
+# // println!("x is five!");
+    println!("x は 5 です!");
 } else {
-    println!("x is not five :(");
+# // println!("x is not five :(");
+    println!("x は 5 ではありません :(");
 }
 ```
 
-If there is more than one case, use an `else if`:
+<!-- If there is more than one case, use an `else if`: -->
+場合分けが複数あるときは、 `else if` を使います:
 
 ```rust
 let x = 5;
 
 if x == 5 {
-    println!("x is five!");
+# // println!("x is five!");
+    println!("x は 5 です!");
 } else if x == 6 {
-    println!("x is six!");
+# // println!("x is six!");
+    println!("x は 6 です!");
 } else {
-    println!("x is not five or six :(");
+# // println!("x is not five or six :(");
+    println!("x は 5 でも 6 でもありません :(");
 }
 ```
 
-This is all pretty standard. However, you can also do this:
+<!-- This is all pretty standard. However, you can also do this: -->
+全くもって普通ですね。しかし、次のような使い方もできるのです:
 
 ```rust
 let x = 5;
@@ -60,14 +79,18 @@ let y = if x == 5 {
 }; // y: i32
 ```
 
-Which we can (and probably should) write like this:
+<!-- Which we can (and probably should) write like this: -->
+次のように書くこともできます（そして、大抵はこう書くべきです）:
 
 ```rust
 let x = 5;
 
 let y = if x == 5 { 10 } else { 15 }; // y: i32
 ```
 
-This works because `if` is an expression. The value of the expression is the
-value of the last expression in whichever branch was chosen. An `if` without an
-`else` always results in `()` as the value.
+<!-- This works because `if` is an expression. The value of the expression is the -->
+<!-- value of the last expression in whichever branch was chosen. An `if` without an -->
+<!-- `else` always results in `()` as the value. -->
+これが出来るのは `if` が式であるためです。その式の値は、選択された分岐中の最後の式の値となります。
+`else` のない `if` では、その値は常に `()` となります。
+